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GMPV – Geochemistry, Mineralogy, Petrology & Volcanology

GMPV1.1 – Advances in measuring and applying triple oxygen isotopes

EGU2020-18899 | Displays | GMPV1.1

Tracing the triple isotope composition of air by high-precision analyses of meteorites, rocks and fossils

Andreas Pack, Meike Fischer, Christian Stübler, Stefan Peters, and Dingsu Feng

High-precision measurements of the triple oxygen isotope ratios (δ18O and Δ‘17O) in terrestrial waters, rocks and minerals opened new and exciting applications in the field of stable oxygen isotope geochemistry. After giving a short overview over measurement techniques and various applications, it will be emphasized on tracing the atmospheric composition in rocks and minerals.

Atmospheric samples from ice cores only date back ~1 Myrs. To obtain information about the atmosphere for the 99.98% of Earth history that is not covered by ice cores, we need to look for rocks. The oxygen isotope composition of the atmosphere younger than 2.4 Gyrs is dominated by molecular oxygen (O2). Molecular O2 is one of few components on Earth that has a mass-independent oxygen isotope signature. The anomaly in 17O provides information about the presence of an ozone layer, the global biosphere primary production, or the atmospheric CO2 mixing ratio. A few rocks and fossils provide information about the 17O anomaly of air O2. Sedimentary sulfates may form by precipitation from SO42- that formed by subaerial oxidation of pyrite. In that process, a part of the oxygen in the sulfate originates from air O2. Mobilizing of the sulfate oxygen can carry this anomaly over to other minerals like Fe oxides. The isotope signature of fossil tooth enamel also provides information about the atmospheric composition. Air O2 is inhaled and used to oxidize carbohydrates and fat to (mainly) CO2 and H2O, which equilibrate with body water. Tooth apatite then precipitates from body water and inherits an anomaly in 17O from the inhaled air O2. Manganese oxides are known to form by oxidation of Mn under participation of O2. If the isotope composition of dissolved O2 in the aqueous environment, in which the manganese oxides form is controlled by air, manganese oxides can be used to trace the composition of air O2. It has been shown that some meteorite impact melts (tektites) have exchanged with ambient air O2. As result of that exchange, they carry a 17O anomaly that may be used to trace the composition of air O2. Also, I-type cosmic spherules have been shown to be indicators for the isotope anomaly of air O2. These spherules form by aerial oxidation of asteroidal metallic Fe,Ni particles and thus can carry the anomaly of air O2. Such recent discoveries open insights into the composition of the Earth atmosphere beyond the 1 Myrs limit from the ice core record.

How to cite: Pack, A., Fischer, M., Stübler, C., Peters, S., and Feng, D.: Tracing the triple isotope composition of air by high-precision analyses of meteorites, rocks and fossils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18899, https://doi.org/10.5194/egusphere-egu2020-18899, 2020.

EGU2020-19830 | Displays | GMPV1.1

Standardizing high precision Δʹ17O data from silicate rocks and minerals

Martin Miller, Andreas Pack, Ilya Bindeman, and Richard Greenwood

The Δʹ17O value of O2 gas (the analyte for nearly all oxygen triple isotope measurements) can currently be measured to a precision of about 6 ppm; significantly better than the precision of the corresponding δ17O and δ18O data. However, reporting Δʹ17O measurements of silicate rocks to this degree of accuracy, relative to (for example) the VSMOW-SLAP line on the ln(1 + δ17O) versus ln(1 + δ18O) plot, poses practical challenges. Regardless of the reference line assigned, Δʹ17O values are still inextricably linked to the δ17O and δ18O calibration of the ‘working standard’ O2 on the VSMOW–SLAP scale. Yet few laboratories have the capability to make such measurements on waters and on silicates. Even when direct calibration to VSMOW and SLAP is possible, there is not yet consensus on the Δʹ17O values of widely used silicate standards such as UWG-2 garnet or San Carlos olivine, when reported to a common reference line.

Fluorination of silicate rocks and minerals, to produce O2 for isotope ratio measurements, requires a different procedure from that used for the fluorination of waters. Thus, there is the possibility of systematic errors being introduced by using a water reference material for reporting δ17O and δ18O data of silicates. Furthermore, fractionation arrays of natural silicates on the three-isotope plot are generally offset from VSMOW, which introduces an additional complication. To eliminate such potential sources of error, some authors have chosen to report δ17O and δ18O data relative to San Carlos olivine (as representative of Earth’s mantle) rather than to VSMOW, in conjunction with a reference line of assigned slope, for characterizing Δʹ17O values. However, a two-point scale, such as VSMOW–SLAP for waters, is preferable to a single point calibration

We have therefore characterized Δʹ17O values (and with inter-laboratory comparison) of two silicates spanning a greater δ18O range than VSMOW–SLAP and suggest that these materials may be used for accurate determinations of silicate Δʹ17O values. Our high-δ18O standard is a flint, designated SKFS, with δ18O = 33.93 ± 0.08 ‰ (standard error) and Δʹ17O = –69 ± 3 ppm relative to the VSMOW-SLAP reference line. This material can therefore be used to calibrate the position of an assigned reference line such that it passes through VSMOW. Alternatively, in combination with our low-δ18O silicate standard, designated as KRS (δ18O = –25.20 ± 0.03, Δʹ17O = –114 ± 2 ppm relative to the VSMOW-SLAP reference line), an empirical two-point silicate reference line may be defined from high precision δ17O and δ18O measurements of these proposed standards. Δʹ17O data of silicate rock and mineral samples reported relative to this reference line are independent of whether the δ17O and δ18O measurements are reported relative to VSMOW or to the ‘working standard’ O2, of any isotopic composition. This confers significant advantages for inter-laboratory comparisons.

How to cite: Miller, M., Pack, A., Bindeman, I., and Greenwood, R.: Standardizing high precision Δʹ17O data from silicate rocks and minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19830, https://doi.org/10.5194/egusphere-egu2020-19830, 2020.

EGU2020-17881 | Displays | GMPV1.1

High-precision triple oxygen isotope analysis of Archean and Proterozoic carbonates

Oliver Jäger, Jakub Surma, Nina Albrecht, Chris S. Marien, Wanli Xiang, Katharina Schier, Michael Bau, Joachim Reitner, and Andreas Pack

Oxygen isotopes are a widely used tracer in the field of paleoceanography and provide unique information on mineral formation and environmental conditions. Carbonate sediments record a shift in δ18O of 10 to 15‰ from the Archean towards higher values in the Phanerozoic. Three different scenarios are suggested to explain this observation: (I) hot Archean oceans, (II) depletion of 18O in Archean oceans compared to present day and (III) diagenetic alteration of the primary isotopic signature [1]. Recent advances in high-resolution gas source isotope ratio mass spectrometry provide a new tool that may allow to decipher the origin of this isotopic shift observed in the early rock record. We performed high-precision 18O/16O and 17O/16O measurements on oxygen ion fragments (16O+, 17O+, 18O+) generated in the ion source from CO2 gas [2]. Isobaric interferences on m/z=17 (16OH+) and m/z=18 (H216O+) are separated by means of high mass resolution. The CO2 gas is first liberated from carbonate samples by orthophosphoric acid digestion and then analyzed on a Thermo Scientific Ultra dual-inlet gas source isotope ratio mass spectrometer [3]. By adding the dimension of 17O/16O to the classical 18O/16O system, equilibrium trajectories of carbonates that are defined by the equilibrium fractionation factor (18aeq) and the triple isotope fractionation exponent (θ) can be predicted as a function of temperature. Minerals that were altered by or formed in meteoric water can be distinguished from those that precipitated in equilibrium with ambient sea water. Therefore, triple oxygen isotope analysis of carbonates does not only hold the potential for a new single-phase paleothermometer, but may also be used to trace the origin of carbonates. Here, we present high-precision triple oxygen isotope data for carbonates from the Pilbara and the Kaapvaal cratons that cover nearly one billion years from the Paleoarchean to the Paleoproterozoic. Marine carbonates from the Phanerozoic complement the dataset. The carbonates were formed in different marine settings, from shallow marine stromatolites to carbonates grown in the interstitial space of basaltic pillows. Phanerozoic carbonates record equilibrium conditions with modern sea water at moderate temperatures. The majority of Precambrian carbonates plot below the predicted equilibrium curve in the δ’18O-Δ‘17O space and do not reflect equilibrium conditions with modern sea water at elevated temperatures that were proposed for the Archean oceans. Modeling the triple oxygen isotope composition of carbonates in equilibrium with sea water, that is depleted in 18O also cannot explain the observed isotopic shift. Further modeling of post-depositional alteration suggests that most carbonates interacted and re-equilibrated with meteoric waters at variable water-rock ratios and temperatures.

[1] Shields and Veizer, 2002, Geochem., Geophy., Geosyst., 10.1029/2001GC000266
[2] Getachew et al., 2019, Rapid Commun. Mass. Spectrom., 10.1002/rcm.847
[3] Eiler et al., 2013, Int. J. Mass. Spectrom., 335, 45-56.

How to cite: Jäger, O., Surma, J., Albrecht, N., Marien, C. S., Xiang, W., Schier, K., Bau, M., Reitner, J., and Pack, A.: High-precision triple oxygen isotope analysis of Archean and Proterozoic carbonates , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17881, https://doi.org/10.5194/egusphere-egu2020-17881, 2020.

EGU2020-21469 | Displays | GMPV1.1

Hydrothermal seawater-basalt exchange reactions traced by triple oxygen and strontium isotope values of fluids and epidotes

David Zakharov, Ryoji Tanaka, Craig Lundstrom, David Butterfield, Mark Reed, and Ilya Bindeman

Hydrothermal circulation of seawater at mid-ocean ridges cools the oceanic crust and modulates the oceanic chemistry over multimillion-year time scales. Recent research on mass-dependent fractionation of triple oxygen isotopes allows us to gain a new insight into the seawater-basalt exchange reactions that occur within the oceanic crust. To understand the systematics of triple oxygen isotope exchange, we present a novel combined dataset for Δ17O and 87Sr/86Sr isotope values measured in modern seawater-derived vent fluids at the Axial Seamount volcano located on the Juan de Fuca Ridge and oceanic epidotes extracted from altered mid-ocean ridge basalts. Upon reaction with fresh oceanic crust, seawater evolves towards the low Mg compositions characteristic of fluids in equilibrium with basalt. In concert with decreasing Mg content and with decreasing 87Sr/86Sr, the vent fluids at Axial Seamount shift towards values that are 0.04 ‰ lower in Δ17O and 2 ‰ higher in δ18O compared to initial seawater. The 87Sr/86Sr and Δ17O values of epidotes extracted from modern hydrothermally altered basalts reveal a trend of isotope exchange similar to the one defined by the fluids. We suggest that epidotes record isotope shifts that were experienced by fluids in the areas of focused flow within the oceanic crust. Both fluids and epidotes display similar trajectories of Δ17O and 87Sr/86Sr shifts which are modeled using a Monte-Carlo simulation of reactive transport in dual-porosity medium. These trajectories provide important constraints on the physical complexity of reactive circulation of seawater within the oceanic crust. We show how the contribution of hydrothermal circulation to the isotope budget of seawater can be changed during geologic history and evaluated based on the studies of fragments of ancient oceanic crust.

How to cite: Zakharov, D., Tanaka, R., Lundstrom, C., Butterfield, D., Reed, M., and Bindeman, I.: Hydrothermal seawater-basalt exchange reactions traced by triple oxygen and strontium isotope values of fluids and epidotes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21469, https://doi.org/10.5194/egusphere-egu2020-21469, 2020.

EGU2020-44 | Displays | GMPV1.1

Using triple water isotopes signatures of surface snow to gauge metamorphism in Antarctica

Mathieu Casado, Amaelle Landais, Ghislain Picard, Laurent Arnaud, Giuliano Dreossi, Barbara Stenni, and Frederic Prie

Water isotopic composition is a key proxy for past climate reconstructions using deep ice cores from Antarctica. As precipitation forms, the local temperature is imprinted in the snowfalls δ18O. However, this climatic signal can be erased after snow deposition when snow is exposed to the atmosphere for a long time in regions with extremely low accumulation. Understanding this effect is crucial for the interpretation of ice core records from the extremely dry East Antarctic Plateau, where post-deposition processes such as blowing snow or metamorphism affect the physical and chemical properties of snow during the long periods of snow exposure to the atmosphere. Despite the importance of these processes for the reliable reconstruction of temperature from water isotopic composition in ice cores, the tools required to quantify their impacts are still missing. Here, we present a first year-long comparison between (a) time series of surface snow isotopic composition including d-excess and 17O-excess at Dome C and (b) satellite observations providing information on snow grain size, a marker of surface metamorphism. Long summer periods without precipitation tend to produce a surface snow metamorphism signature erasing the climatic signal in the surface snow δ18O. Using a simple model, we demonstrate that d-excess and 17O-excess allow the identification of the latent fluxes induced by metamorphism, and their impact on surface snow isotopic composition. In turn, their measurements can help improve climate reconstructions based on δ18O records ice by removing the influence of snow metamorphism.

How to cite: Casado, M., Landais, A., Picard, G., Arnaud, L., Dreossi, G., Stenni, B., and Prie, F.: Using triple water isotopes signatures of surface snow to gauge metamorphism in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-44, https://doi.org/10.5194/egusphere-egu2020-44, 2020.

EGU2020-6641 | Displays | GMPV1.1

Factors controlling the triple oxygen isotope composition of grass leaf water and phytoliths: insights for paleo-environmental reconstructions

Anne Alexandre, Clément Outrequin, Christine Vallet-Coulomb, Amaelle Landais, Clément Piel, Sébastien Devidal, Christophe Peugeot, Théodor Ouani, Simon Afouda, Martine Couapel, Corinne Sonzogni, Jean-Charles Mazur, Frédéric Prié, and Elizabeth Webb

The oxygen isotope signature of leaf water is used to trace several processes at the soil-plant-atmosphere interface. During photosynthesis, it is transferred to the oxygen isotope signature of atmospheric CO2 and O2, which can be used for reconstructing past changes in gross primary production. The oxygen isotope signature of leaf water additionally imprints leaf organic and mineral compounds, such as phytoliths, used as paleoclimate and paleovegetation proxies when extracted from sedimentary materials.

Numerous experimental and modelling studies were dedicated to constrain the main parameters responsible for changes in the δ18O of leaf water. Although these models usually correctly depict the main trends of 18O-enrichment of the leaf water when relative humidity decreases, the calculated absolute values often depart from the observed ones by several ‰. Moreover, the δ18O of leaf water absorbed by plants is dependent on the δ18O value of meteoric and soil waters that can vary by several ‰ at different space and time scales. These added uncertainties make our knowledge of the parameters responsible for changes in the δ18O of leaf water and phytoliths flawed.

Changes in the triple oxygen isotope composition of leaf water, expressed by the 17O-excess, are controlled by fewer variables than changes in δ18O. In meteoric water the 17O-excess varies slightly as it is weakly affected by temperature or phase changes during air mass transport. This makes the soil water fed by meteoric water and the atmospheric vapour in equilibrium with meteoric water changing little from a place to another. Hence the 17O-excess of leaf water is essentially controlled by the evaporative fractionation. The latest depends on the ratio of vapor pressure in the air to vapor pressure in the stomata intercellular space, close to relative humidity. Leaf water evaporative fractionation can lead to 17O-excess negative values that can exceed most of surficial water ones.

Here we present the outcomes of several recent growth chamber and field studies, for the purpose of i) refining the grass leaf water and phytoliths δ18O and 17O-excess modelling, ii) assessing whether the δ18O and 17O-excess of grass leaf water can be reconstructed from phytoliths, and iii) examining the precision of the 17O-excess of phytoliths as a new proxy for past changes in continental atmospheric relative humidity. Atmospheric continental relative humidity is an important climate parameter poorly constrained in global climate models. A model-data comparison approach, applicable beyond the instrumental period, is essential to progress on this issue. However, there is currently a lack of proxies allowing quantitative reconstruction of past continental relative humidity. The 17O-excess signature of phytoliths could fill this gap.

How to cite: Alexandre, A., Outrequin, C., Vallet-Coulomb, C., Landais, A., Piel, C., Devidal, S., Peugeot, C., Ouani, T., Afouda, S., Couapel, M., Sonzogni, C., Mazur, J.-C., Prié, F., and Webb, E.: Factors controlling the triple oxygen isotope composition of grass leaf water and phytoliths: insights for paleo-environmental reconstructions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6641, https://doi.org/10.5194/egusphere-egu2020-6641, 2020.

EGU2020-7701 | Displays | GMPV1.1

Control on millennial scale events (H-events) inferred from triple oxygen isotope ratios of speleothems from a Northeast Indian cave

Sasadhar Mahata, Pengzhen Duan, Lijuan Sha, Jonathan Baker, Gayatri Kathayat, Xiyu Dong, Baoyun Zong, Youfeng Ning, Haiwei Zhang, and Hai Cheng

The carbonate 17O anomaly (Δ17O) has recently been developed as a geochemical proxy for estimating the relative humidity of moisture at the source point of evaporation, which can be a vital tool in paleoclimate research. Speleothem Δ17O variability in particular may provide a quantitative constraint on moisture regimes at millennial and orbital timescales—far longer than can be addressed by analyzing 17O in other materials, such as tree rings. Modern observations and calibration studies have established a robust negative correlation between the Δ17O (17O excess) of rainfall and relative humidity, so that Δ17O is enhanced during arid conditions at the moisture source. Herein, we report novel triple oxygen isotope data across the Last Glacial in speleothems collected from Cherrapunji Cave, northeastern India. Triple oxygen isotope measurements were obtained by an O2-CO2 Pt-catalyzed oxygen-isotope equilibration method. Preliminary results suggest lower Δ17O in speleothems during cold periods (e.g. Heinrich Events), which would imply higher relative humidity over the oceanic moisture source. Importantly, higher source relative humidity does not necessarily imply changes in precipitation amount at the cave site. While it is possible that a substantial geographic shift in the moisture source region (or additional contributions from secondary sources) could obfuscate the Δ17O signal, we argue that this explanation is unlikely for our study site. Alternatively, we cannot exclude the effects of excessive moisture recycling in the tropical ocean, which can enhance the 17O anomaly in cloud vapor (particularly if combined with large temperature swings) and thereby alter speleothem Δ17O. To refine our interpretation of the Δ17O signal in Cerrapunji Cave samples, we investigate multiple cold periods, as well as coeval samples from the Asian Summer Monsoon region. Finally, we compare our results with novel data from westernmost Asia, where temperature variations during cold events are more likely to be accompanied by large shifts in predominant moisture source, due to the migration of wintertime westerlies.

How to cite: Mahata, S., Duan, P., Sha, L., Baker, J., Kathayat, G., Dong, X., Zong, B., Ning, Y., Zhang, H., and Cheng, H.: Control on millennial scale events (H-events) inferred from triple oxygen isotope ratios of speleothems from a Northeast Indian cave, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7701, https://doi.org/10.5194/egusphere-egu2020-7701, 2020.

EGU2020-14088 | Displays | GMPV1.1

Accuracy and reproducibility of the triple oxygen isotope measurement of silicate micro-samples by laser-fluorination-IRMS

Martine Couapel, Corinne Sonzogni, Anne Alexandre, and Florence Sylvestre

Recent studies showed that the 17O-excess of plant leaf biogenic silicates (phytoliths) can be used to quantify the atmospheric relative humidity occurring during leaf water transpiration. The 17O-excess vs ∂18O signature of phytoliths can also be used to trace back to the signature of leaf water. In a similar way, the signature of lacustrine diatoms is expected to record the signature of the lake water in which they formed. Therefore, the triple oxygen isotope composition of biogenic silicates extracted from well-dated sedimentary cores may bring new insights for past climate and hydrological reconstructions. However, for high time-resolution reconstructions, we need to be able to measure microsamples (300 to 800 µg) of biogenic silica. In another context, the triple oxygen isotope composition of micro-meteorites constitutes an efficient tool to determine their parent-body. In this case too, micro-samples need to be handled.

Here we report the results of new ∂18O and ∂17O measurements of macro- and micro-samples of international and laboratory silicate standards (e.g. NBS28 quartz, San Carlos Olivine, Boulangé quartz, MSG phytoliths and PS diatoms). Molecular O2 is extracted from silica and purified in a laser-fluorination line, passed through a 114°C slush to condense potential interfering gasses and sent to the dual-inlet Isotope Ratio Mass Spectrometer (IRMS) Thermo-Scientific Delta V. In order to get sufficient 34/32 and 33/32 signals for microsamples the O2 gas is concentrated within the IRMS in an additional auto-cooled 800 ml microvolume tube filled with silica gel. Accuracy and reproducibility of the ∂18O, ∂17O and 17O excess measurements are assessed. Attention is payed to determine the concentration from which O2 gas yields offsets in ∂18O, ∂17O and 17O-excess are measured and whether these offsets are reproducible and can be corrected for.

How to cite: Couapel, M., Sonzogni, C., Alexandre, A., and Sylvestre, F.: Accuracy and reproducibility of the triple oxygen isotope measurement of silicate micro-samples by laser-fluorination-IRMS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14088, https://doi.org/10.5194/egusphere-egu2020-14088, 2020.

EGU2020-5798 | Displays | GMPV1.1

Recent developments and applications of triple oxygen isotope measurements by secondary ion mass spectrometry

Yves Marrocchi, Johan Villeneuve, Paula Peres, and Firmino Fernandes

Oxygen isotopes are powerful proxies that are commonly used to decipher the formation of terrestrial and extraterrestrial rocks. Most of modern scientific approaches imply the determination of the oxygen isotopic composition at the mineral scale, thus requiring instruments enable to perform in situ, multi-collection, isotopic analyses in complex mineralogical assemblages and zoned minerals. Among them, large-geometry secondary ion mass spectrometer (LG-SIMS) is the most versatile with unique advantages such as (i) high spatial resolution (10–20 μm beam diameter and 1–2 μm depth); (ii) high sensitivity (detection limits below the ppm level for most elements) and (iii) high mass-resolution analysis allowing to remove most isobaric interferences (Villeneuve et al., 2019). Thanks to these capabilities, analytical uncertainties were significantly reduced for oxygen isotopes and reproducibilities much better that 1 ‰ on d17O and d18O are commonly obtained (e.g., Vacher et al. 2016; Marrocchi et al., 2018). Reaching such precisions is, however, linked to the use of 1011 Ω Faraday Cups (FCs) that require minimum count rates of > 106 cp/s for reaching permil precisions. This implies performing measurements with relatively large primary beam (i.e., 15-20 μm) that limits the minerals that can be targeted, especially in extraterrestrial samples (e.g., chondrule olivine crystals, Marrocchi et al., 2019).

Latest generation LG-SIMS instruments have been recently equipped with 1012 Ω FCs that enable isotopic measurements to be performed at count rates significantly lower (i.e., 3 × 105 cp/s) while maintaining good precision. This implies that high-precision oxygen isotopic measurements can be now performed with a less intense and smaller primary beam (~1 nA; 5 μm), In this contribution, we will report the specific characteristics of measurements using 1012 Ω FCs and the reproducibilities obtained for oxygen isotope measurements. Few scientific examples where the use of 1012 Ω FCs can represent a significant beakthrough will also be presented.

Marrocchi Y., Bekaert D.V. & Piani L. (2018). Origin and abundance of water in carbonaceous asteroids. Earth and Planetary Science Letters 482, 23-32.

Marrocchi Y., Euverte R., Villeneuve J., Batanova V., Welsch B., Ferrière L. & Jacquet E. (2019) Formation of CV chondrules by recycling of amoeboid olivine aggregate-like precursors. Geochimica et Cosmochimica Acta 247C, 121-141.

Villeneuve J., Chaussidon M., Marrocchi Y., Deng Z. & Watson B.E. (2019). High-precision silicon isotopic analyses by MC-SIMS in olivine and low-Ca pyroxene. Rapid Communication in Mass Spectrometry 33, 1589-1597.

Vacher L.G., Marrocchi Y., Verdier-Paoletti M., Villeneuve J. & Gounelle M. (2016) Inward radial mixing of interstellar water ices in the solar protoplanetary disk. The Astrophysical Journal Letters, 826, 1-6.

 

How to cite: Marrocchi, Y., Villeneuve, J., Peres, P., and Fernandes, F.: Recent developments and applications of triple oxygen isotope measurements by secondary ion mass spectrometry , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5798, https://doi.org/10.5194/egusphere-egu2020-5798, 2020.

EGU2020-20088 | Displays | GMPV1.1

Improved precision and throughput for 17O-excess measurements on water with Cavity Ring-Down Spectroscopy

Magdalena E. G. Hofmann, Zhiwei Lin, Thomas Doherty, Jonathan D. Bent, and Gregor Lucic

Triple oxygen isotope data (denoted as 17O-excess) have been used to constrain meteorological processes, plant fractionation processes, animal metabolism, and a variety of other physical and chemical processes. Measurement precision is key in order to successfully apply this promising new tracer to a range of scientific questions. Up to date, the highest measurement precision for 17O-excess on water was achieved by converting water to O2 and subsequent mass spectrometric analysis of O2 (Barkan and Luz, 2005). This approach allows to reach a measurement precision of about 5-6permeg. However, it is very difficut to setup and only a few laboratories worldwide succesfully use this methodology. A far simpler approach is to use Cavity Ring-Down Spectroscopy (CRDS), i.e. the Picarro L2140-i analyzer that measures δ18O, δ17O, δD and determines 17O-excess. To date, the 17O-excess measurement precision of CRDS was limited to 10-15permeg. Here, we will present a new metholodology that allows to reach a similar or even better precision compared to the mass spectrometric approach. The improved methodology does not require any hardware changes but is solely based on modifications of the injection procedure. 

How to cite: Hofmann, M. E. G., Lin, Z., Doherty, T., Bent, J. D., and Lucic, G.: Improved precision and throughput for 17O-excess measurements on water with Cavity Ring-Down Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20088, https://doi.org/10.5194/egusphere-egu2020-20088, 2020.

EGU2020-6191 | Displays | GMPV1.1

Can we not use the Δ value to measure a triple isotope system?

Huiming Bao and Xiaobin Cao

Can we not use the Δ value to measure a triple isotope system?

 

Huiming Bao1 ,2, 3 and Xiaobin Cao1 ,2

 

1 International Center for Isotope Effects Research, Nanjing University, Nanjing 210023, P. R. China

2 School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, PR China

3 Department of Geology and Geophysics, Louisiana State University, E235 Howe Russell Kniffen, Baton Rouge, LA 70803

 

In a triple isotope system, taking oxygen for example, the deviation of the δ17O (or δ’) from a defined δ17O-δ18O relationship is measured by the term Δ, defined as the value of δ17O - C×δ18O, in which “C” is a reference slope number. The use of Δ has generated two problems. First, there is a spectrum of C values currently being adopted in the community, for reasons of end-member cases (e.g. 0.5305 at high-temperature limit), legacy (0.52), or compound-specificity (e.g. 0.528 for water cycle or 0.524 for silicates). These practices have brought confusions especially when we deal with small Δ values and when we must compare Δ values among different compounds. A second, more serious problem is the lack of appreciation that a Δ value scales with its corresponding δ18O value. That means even for the same process we may get different Δ values depending on the magnitude of fractionation and/or laboratory references used.

A pair of radial-angular parameters in a polar coordinate system or a pair of δ18O and δ17O in Cartesian space uniquely describe a triple isotope data point in 2D space. Either of the two ways would thaw any debates on the choice of reference slope value C necessary for calculating the Δ. In addition, a polar coordinate system is usually preferred when studying behaviors centering around an origin, in this case, isotope composition deviating from a reference point (0, 0). The angular coordinate φ of a triple isotope composition stays the same for the same fractionation process regardless of its radial coordinate r which is determined by δ18O and δ17O values. Thus, the use of a polar coordinate (r, φ) to describe a triple isotope composition in 2D space would avoid the δ18O scaling issue for Δ values of the same process. Unfortunately, polar coordinate does not offer straightforward representation of process-specific δs or fractionation factors. Using just a pair of δ18O and δ17O values to describe a triple isotope system also eliminates additional symbols. Unfortunately, the direct use of the δ18O and δ17O presents an apparently larger uncertainty for a data point than the other approaches. And it could not take advantage of the use of an accurate Δ value in case when the analytical yield is not 100%.

The limitations of a polar coordinate system or a pair of δ18O and δ17O in Cartesian space outweigh their advantages and we are left with no better alternative than the use of Δ. Therefore, when reporting small Δ values, we must report their corresponding δ18O values as well to avoid scaling bias when dealing with small Δ values.

 

How to cite: Bao, H. and Cao, X.: Can we not use the Δ value to measure a triple isotope system?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6191, https://doi.org/10.5194/egusphere-egu2020-6191, 2020.

EGU2020-5752 | Displays | GMPV1.1

Spatiotemporal Δ17O variability in the rock record

Matthew Warke, Ross Pettigrew, David Millward, Robert Raine, Stuart Clarke, Yongbo Peng, Huiming Bao, and Mark Claire

The  Δ17O value of sedimentary sulfate can provide a direct, stable, geological archive of atmospheric-biospheric evolution. Negative Δ17O values in gypsum/anhydrite are inherited from the negative Δ17O value of atmospheric O2 which is transferred to sulfate during sulfide weathering. The magnitude of the O2 Δ17O value reflects pCO2, pO2 and gross primary productivity, hence modelling of the geological Δ17O record has led to estimates of changing atmospheric composition and primary productivity over Earth history. However, sulfate Δ17O values represent a conservative estimate of atmospheric Δ17O values as the magnitude of negative Δ17O in sulfate can be diluted (or erased) through sulfur cycling. As sulfate is transported away from the site of sulfide oxidation the likelihood of this happening increases.

Although this effect is acknowledged, the extent to which Δ17O values may vary within and between palaeoenvironments, and how evaporite sedimentology may affect stratigraphic interpretations of Δ17O values, remains unclear. We present the preliminary results of two case-studies probing the spatiotemporal variability of Δ17O values.

Case-study 1: temporally correlative Tournaisian (Lower Mississippian) evaporites within Carboniferous rift basins of Britain and Ireland were deposited in a range of settings: coastal wetland (Ballagan Fm.); supratidal sabkha on margin of a restricted basin (Ballycultra Fm.); and coastal sabkha on open ocean margin (Middleton Dale Anhydrite Fm.) All three settings plot on a positive slope in d34S vs Δ17O space with values ranging between δ34S ≈ +15 ‰, Δ17O ≈ -0.08 ‰ and δ34S ≈ +24 ‰, Δ17O ≈ -0.2 ‰. We discuss whether this trend (and intraformational trends) represents a spatial variability in sulfate Δ17O as controlled by fluctuating fluvial and marine dominance in evaporite depositional environments, or whether this might represent a temporal change in δ34S and Δ17O.   

Case study 2: non-marine evaporites of the early Permian Cedar Mesa Sandstone (CMS) Formation in Utah were deposited in continental saline pans in an erg-margin setting that fluctuated through arid and humid cycles. These evaporites record negative Δ17O values as low as -270 per meg, however δ34S values lie along the marine curve. We interpret the signal preserved in the CMS as recycling of the underlying marine evaporites of the late Carboniferous Paradox Formation which have been uplifted on the basin margin. Hence, we discuss how in non-marine settings the recycling of evaporites can decouple the age of the succession from the age of the atmospheric Δ17O signal.

How to cite: Warke, M., Pettigrew, R., Millward, D., Raine, R., Clarke, S., Peng, Y., Bao, H., and Claire, M.: Spatiotemporal Δ17O variability in the rock record, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5752, https://doi.org/10.5194/egusphere-egu2020-5752, 2020.

EGU2020-22196 | Displays | GMPV1.1

1-D photochemical model predicts oxygen isotope anomalies in early Earth atmospheres

Bethan Gregory, Mark Claire, and Sarah Rugheimer

Atmospheric oxygen and ozone over geological time have been constrained using various geochemical proxies and modelling studies, but ambiguity remains. Triple oxygen isotope measurements from Phanerozoic and Proterozoic rocks (e.g. Crockford et al., 2019) provide a direct record of ancient atmospheric composition, and as such are an exciting novel proxy. The only known source of mass-independent fractionation of oxygen isotopes (O-MIF) on Earth is in the formation of stratospheric ozone. A large positive O-MIF signal is imparted to ozone, while the larger reservoir of oxygen gains a much smaller negative O-MIF signal. These species interact with other gases in the atmosphere, and oxidised end products including nitrate, sulphate and perchlorate can persist in various geological archives such as ice, arid desert soil, and marine evaporites. As a result, the magnitude of the O-MIF signature detected in the geological record could be used to quantify levels of atmospheric ozone (and closely-related molecular oxygen) over certain time intervals. Here we develop a one-dimensional photochemical model to incorporate the three isotopes of oxygen, in order to trace oxygen isotope anomalies from stratospheric ozone through other atmospheric species, and into the geological record. This model, ‘Atmos,’ has been calibrated over 40 years to provide credible estimates of atmospheric composition deviating from the modern. We use the model to show the lowest oxygen levels at which the anomaly can be produced and transferred, putting a potential lower limit on oxygen levels for parts of the Phanerozoic and mid-Proterozoic.

Reference:

Crockford, P.W., Kunzmann, M., Bekker, A., Hayles, J., Bao, H., Halverson, G.P., Peng, Y., Bui, T.H., Cox, G.M., Gibson, T.M. and Wörndle, S., 2019. Claypool continued: Extending the isotopic record of sedimentary sulfate. Chemical Geology.

How to cite: Gregory, B., Claire, M., and Rugheimer, S.: 1-D photochemical model predicts oxygen isotope anomalies in early Earth atmospheres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22196, https://doi.org/10.5194/egusphere-egu2020-22196, 2020.

EGU2020-17738 | Displays | GMPV1.1

Triple oxygen isotope constraints on the origin of ocean island basalts

Xiaobin Cao, Huiming Bao, and Yongbo Peng

 

Understanding the origin of ocean island basalts (OIB) has important bearings on Earth’s deep mantle. Although it is widely accepted that subducted oceanic crust, as a consequence of plate tectonics, contributes material to OIB’s formation, its exact fraction in OIB’s mantle source remains ambiguous largely due to uncertainties associated with existing geochemical proxies. We have shown, through theoretical calculation and examining published data, that unlike many known proxies, triple oxygen isotope compositions (i.e. Δ17O) in olivine samples are not affected by crystallization and partial melting. This unique feature allows olivine Δ17O values to identify and quantify the fractions of subducted ocean sediments and hydrothermally altered oceanic crusts in OIB’s mantle source. In this work, new Δ17O measurements for OIB will be presented, and the implications will be discussed. 

 

How to cite: Cao, X., Bao, H., and Peng, Y.: Triple oxygen isotope constraints on the origin of ocean island basalts , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17738, https://doi.org/10.5194/egusphere-egu2020-17738, 2020.

Tracing the sources of pollutants in Ganga river water using conventional and non-conventional isotope analysis in nitrates

 

Abhayanand S. Maurya1, Amzad H. Laskar2, Nityanand S. Maurya3, Mao-Chang Liang4,

 

1Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India

2Geosciences Division, Physical Research Laboratory, Ahmedabad 380009, Gujarat, India

3Department of Civil Engineering, National Institute of Technology Patna, India

4Institute of Earth Sciences, Academia Sinica, Taiwan

 

Ganga is the largest river in India providing fresh water to ~40 % of India’s population which is more than any other river in the world. It is also one of the most polluted rivers in the world. Pollution, mainly from human and industrial wastes in the Ganga poses significant threats to human health and environment. This is an attempt to identify and quantify the contribution of different sources in the river water pollution using stable isotopes in nitrate (NO3-). We measured non-conventional triple oxygen isotopes (∆17ONO317ONO3-λδ18ONO3) along with the conventional isotopes (δ15N and δ18O) in NO3- and concentrations of major ions and metals (both heavy and trace ones) in Ganga river water to understand the sources and contribution from different pollution sectors. We also measured stable water isotopes (δD and δ18O) to understand the secondary processes such as in stream evaporation and inflow over the course of the river.  Water samples were collected from multiple locations starting from the clean water in the upstream region to all the way to the estuaries before the onset of monsoon, to best capture anthropogenic signals. ∆17O in NO3- is used to partition the atmospheric depositions from other sources such as human and industrial wastes and δ15N and δ18O values are used to partition the contribution of pollutants from different land sources such as municipal wastes and agricultural fertilizers. ∆17O in NO3- is also used to understand reaction processes which affect the isotopic composition such as nitrification, denitrification, volatilization, assimilation and mineralization as those processes mostly follow mass dependent fractionation without affecting ∆17O but influence the conventional isotopic compositions. We will present the results along with some recommendations for reducing the pollution level of the Ganga water.

 

How to cite: Maurya, A. S.: Tracing the sources of pollutants in Ganga river water using conventional and non-conventional isotope analysis in nitrates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2854, https://doi.org/10.5194/egusphere-egu2020-2854, 2020.

EGU2020-6734 | Displays | GMPV1.1

Triple oxygen isotope variations in precipitation from southeast China and its hydrological significance

Pengzhen Duan, Sasadhar Mahata, Lijuan Sha, and Hai Cheng

    High precision triple oxygen isotope measurement of meteoric water is a newly added tracer in hydrological and paleoclimate research. However, it is prerequisite to study the controls on precipitation 17O-excess for proper application of it. Here we report two years highly precise precipitation data from Nanjing, a southeast China station dominated by Asian monsoon. All the water isotopes (δ17O, δ18O and δD) reported here are based on mass spectrometer measurements and optical measurements (cavity ring-down spectroscopy). Nanjing receives moisture from different vapor sources and experiences different rainout mechanisms during various monsoonal sessions. Combined use of above parameters can help us to delineate processes occurring during evaporation, transport, condensation and re-evaporation. Year to year 17O-excess variability is observed in the obtained dataset and no notable seasonal variation is observed. However, the 17O-excess seasonal amplitude is little larger in the first year than the subsequent year. So far, it is known that the precipitation 17O-excess depends on three values: 17O-excess of the source water bodies, amount of 17O-excess gain during evaporation and 17O-excess loss during raindrops evaporation. During dry months 17O-excess gain is balanced by 17O-excess loss, which might lead to the near absence of seasonal cycle at Nanjing. From the comparison of observed data and model simulation, the amount of re-evaporation on falling raindrop is estimated to be about 10% at Nanjing. In addition, correlation with available meteorological parameters has been discussed. Except temperature no significant correlation has been found with other metrological variables (relative humidity and rainfall amount). This study will serve as a baseline to understand some of issues in paleoclimate that have puzzled the scientific community for years.

How to cite: Duan, P., Mahata, S., Sha, L., and Cheng, H.: Triple oxygen isotope variations in precipitation from southeast China and its hydrological significance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6734, https://doi.org/10.5194/egusphere-egu2020-6734, 2020.

EGU2020-13543 | Displays | GMPV1.1

Contribution of the triple oxygen isotope composition of precipitation to the identification of surface-atmosphere interactions in the sub-humid part of West Africa

Christine Vallet-Coulomb, Anne Alexandre, Christophe Peugeot, Abdoukarim Alassane, Vinel Gbewezoun, Martine Couapel, Clément Outrequin, Théodore Ouani, and Simon Afouda

This study aims at evaluating the information carried by the 17O-excess composition of precipitation in the sub-humid part of West Africa. Located at the southern border of the Sahelian band, the so-called “Sudanian Climatic Zone”, characterized by annual precipitation of 1200-1400mm, plays a crucial role in providing water to large African watersheds such as the Niger river’s one, and the Lake Chad catchment. Surface-atmosphere interactions were shown to influence convective processes in the semi-arid Sahelian band, with positive feedbacks between vegetation land cover and rainfall. Less focus has been put on the more humid Sudanian Zone, although surface-atmosphere interactions may have an important influence on the control of rainfall variations, and therefore on water resource availability in these watersheds.

The stable isotope composition of precipitation reflects the combination of different processes associated with phase changes over the atmospheric water cycle, from the initial water vapor formation above the ocean to the raindrop on the ground surface. Classical tracers (δ18O, δ2H, and d-excess) are affected by multiple factors (i.e. Rayleigh process, temperature, humidity) changing during these successive steps. In contrast, 17O-excess variations mainly records evaporation processes controlled by the humidity conditions that prevail during phase change. There are few available 17O-excess studies focusing on precipitation in tropical and sub-tropical areas. They show that the 17O-excess in precipitation provides information on 1) relative humidity at oceanic moisture sources, and 2) secondary processes, such as raindrop re-evaporation. The contribution of vapor of continental origin, produced either by plant transpiration or soil water evaporation, should additionally affect the 17O-excess signature of precipitation, although no data are available so far to evaluate the magnitude of this process.

For the study presented here, we collected precipitation from two sampling stations, both located in Benin and affected by a similar oceanic moisture source in the Gulf of Guinea. The first station (lat. 6°26’ N; long. 2°21’ E) is located along the coast and is essentially subject to oceanic influence. The second station (lat. 9°44’ N; long. 1°34’ E) is located 400 km inland and may be additionally affected by continental vapor recycling. The stable isotope composition of rainfall samples (δ2H, δ18O and δ17O) are measured on a WS-CRDS Picarro L2140-i, using three replicates per sample. Comparison between those two records allow to investigate how humidity at the oceanic source, raindrop re-evaporation and continental vapor­ contribute to the 17O-excess signature of precipitation.

How to cite: Vallet-Coulomb, C., Alexandre, A., Peugeot, C., Alassane, A., Gbewezoun, V., Couapel, M., Outrequin, C., Ouani, T., and Afouda, S.: Contribution of the triple oxygen isotope composition of precipitation to the identification of surface-atmosphere interactions in the sub-humid part of West Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13543, https://doi.org/10.5194/egusphere-egu2020-13543, 2020.

EGU2020-7795 | Displays | GMPV1.1

Triple oxygen (16O, 17O, 18O) and hydrogen (1H, 2H) isotope analyses of rainfall events in Central-South Pyrenees.

Reyes Giménez, Fernando Gázquez, Miguel Bartolomé, and Ana Moreno

Few studies of rainfall isotopic composition are available in the northern Iberian Peninsula, and up to now none of them has provided detailed analyses of the triple oxygen isotopes (d17O,  d18O and derived parameter 17Oexcess), preventing from the complete understanding of some atmospheric processes and their relationship with the current climate in this region. This information, together with the characterization of dripwater isotopic composition once transferred throughout the epikarst, is essential to the correct interpretation of paleoclimate records based on speleothem isotopic data.

We provide the first database of triple oxygen and hydrogen stable isotopes of rainwater in Central-South Pyrenees. We characterize local rainfall isotopic variability in a high altitude site and identify the main factors controlling the isotopic composition of rainwater. The samples were collected on a rainfall-event basis from July 2017 to June 2019 (n=216) at the interpretation center of “Las Güixas” touristic cave in Villanúa (Huesca, Spain), where other monitoring surveys are in progress. This site (42º40’59’’N; 0º31’55’’W; 957 m a.s.l.) is characterized by a transitional Mediterranean – Oceanic climate with a highly contrasted seasonality, mean annual temperature of 10ºC and mean annual precipitation of 1100 mm. We analyzed d17O, d18O and dD, and derived parameters 17Oexcess and d-excess in rainwaters using a Picarro L2140-i analyzer at the University of Almería (Spain), with mean precisions (1-standard error) of 5 per meg for 17Oexcess and 0.1‰ for d-excess. Meteorological variables (temperature, RH and rainfall amount) were monitored (every 10 min) at the sampling site during the length of this study.

During the two-years monitoring period, d18O ranged from -21.7 to 8.7‰ and dD did from -170.8 to 34.1‰, with average values of -7.4‰ and -52.3‰, respectively. The 17Oexcess averaged 21±24 per meg and the mean d-excess was 7.1±7.7‰. The local meteoric water line is defined by dD= 7.3·d18O+1.9 (R2=0.96) and d´17O= 0.524·d´18O+0.0088 (R2=1). The d17O, d18O and dD values were higher during summer (June to September; -2.1, -3.9 and -26.6‰, respectively; n=68) and were lower during the rest of the year (-4.7, -9.0 and -63.8‰, respectively; n=164). In contrast, the 17Oexcess and d-excess were lower during summer (3 per meg and 4.6‰, respectively) and higher (29 per meg and 8.2‰, respectively) during the remaining months. We found that the isotopic parameters are weakly correlated with rainfall amount during each event, but they strongly depend on seasonal changes in air temperature and relative humidity. The extremely low 17Oexcess and d-excess values observed in summer (down to -75 per meg and -35.6‰, respectively), cannot be explained by particular conditions at the source of moisture during water vapor formation, but by local meteorological parameters and rain drops re-evaporation during rainfall events.

Further processing of this database will consider other influencing factors in the isotopic composition of rainfall events, such as changes in the source moisture, synoptic pattern and type of rainfall, to further understand the complexity of atmospheric processes through the information stored in the triple oxygen isotopes of rainfall, with application to future 17Oexcess studies in speleothems.

How to cite: Giménez, R., Gázquez, F., Bartolomé, M., and Moreno, A.: Triple oxygen (16O, 17O, 18O) and hydrogen (1H, 2H) isotope analyses of rainfall events in Central-South Pyrenees., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7795, https://doi.org/10.5194/egusphere-egu2020-7795, 2020.

EGU2020-6938 | Displays | GMPV1.1

Gypsum speleothems record the triple oxygen (δ17O and δ18O) and hydrogen (δ2H) isotopic composition of cave dripwater: potential paleoenvironmental implications

Fernando Gazquez, Veronica Chiarini, Andrea Columbu, Jo De Waele, Philippe Audra, Didier Cailhol, Marco Vattano, Giuliana Madonia, Alena Giesche, José-María Calaforra, and David A. Hodell

Gypsum (CaSO4∙2H2O) speleothems (i.e. stalactites, stalagmites, etc.) in caves form frequently through dissolution of the gypsum host-rock by seepage water and subsequent secondary mineral re-precipitation from gypsum-saturated solutions [1]. Gypsum takes its structurally-bound hydration water (GHW) from the liquid; the isotopic composition (δ17O, δ18O and δ2H) of GHW reflects that of cave dripwater at the time of mineral crystallization, with insignificant effect of temperature on the liquid-GHW isotope fractionation factors [2]; therefore, GHW may be used to reconstruct the isotopic composition of paleo-dripwater in caves. Here we investigate the triple oxygen and hydrogen isotopic composition of GHW in speleothems from circum-Mediterranean gypsum caves, including the gypsum karsts of Emilia Romagna (NE Italy), Sorbas (SE Spain), Sicily and Mesaoria (Cyprus), all of them hosted in gypsum of Messinian age (ca. 5.5 Ma). The climatic settings of the studied caves range from semiarid (i.e. Sorbas and Mesaoria, <300 mm·yr-1) to relatively wet (i.e. Emilia Romagna and Sicily >600 mm·yr-1).

Our results reveal that most gypsum speleothems in these caves precipitated from unevaporated solutions (e.g. d-excess >8‰ and 17Oexcess >10 per meg), with isotopic compositions similar to those of local meteoric/seepage waters and close to the local meteoric water lines (LMWL) of each region. Gypsum crystallization in absence of evaporation can be explained by the mechanism known as Ostwald ripening [3], a solution-mediated recrystallization under constant temperature by which older crystals (i.e. Messinian gypsum) dissolve to feed new crystals (i.e. gypsum speleothems). Only GHW in speleothems from the Sorbas caves show evidence for solution evaporation prior mineral precipitation. Gypsum speleothems in several caves of Emilia Romagna crystallized from unevaporated waters with significantly different triple oxygen and hydrogen isotopic compositions (e.g. Ca´ Castellina cave: δ18O=-8.3±0.3‰, δ2H=-55.2±1.7‰, 17Oexcess=33±9 per meg; Abisso Bentini cave: δ18O=-10.6±0.3‰, δ2H=-73.4±1.8‰, 17Oexcess=47±13 per meg). In absence of chronological data, this can be interpreted as (1) gypsum speleothems formed in different climatic periods or (2) do at present from waters that seepage into the epikarst during different times of the year. Either way, gypsum records the mean isotopic composition of seepage water under distinct environmental conditions in this region.

The δ18O and 17Oexcess values across the entire dataset are negatively correlated, unlike δ18O and d-excess values that are, positively correlated for δ18O<-6‰ and negatively correlated for δ18O>-6‰. We suggest that the different behaviors of 17Oexcess and d-excess derive from their distinct sensitivities to environmental parameters (i.e. RH and temperature) during formation of water vapor at the moisture source of rain and local effects during rainfall events in each area. We conclude that gypsum speleothems of known ages may be useful as archives for triple oxygen and hydrogen isotope reconstructions of paleo-rainfall.

[1] Gázquez et al. 2017. Chemical Geology, v. 452, p. 34–46; [2] Gázquez et al. 2017. Geochimica et Cosmochimica Acta, v. 198, p. 259–270; [3] Kahlweit, 1975. Advances in Colloid and Interface Science, v. 5, p. 1–35.

How to cite: Gazquez, F., Chiarini, V., Columbu, A., De Waele, J., Audra, P., Cailhol, D., Vattano, M., Madonia, G., Giesche, A., Calaforra, J.-M., and Hodell, D. A.: Gypsum speleothems record the triple oxygen (δ17O and δ18O) and hydrogen (δ2H) isotopic composition of cave dripwater: potential paleoenvironmental implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6938, https://doi.org/10.5194/egusphere-egu2020-6938, 2020.

EGU2020-6791 | Displays | GMPV1.1

A novel application of triple oxygen isotope ratios of speleothems

lijuan sha, Sasadhar Mahata, Pengzhen Duan, Boaz Luz, Pu Zhang, Jonathan Baker, Baoyun Zong, Youfeng Ning, Yassine Ait Brahim, Haiwei Zhang, R. Lawrence Edwards, and Hai Cheng

Triple oxygen isotope compositions have become one of critical proxies in characterizing a wide range of geochemical and hydroclimate processes. However, Δ17O (carbonate 17O anomaly) has only been barely used in the last decade because it is difficult to measure δ17O of natural samples to a sufficient precision in order to resolve small natural variability. In this study, we present triple oxygen isotope data from speleothems obtained by an O2-CO2 Pt-catalyzed oxygen-isotope equilibration method. The high precision (9 per meg or better, 1σ SD) of our new speleothem Δ17O data is sufficient to resolve subtle hydroclimatic signals. Based on this method, we established triple-oxygen-isotope records of TON cave in westerly region since the last 135ka, providing the evolution history of water vapor source and water vapor cycle in the orbit-millennium scale atmospheric precipitation in the Central Asia. In addition, the triple-oxygen-isotope records of speleothem from Asian and South American monsoonal regions were established in the key periods, such as glacial and interglacial periods. Our speleothem Δ17O data indicate a 20 per meg difference between Marine Isotope Stage 5d and 5e in samples from Central Asia, suggesting a shift in moisture source and/or fractionation history. Unexpectedly, there were no measurable Δ17O differences between glacial and interglacial samples from both the South American (western Amazon) and Asian (southern China) monsoon domains, implying consistent moisture-source conditions across glacial and interglacial cycles, at least in terms of relative humidity. Speleothem Δ17O data may thus provide new and important constraints for understanding regional and global hydroclimate dynamics.

How to cite: sha, L., Mahata, S., Duan, P., Luz, B., Zhang, P., Baker, J., Zong, B., Ning, Y., Ait Brahim, Y., Zhang, H., Edwards, R. L., and Cheng, H.: A novel application of triple oxygen isotope ratios of speleothems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6791, https://doi.org/10.5194/egusphere-egu2020-6791, 2020.

Here, we describe a system for measuring triple oxygen and hydrogen isotopic ratios of both the liquid and vapour during evaporation of water in a dry gas stream (N2 or dry air) at constant temperature and relative humidity (RH).  The hardware consists of a polymer glove box (COY), peristaltic pump (Ismatec), and Picarro L2140-i cavity ring-down laser spectrometer (CRDS) with Standard Delivery Module (SDM). Liquid water from the evaporation pan is sampled via a closed recirculating loop and syringe pump that delivers a constant rate of water to the vaporizer, maintaining a constant concentration of water vapour in the cell (20,000 ±103, 1 s.d.) over an injection cycle. Liquid measurements alternate with vapour from the glove box which is introduced to the CRDS using a diaphragm gas pump. Important for high-precision measurements, both cavity pressure and outlet valve stability are maintained throughout the liquid injection and subsequent vapour phase. Experiments are bookended by two in-house standards which are calibrated to the SMOW-SLAP scales. An additional drift corrector is introduced periodically.

 

To test the precision and stability of the liquid injections, we sampled from an isotopically homogeneous volume of water and introduced it to the cavity over a period of ~48h. To minimise the standard deviation derived from noise, we chose an optimum integration time of ~2000s (~33 minutes) based on σAllan minimisation. Therefore, for combined liquid-vapour experiments we use an injection/vapour sampling window of 40-minutes (140ug water is consumed per injection), which provides a data collection period of 33-minutes after a 7-min waiting time for equilibration.

 

Across a single liquid injection, the mean standard error for d17O, d18O, and dD is 0.008‰, 0.007‰, and 0.02‰, respectively. For the vapour phase equivalent, the mean standard error for d17O, d18O, and dD is 0.01‰, 0.009‰, 0.03‰ respectively. For the d-excess in the liquid and the vapour across one 33-minute cycle, the standard error is 0.07‰ and 0.08‰, respectively. For the O17-excess in the liquid and the vapour across one 33-minute cycle, the standard error is 6 per meg and 8 per meg, respectively.

 

A single evaporation experiment produces in excess of 100,000 measurements each of d17O, d18O, and dD for both the evaporating liquid and resulting vapour. These measurements result in 95% confidence limits for the slope of ln(d17O+1) vs ln(d18O+1) of ±0.0002 and ±0.0003 for the liquid and vapour, respectively.  For the slope of ln(dD+1) vs ln(d18O+1) we obtain a 95% confidence interval of ±0.001 and ±0.002 for the liquid and vapour, respectively. The experimental method permits measurement of fractionation of triple oxygen and hydrogen isotopes of water under varying experimental conditions (e.g., RH, temperature, turbulence) at very high precision. It will be useful for testing numerical models of evaporation and conducting experiments to simulate evaporation and isotopic equilibration in natural systems. An application to closed-basin lakes will be presented.

How to cite: Brady, M. and Hodell, D.: Continuous and simultaneous measurement of triple oxygen and hydrogen isotopes of liquid and vapour during evaporation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16872, https://doi.org/10.5194/egusphere-egu2020-16872, 2020.

EGU2020-21681 | Displays | GMPV1.1

A paleoclimatic perspective of triple oxygen isotopes from gypsum in Holocene Thar Desert playa lakes

Alena Giesche, Yama Dixit, Fernando Gázquez, Thomas Bauska, Matthew Brady, Vikas K. Singh, Ravindra N. Singh, Cameron A. Petrie, and David A. Hodell

The Thar Desert (NW India) has numerous evaporative saline playa lakes. Some are still active and others are dry and preserve up to several meters of sedimentary deposits. These deposits feature a variety of evaporite minerals, including the hydrated mineral gypsum (CaSO4 2H2O). Assuming no secondary exchange, the isotopic composition of the gypsum hydration water preserves the δ18O, δ17O and δ D of palaeolake water at the time of gypsum formation. This method provides a way to understand the hydrologic balance in a part of the world where it is typically very difficult to obtain palaeoclimate records. Our 36-hour pan evaporation experiment on site shows that triple oxygen isotopes track changes in evaporative conditions, which vary diurnally due to fluctuating temperature and relative humidity, and appear to reflect night-time condensation. We present new palaeohydrological records from two dry playas (Karsandi, Khajuwala) and one active playa (Lunkaransar) in the Thar Desert using the triple oxygen and hydrogen isotopic composition of gypsum hydration water. Results show that a source of water maintained active playa lake basins in the central Thar Desert for much of the Holocene, either by enhanced direct precipitation and/or fluvial sources. The derived 17O-excess and d-excess data potentially enable modelling of past changes in relative humidity, once other parameters (windiness, evaporation/inflow, etc.) are set.

How to cite: Giesche, A., Dixit, Y., Gázquez, F., Bauska, T., Brady, M., Singh, V. K., Singh, R. N., Petrie, C. A., and Hodell, D. A.: A paleoclimatic perspective of triple oxygen isotopes from gypsum in Holocene Thar Desert playa lakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21681, https://doi.org/10.5194/egusphere-egu2020-21681, 2020.

Gypsum crystals capture the isotopic composition (δ2H, δ17O, and δ18O) of ambient water in their structurally bonded water and may serve as a useful tool to reconstruct paleoclimate. Relative humidity, water temperature, wind speed, along with the isotopic composition of atmospheric vapor and inflowing water control, to a variable degree, the relative proportion of equilibrium and kinetic isotope fractionation during evaporation, and, thus, ultimately determine the d-excess and 17O-excess of gypsum-bonded water. Here, we demonstrate that the respective best fit of these variables through measured gypsum-bonded water isotope data using the classic Craig-Gordon evaporation model provides apparent absolute values for the fundamental climate mean state variables humidity and temperature and an empirical wind speed parameter of the geologic past.

In this proof-of-concept study, we sampled gypsum crystals within individual stratigraphic units of Pliocene lacustrine deposits from the Atacama Desert, extracted their structurally bonded water, and analyzed the hydrogen and triple oxygen isotope composition. The spread of measured isotope data within each sampled stratigraphic unit suggests variable degrees of evaporation between individual gypsum samples along a common evaporation trajectory. We used the Craig-Gordon evaporation model together with a Monte Carlo simulation to determine the limits of climate mean state variables that fit the measured isotopic data.

Our results demonstrate that primary isotope signatures of marine and continental source waters are preserved in structurally bonded gypsum waters. The data coherently suggest a slightly warmer (18-35°C), less windy and much more humid (50-75%) climate for the Pliocene Atacama, which is consistent with marine records and global circulation climate models that agree on “permanent El Niño” conditions for the Pliocene in the equatorial East Pacific.

Under the assumption that mixing of different brines or multiple sources is insignificant - as would be evident from scattering of isotopic data below the evaporation trajectory in 17O-excess over δ18O – the combined hydrogen and triple oxygen isotope analyses of gypsum-bonded water provides a powerful tool to quantify past mean states of humidity and temperature, and to estimate paleo-wind conditions.

How to cite: Voigt, C., Herwartz, D., and Staubwasser, M.: Quantitative reconstruction of past climate mean states in the Atacama Desert using hydrogen and triple oxygen isotopes of gypsum hydration water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18469, https://doi.org/10.5194/egusphere-egu2020-18469, 2020.

EGU2020-18698 | Displays | GMPV1.1

Identifying water availability in the Atacama Desert (Chile) by triple oxygen isotope analyses of sulfates

Swea Klipsch, Daniel Herwartz, and Michael Staubwasser

Ca-Sulfates (Gypsum and Anhydrite) are the most common salts accumulating in the soil of the Chilean Atacama Desert. Sulfate sources include sea spray, redeposition of playa sediments, terrestrial weathering, and deposition of sulfate formed in the atmosphere (secondary atmospheric sulfate = SAS). Sulfate from sea spray, playa lakes, and terrestrial weathering have a triple oxygen isotope composition (Δ17OSO4) at or slightly below zero reflecting reaction with water and oxygen. Positive Δ17OSO4 are generally the result of atmospheric SO2 oxidation by ozone or hydrogen peroxide. Sulfate oxygen is only altered with ambient water by cycling through biological activity resulting in Δ17OSO4 ≈ 0‰. Therefore, Δ17OSO4 aids in quantifying the relative contribution of SAS to the desert soil and in identifying bioactivity and water availability in the hyperarid Atacama Desert. The spatial quantification of different sulfate sources may serve to improve the understanding of sulfate deposition in this region.

Samples were analysed by continuous flow IRMS using the pyrolysis of Ag2SO4 to determine Δ17OSO4 from O2. An optimized sample preparation to form clean silver sulfate and intra-day calibration against two in-house standards resulted in an external reproducibility of 0.12‰. An inter laboratory comparison including data derived from the laser-fluorination method confirmed the accuracy of our analyses.

We analyzed desert soil surface samples from four E-W transects in the Atacama Desert reaching from the Pacific coast across the Coastal Cordillera, the Central Depression, and up the alluvial fans protruding from the Pre-Andean Cordillera. Transects begin at Pisagua (19.5°S), Salar Grande (21.0°S), Antofagasta (24.0°S), and Paposo (25.0°S). Values of Δ17OSO4 vary between 0.3 and 1.1‰. The lowest Δ17OSO4 values were measured near Salar Grande and on the Pre-Andean alluvial fans. The highest Δ17OSO4 values are observed at the highest altitudes in the Coastal Cordillera - east of Paposo - well above the coastal fog zone (> 1200 m). At similar or higher altitudes on the Pre-Andean fans, Δ17OSO4 converge towards zero.

The spatial distribution is the result of source contributions and subsequent biological reset. Positive Δ17OSO4 values throughout suggest a significant contribution from SAS. We quantified sea spray contributions using Cl- concentration, which drop dramatically above the fog-zone [1]. Furthermore, salt distribution suggests minimal weathering and redistribution in recent times. The lowest contribution from such near zero Δ17OSO4 sulfate sources are expected in the Coastal Cordillera, which is consistent with our data. Within the Coastal Cordillera there is a north to south Δ17OSO4 trend, which is also an elevation trend. Increased water availability from fog at lower elevations facilitates more efficient resetting of Δ17OSO4 via microbial activity. These observations suggest that the driest place in the Atacama Desert is situated within the Coastal Cordillera above the fog zone.

[1] Voigt et al. (2020) Global and Planetary Change 184

How to cite: Klipsch, S., Herwartz, D., and Staubwasser, M.: Identifying water availability in the Atacama Desert (Chile) by triple oxygen isotope analyses of sulfates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18698, https://doi.org/10.5194/egusphere-egu2020-18698, 2020.

EGU2020-20136 | Displays | GMPV1.1

The multi oxygen isotope analyses on black crust from Sicily highlight the volcanic emission influence from Mount Etna on urban areas

Adeline Aroskay, Erwan Martin, Slimane Bekki, Giuseppe Montana, Luciana Randazzo, and Pierre Cartigny

This study reports on measurements of Δ17O (derived from the triple oxygen isotopes) in sulphate from black crust sampled in Sicily. Atmospheric oxidants, such as O3, H2O2, OH and O2 carry specific 17O-anomalies, which are partly transferred to the sulphate during sulphur gas (e.g. SO2) oxidation. Hence, the Δ17O in sulphate can be used as a tracer of sulphur oxidation pathways. So far, this method has been mostly applied on sulphate from aerosols, rainwaters, volcanic deposits and ice cores. Here we propose a new approach, that aims to investigate the dominant oxidants of gaseous sulphur precursors into sulphate extracted from black crust material. Black crusts are mostly found on building/monument/sculpture and are the result of the reaction between sulphur compounds (SO2, H2SO4) and carbonate (CaCO3) from the substrate, which leads to the formation of gypsum (CaSO4, 2H2O). Sicilian black crust from sites under different emission influences (anthropogenic, marine and volcanic) were collected. Multi oxygen and sulphur isotope analyses were performed to better assess the origins of black crust sulphate in these different environments. This is crucial for both a better understanding of the sulphur cycle and the preservation of historical monument.

Multi sulphur isotopes show mostly negative values ranging from -0.4 ‰ to 0.02 ‰ ± 0.01 and from -0.59 ‰ to 0.41‰ ± 0.3 for Δ33S and Δ36S respectively. This is unique for natural samples and different from sulphate aerosols measured around the world (Δ33S > 0‰). This tends to indicate that sulphate from black crust is not generated by the same processes as sulphate aerosols in the atmosphere. Instead of SO2 oxidation in the atmosphere, dry deposition of SO2 and its oxidation on the substratum is preferred. The multi oxygen isotopes show a clear dependence with the geographical repartition of the samples. Indeed, black crusts from Palermo (the biggest Sicilian city) show small 17O-anomalies ranging between -0.16 ‰ to 1.02 ‰ with an average value of 0.45 ‰ ± 0.26 (n=12; 2σ). This is consistent with Δ17O values measured in black crust from the Parisian Basin (Genot et al., 2020), which are also formed in an environment influenced by anthropogenic and marine emissions. On the other hand, samples from the eastern part of the Mount Etna region, which are downwind of the volcanic emissions, show the highest 17O-anomalies ranging from 0.48 ‰ to 3.87 ‰ with an average value of 2.7 ‰ ± 0.6 (n=11; 2σ).

These results indicate that volcanic emissions influence the oxygen isotopic signature of black crust sulphate. In standard urban areas, SO2 deposited on the substratum is mostly oxidised by O2-TMI and H2O2 to generate the black crust. Yet, under the influence of volcanic emissions, O3 may play the main role in the SO2 oxidation.

How to cite: Aroskay, A., Martin, E., Bekki, S., Montana, G., Randazzo, L., and Cartigny, P.: The multi oxygen isotope analyses on black crust from Sicily highlight the volcanic emission influence from Mount Etna on urban areas , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20136, https://doi.org/10.5194/egusphere-egu2020-20136, 2020.

GMPV1.2 – Advances in microanalysis: new horizons in micro and nanoscale chemistry

EGU2020-11454 | Displays | GMPV1.2 | Highlight

Dating habitability with nanoscale measurements of Early Mars

Desmond Moser

The earliest known physical records of Mars and Earth lie in microscopic grains of zirconium-rich geochronology minerals such as zircon and baddeleyite. The reconstruction of the pressure and temperature histories of these phases is one of the few ways in which we can bracket the onset of conditions permissive of microbiota survival, and requires an integration of several nanoscale measurement techniques. This presentation will overview a recent, detailed investigation of zircons and baddeleyite from Mars [1], the earliest known from planets to date, as well as comparator studies of thermally and/or shock metamorphosed samples from the Earth and Moon. The approach is to spatially correlate measurements of the chemical and orientation microstructure of individual grains in order to characterize thermal, shock and diffusion history and better interpret U-Pb geochronology data. Also revealed are proxies for high temperature metamorphism such as nanoclusters of Pb and trace elements and nanoveins of impact melt as well as trace elements introduced through subsequent lower-temperature hydrothermal metamorphism. The techniques required include electron microscopy and cathodoluminescence (CL), Electron Backscatter Diffraction (EBSD), Transmission Kikuchi diffraction (TKD), mass spectrometry, and Atom Probe Tomography (APT). The Mars records were collected from a population of zircon and baddeleyite grains within five meteoritic fragments of polymict breccia (e.g. NWA 7034, NWA 7475). These data were compared to those from analogue sites of heavily bombarded Archean crust such as the central uplift of the Vredefort structure of South Africa, the Earth’s largest and oldest recognized impact crater, the Sudbury impact structure in Canada, and Apollo samples of the lunar regolith. The Mars population of grains reveals little evidence of the nanofeatures of heavily bombarded and heated crust, and no exposure to life-limiting pressures or temperature since crystallization 4.48 billion years ago. The conclusion is that global, planet-shaping bombardment effects on Mars, such as those which created its distinctive hemispheric dichotomy, had ceased by the time these grains and their associated crust crystallized. It follows that Mars entered a window of habitable conditions very early in solar system history, a pathway likely mirrored by the Earth. In this way nanoscale measurements, required to investigate microscopic mineral grains, serve as important tools for reconstructing important time periods in planetary evolution and abiogenesis.

Reference:

[1] DE Moser, GA Arcuri, DA Reinhard, LF White, JR Darling, IR Barker, DJ Larson, AJ Irving, FM McCubbin, KT Tait, J Roszjar, A Wittmann, C Davis (2019) Decline of giant impacts on Mars by 4.48 billion years ago and an early opportunity for habitability. Nature Geoscience 12,  522–527.

How to cite: Moser, D.: Dating habitability with nanoscale measurements of Early Mars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11454, https://doi.org/10.5194/egusphere-egu2020-11454, 2020.

EGU2020-1169 | Displays | GMPV1.2 | Highlight

Lunar Phosphates Record Impact Cratering Events at Micro to Nano Scales

Ana Cernok, Lee White, Kimberly Tait, Mahesh Anand, Sandra Kamo, Martin Whitehouse, and James Darling

The field of planetary mineralogy has greatly benefited from recent studies of accessory minerals that utilise µm- and nm-scale analytical techniques such as EBSD, APT, TEM and SIMS. Apatite and merrillite have been of particular interest, as they record vital information on the volatile content, U-Pb ages and trace-element composition of various planetary materials. However, the extent to which shock-deformation, pervasive among all planetary materials, affects the distribution of these valuable geochemical tracers is still poorly understood. Here we focus on exploring the U-Pb and Pb-Pb ages of apatite and merrillite in a set of variably shocked lunar rocks, building on previous nanostructural analyses of the phosphates.

We carried out U-Pb and Pb-Pb analyses of phosphates in Apollo 17 samples of the Mg-suite rocks (76535, 76335, 76255, 72255, 78235 and 78236) using the CAMECA 1280 ion microprobe at the NordSIMS facility (Stockholm). In addition, we applied a recently developed approach of conducting high-precision U-Pb and Pb-Pb analyses by ID-TIMS of extracted phosphate grains (Jack Satterly Lab, University of Toronto). For this purpose, individual ~50x50x30 µm crystals of apatite and merrillite were extracted directly from thin sections using a Xe+ plasma FIB.

As determined by SIMS, 207Pb/206Pb systematics of the unshocked or weakly shocked apatite in 76535 and 76335 is undisturbed, implying cooling of the rock below the closure temperature of Pb diffusion in apatite (~450°C) at ~4.2 Ga, ~100 Ma younger than what is interpreted as the rock’s crystallization age. Phosphates that experienced similar levels of deformation but were in proximity or in direct contact with the impact melt in samples 76255 and 72255 show almost complete age resetting (~3.92 Ga). The SIMS determined age of 16 phosphates in sample 76255 is 3922.2 ± 6.7 Ma (2σ) and agrees with the previously published 207Pb/206Pb phosphate ages of impact melt breccias found within the same boulder and was interpreted as the timing of the Imbrium impact. These recrystallized phosphates yield comparable TIMS Pb-Pb ages (3917.8 ± 1.8 Ma and 3921.0 ± 1.3 Ma, 2σ) with significantly lower internal uncertainties than that of the individual SIMS measurements and may represent multiple impact-events close to the Imbrium event.

SIMS U-Pb analyses of highly shocked phosphates (78235 and 78236) reveal a discordia line with an upper intercept of ~4.2 Ga and a lower intercept of ~0.5 Ga. We interpret this new, younger age as a minor thermal event that reactivated existing shock-induced nm-scale grain boundaries, as visualised by APT, within the apatite population to allow for Pb-loss at ~0.5 Ga. We propose a small crater located near the Apollo 17 landing site as a possible source of this sample.

By correlating micro- to nanostructural characterization with in-situ age systematics we show that apatite and merrillite are powerful thermochronometers that provide a new approach to dating which has the potential to discriminate between temporally similar events. This can greatly aid in unravelling the bombardment record of solar system and be helpful when dealing with samples of limited availability (e.g. space return missions).

How to cite: Cernok, A., White, L., Tait, K., Anand, M., Kamo, S., Whitehouse, M., and Darling, J.: Lunar Phosphates Record Impact Cratering Events at Micro to Nano Scales , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1169, https://doi.org/10.5194/egusphere-egu2020-1169, 2020.

EGU2020-11139 | Displays | GMPV1.2 | Highlight

Combined multi-isotopic and (S)TEM study of pre-solar silicates to probe the solar system’s prenatal history

Luc Lajaunie, Manish N. Sanghani, William D.A. Rickard, José. J. Calvino, Kuljeet K. Marhas, and Martin Bizzarro

Introduction Primitive extraterrestrial materials like carbonaceous chondrite matrices and interplanetary dust particles contain tiny dust grains that were formed in the winds of red giant branch, or asymptotic giant branch stars (AGB) and in the ejecta of novae and supernovae (SNe) explosions before the formation of our solar system. Presolar grains survived all the processes that created our solar system and carry the signatures of their parent stellar sources. Correlating isotopic data of individual presolar silicates with microstructural and chemical analyses obtained by STEM, provides a unique opportunity to provide better insights into physiochemical conditions of grain formation in stellar environments, grain alteration in the interstellar and parent body processes and also helps constraining various astrophysical grain condensation models. In this work, isotopic, structural and chemical analysis of nine presolar silicate grains from the CH3/CBb3 chondrite Isheyevo and CR2 chondrite NWA801 are reported.

Experimental Presolar oxygen anomalous grain search using oxygen isotope imaging was done in-situ using NanoSIMS50 ion microprobe and five grains from AGB and four grains from SNe, were selected for (S)TEM investigations. The TEM lamellas were prepared using a TESCAN LYRA3 FIB-SEM at Curtin University. Structural and chemical analysis of presolar grains were performed by combining high-resolution scanning TEM imaging, spatially-resolved electron energy-loss spectroscopy (EELS) and spatially-resolved energy-dispersive X-ray spectroscopy (EDS) by using a FEI Titan Cubed Themis 60-300 microscope at Cádiz University which was operated at 200 kV. EDS quantification was corrected by using a standard reference sample of known composition and density and by taking into account the thickness of the probed area by using low-loss EELS. EELS spectrum images for fine structures (mostly, O-K, Si-L2,3 and Fe-L2,3 edges) analyses were acquired with the monochromator excited allowing an energy resolution of about 0.4 eV. After denoising using principal components analysis and removal of the multiple scattering, we were able to map the heterogeneities related to the Fe oxidation state and to the oxygen local chemical environment. This allowed us to compare the degree of aqueous alteration of the grain with the surrounding rim and matrix grains.

Results TEM and STEM data have revealed a strong heterogeneity and a broad range of structural and chemical compositions of the grains that enabled us to compare the stellar grain condensation environments (e.g. AGB stars and SNe), and suggest widely varying formation conditions for the presolar silicates identified in this study. Only one of the grains originally condensed as an amorphous grain has shown preferential sputtering of Mg, indicating that Mg-rich amorphous grains are not preferentially destroyed. Several grains are found with signatures that represent interstellar, nebular and parent body alteration. An oldhamite-like grain within a presolar enstatite grain is probably the first observation of an oldhamite grain as a seed grain for the condensation of an enstatite grain in stellar atmospheres. All these results, which will be discussed in detail, point out the importance of coordinated isotopic, microstructural and chemical studies of presolar silicates to investigate the processes that may have played a role in shaping our solar system.

How to cite: Lajaunie, L., Sanghani, M. N., Rickard, W. D. A., Calvino, J. J., Marhas, K. K., and Bizzarro, M.: Combined multi-isotopic and (S)TEM study of pre-solar silicates to probe the solar system’s prenatal history, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11139, https://doi.org/10.5194/egusphere-egu2020-11139, 2020.

EGU2020-826 | Displays | GMPV1.2

The Raman Estimation of the Composition of Clinopyroxene Inclusions in Natural Diamonds

Anastasiya Kalugina and Dmitriy Zedgenizov

Clinopyroxenes (Na,Mn,Ca,Fe2+)1-2(Mg,Al,Cr,Fe3+)1-2[(Al,Si)2O6] are common inclusions in natural diamonds of either peridotitic or eclogitic paragenesis. The variety of the composition of pyroxene inclusions in diamonds records the chemical and the physical conditions of the mantle. New approach based on Raman spectroscopy data on 43 pyroxene inclusions in diamonds from Yakutian province (the Siberian craton) and their chemical analyses are provided in this study.

Raman spectroscopy is a high-resolution and non-destructive method used to detect the compositional and the structural characteristics of materials and minerals including high-pressure crystal inclusions in diamonds. Raman spectra of clinopyroxene inclusions in diamonds were collected by Horiba LabRAM HR800 Raman spectrometer with 532-nm laser. In addition, the compositional analyses were obtained by EPMA (JEOL JXA-8100) to correlate chemical variations with specific spectral features and Raman shifts.

Clinopyroxene inclusions show variations of chemical content in the wide ranges: SiO2 54.1-55.9 wt.%, Al2O3 0.31-4.11 wt.%,  Cr2O3 0.32-5.73 wt.%, FeO 1.81-3.49 wt.%, MgO 13.4-18.3 wt.%, CaO 16.1-22.9 wt.%, Na2O 0.28-3.82 wt.% for peridotitic type and SiO2 52.4-56.8 wt.%, Al2O3 4.46-17.8 wt.%,  Cr2O3 <0.29 wt.%, FeO 2.22-11.4 wt.%, MgO 5.65-15.1 wt.%, CaO 9.81-17.1 wt.%, Na2O 3.118-8.121 wt.% for eclogitic type.

Generally, the inclusions yield Raman spectra with four high-intense modes (ν3, ν3’, ν4, ν11, ν17). Observed relative intense of most of these modes (except ν11) depend on changing of crystal orientation. The ν11-mode belongs to the Si-O stretching vibrations of bridging oxygen atoms (Si-Obr). The recorded position of this mode varies in the ranges 665.6-675.1 cm-1 for peridotitic type inclusions and 673.7-688.2 cm-1 for eclogitic type inclusions. One of the factors controlling the shifts of position frequencies of n11-mode is composition.

Peridotitic clinopyroxenes display strong linear correlations between the shifts of position of the ν11-mode and contents of Al2O3 (correlation coefficient r = 0.94), FeO (correlation coefficient r = 0.68), MgO (correlation coefficient r = -0.52), CaO (correlation coefficient r = -0.69), Na2O (correlation coefficient r = 0.92). Eclogitic clinopyroxenes show linear correlations between the shifts of position of the n11-mode and contents of Al2O3 (correlation coefficient r = 0.95), FeO (correlation coefficient r = -0.64), MgO (correlation coefficient r = -0.85), CaO (correlation coefficient r = -0.59), Na2O (correlation coefficient r = 0.84). The most expressed correlations can be used for estimation of composition of inclusions in diamonds only by Raman spectroscopy data without destruction of diamond-host and for identification of clinopyroxenes from potentially diamondiferous mantle rocks.

Acknowledgments: Russian Science Foundation (16-17-10067) supported this work.

How to cite: Kalugina, A. and Zedgenizov, D.: The Raman Estimation of the Composition of Clinopyroxene Inclusions in Natural Diamonds , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-826, https://doi.org/10.5194/egusphere-egu2020-826, 2020.

EGU2020-3473 | Displays | GMPV1.2 | Highlight

Nanoscale compositional segregation in complex In-bearing sulfides: Results from atom probe tomography and transmission kikuchi diffraction

Joachim Krause, Steven M. Reddy, William D. A. Rickard, David W. Saxey, Denis Fougerouse, and Matthias E. Bauer

Indium-bearing sphalerites from the Hämmerlein skarn deposit, located in the western Erzgebirge (Germany), show complex distribution patterns of major and minor elements on a micrometer to sub-micrometer scale. However, with the spatial resolution of traditional analytical methods, such as SEM-based image analysis and field emission electron probe microanalysis (FE-EPMA), many features in these spalerites cannot be resolved. It remains unclear whether Cu, In and Fe are in solid solution in the sphalerite, are concentrated in nanoparticles or form discrete phases.

Atom probe tomography combined with transmission kikuchi diffraction has been used to resolve both the compositional heterogeneity and the nanostructure of these complex In-Cu-Fe-sphalerites. The obtained data indicate a complex structure with micro- to nanometer sized, plate-shaped inclusions of chalcopyrite in the sphalerite. In addition, a nanometer scale In-Cu-sulfide phase forms plate-like segregations in the sphalerite. All types of segregations have similar crystal structure and record the same crystal orientation indicating that they likely formed by exsolution.

The results indicate that complex sulfides containing cations of more than one element as minor or major constituents may represent discrete, exsolved phases, rather than solid solutions or being concentrated in nanoparticles. This heterogeneous nature will affect the nanoscale properties of the sphalerite, which may have implications for the economic extraction of precious elements such as In, when processing these minerals for beneficiation. Furthermore these nanoscale properties will open up new perspectives on formation processes of In-Cu-Fe-sphalerites, which might be relevant for other chemically complex minerals as well.

 

How to cite: Krause, J., Reddy, S. M., Rickard, W. D. A., Saxey, D. W., Fougerouse, D., and Bauer, M. E.: Nanoscale compositional segregation in complex In-bearing sulfides: Results from atom probe tomography and transmission kikuchi diffraction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3473, https://doi.org/10.5194/egusphere-egu2020-3473, 2020.

EGU2020-20427 | Displays | GMPV1.2

Innovative Detection Strategies on Large Geometry SIMS open new challenging applications for light isotope ratio analyses

Paula Peres, Emilie Thomassot, Etienne Deloule, Nordine Bouden, and Firmino Fernandes

Large Geometry Secondary Ion Mass Spectrometry (LG-SIMS), operating in multicollection mode, allows high precision light isotope ratio measurements at high lateral resolution (tens of μm down to sub-μm range). For some challenging applications involving fine scale analysis of low abundance isotopes (i.e. 17O or 36S) or low-concentration elements (i.e. nitrogen in diamonds) measurement of low signal intensities is required. Traditionally, count rates between the upper level of pulse counting systems ~105 c/s and the lower level of Faraday Cup (FC) measurements ~106 c/s are considered to be in a “gap area” where neither detection protocol can achieve performance better than the 1‰ level.

Faraday Cup detectors (FC) offer high precision with no need for gain monitoring, however the uncertainty of FC measurements depends on the signal to noise ratio. One approach for measuring low signal intensities is to use FCs coupled to electrometers with high ohmic resistors. CAMECA LG-SIMS can now be equipped with low noise 1012 Ω resistor FC preamplifier boards for measuring signal intensities down to the ~ 3 x 105 c/s range with precision better than the 0.5‰ level (1SD).

For measurement of low-abundance isotopes, a complementary approach consists of using discrete-dynode pulse counting electron multiplier (EM) detectors, for which drift and aging effects are minimized using a fast automated EM high voltage adjustment routine.

During this PICO presentation, we will discuss the relevance of the detector choice (FC 1012 Ω vs EM) for few examples of innovative applications.

Example of mass independent fractionation:

In addition to classical isotopic ratio measurements (e.g. δ13C, δ15N, δ18O or δ34S), for which the instrumental mass fractionation (IMF) correction is mostly limited by the natural heterogeneity (chemical and isotopic) of the reference material, SIMS is particularly well suited for the measurement of mass independent fractionation (MIF, e.g. ∆33S, ∆36S and ∆17O). Along with classical geochemical processes, the degree of isotopic fractionation scales with the difference in mass of the isotopes involved (i.e. δ33S ≈ 0.515 * δ34S). MIF refers to non-conventional ratios that depart from these mass dependent rules. As instrumental mass fractionation has been shown to be strictly mass dependent, MIF measurements are not subject to IMF correction and are therefore measured directly. The use of SIMS in this specific case is particularly well suited and allows to fully explore the rich phenomenology of MIF source processes. We will discuss the advantages and disadvantages of using FC 1012 Ω for the minor Sulphur isotope (36S) measurement.

Carbon and Nitrogen in diamond:

We will also show a recent analytical development aiming to measure δ13C in diamonds at mass resolution of ~5000 (allowing the full separation of 13C- and 12CH-) as well as N-content and N-isotopes in diamonds at a mass resolution of ~9000 (full separation of 12C14N- and 13C13C-).  For this purpose, the use of FC 1012 Ω greatly improves the data quality and allows the simultaneous measurement of N-content and δ15N.

How to cite: Peres, P., Thomassot, E., Deloule, E., Bouden, N., and Fernandes, F.: Innovative Detection Strategies on Large Geometry SIMS open new challenging applications for light isotope ratio analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20427, https://doi.org/10.5194/egusphere-egu2020-20427, 2020.

Monazite((Ce,Y,La,Th)PO4) is an important phosphate mineral and is one of the widely used minerals for U-Th-Pb dating in geochronology. In this study, we have examined the crystallinity, the valence and coordination of radiogenic Pb in a natural RW-1 monazite standard (ThO2 up to13.5 wt% and Pb up to ~5000 ppm) with a 207Pb/235U age of 904.15 ± 0.26 Ma from a Norwegian pegmatite by using laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy(XPS) and transmission electron microscopy (TEM). The Raman spectrum analysis revealed that this monazite is well crystalline and is not damaged by α-particles. The results of XPS and TEM suggest that the radiogenic Pb produced by the α-decay of U and Th is divalent and radiogenic Pb atom substitutes the Ce-site within the monazite crystal lattice. The qualitative analyses conducted on the HAADF-STEM data reveal heterogeneous distribution of radiogenic Pb within the monazite crystal lattice. This is the first work on the determination of the oxidation state, the atomic location and distribution of radiogenic Pb in a natural monazite (CePO4). The deeply study of radiogenic Pb in monazite at the nanoscale and atomic scale provides a good insight for us to understand the mechanisms of nano-isotopic mobility and the nano-geochronology  that has been poorly understood so far.

How to cite: Tang, X., Li, Q., and Gu, L.: The study of the valence and distribution of radiogenic Pb in monazite by using XPS and TEM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12962, https://doi.org/10.5194/egusphere-egu2020-12962, 2020.

EGU2020-64 | Displays | GMPV1.2

Dislocation nucleation at nanoscale fluid inclusions: Direct observation from atom probe tomography data of naturally deformed pyrite

Renelle Dubosq, Anna Rogowitz, Kevin Schweinar, Baptiste Gault, and David Schneider

In recent years, increasing developments in microscopy and microanalysis have allowed for the direct observation of nanoscale crystalline defects (i.e. dislocations). These defects are particularly important in naturally deformed materials yet this avenue of research remains understudied within the Earth Sciences. Dislocations can now be documented through the use of new and innovative structural and chemical analytical techniques such as electron channeling contrast imaging (ECCI), transmission electron microscopy, and atom probe tomography (APT). The presence and migration of dislocations in crystalline materials, including their role in trace element mobility, play a vital function in the way these materials respond to an applied stress. However, the mechanisms by which dislocations nucleate in minerals remain poorly understood. Prevailing models for dislocation nucleation include generation by Frank-Read sources, stress localization at crack-tips, atomic segregation, and free surface nucleation by critical stress-gradient criterion. Based on recent APT data from naturally-deformed pyrite, combined with electron backscatter diffraction (EBSD) mapping and ECC imaging, we propose a new nucleation mechanism where dislocations are generated by the local stress field in the vicinity of fluid inclusions. The investigated sample consists of a polycrystalline pyrite aggregate within a black shale host rock that has witnessed a peak temperature of 300°C. The combined EBSD and ECCI results reveal crystal plasticity in the form of lattice misorientation up to 8.5° and low-angle grain boundary development. APT data reveals nanoscale fluid inclusions enriched in As, O (H2O), Na and K as well as As- and Co-rich dislocations linked by fluid inclusions. This new model is the first documentation with APT methods of fluid inclusions (voids) in minerals, nanoscale features that are commonly misinterpreted as element clusters or chemically-enriched crystal-defects. The combined data has significant trans-disciplinary implications to the geosciences (structural geology, geochemistry, economic geology, geochronology), the material sciences (metals, ceramics, polymers), and analytical microscopy. Within geochronology voids and dislocations such as these in dated minerals may host elements or isotopes that negatively affect their age. Within ore deposit geology, voids in precious metal-hosting minerals may act as the necessary traps to structurally prevent the metals (gold, silver, copper) from migrating or diffusing out of the host mineral. In material sciences, the presence of such crystalline features can either limit or enhance the performance of engineering materials. Thus, performing APT analysis on crystalline material can help us better understand and predict their physical properties.

How to cite: Dubosq, R., Rogowitz, A., Schweinar, K., Gault, B., and Schneider, D.: Dislocation nucleation at nanoscale fluid inclusions: Direct observation from atom probe tomography data of naturally deformed pyrite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-64, https://doi.org/10.5194/egusphere-egu2020-64, 2020.

EGU2020-2019 | Displays | GMPV1.2

Coupled and combined analyses for unambiguous iron-oxy hydroxides characterization: from laboratory to industrial use

Beate Orberger, Christiane Wagner, Yassine El Mendili, Daniel Chateigner, Stéphanie Gascoin, Henry Pillière, Michel Fialin, Nicolas Rividi, Omar Boudouma, and Richard Wirth

Natural and synthetic iron oxides and iron hydroxides are important minerals for many industrial sectors (e.g. steel making, colors, pigment for coating, electronics, catalysis, soil, waste water and gas treatments, and medicine). In natural environments, such as iron ore mines or iron rich soils (laterites or bauxites), iron oxy-hydroxide associations are complex and evolve related to varying physico-chemical conditions, including. biological interactions. For efficient resource use, unambiguous multiscale characterization is indispensable. Synthetic iron oxides, produced for medical and electronic sector, needs to be failure-free pure phases, thus a continuous quality control is required. Complex iron oxy-hydroxide association can be related to various processes, topotactic transition, pseudomorphosis by substitution and alteration paramorphosis, and corrosions, leading to massive, porous, fibrous and acicular textures or poorly crystalline crusts.

We present examples from iron ore deposits, where coupling of X-Ray diffraction (XRD) with scanning electron microscopy (SEM) and micro-Raman spectroscopy is a powerful tool to distinguish hematite, maghemite and magnetite at grain scale. Oxygen analyses by electron microprobe at (EMPA) fixed carbon coating thickness help to distinguish magnetite and hematite, and contribute with quantitative trace element analyses to chemically differentiate both oxides. At micro- and nano-scale, Transmission Electron Microprobe analyses coupled to X-Ray Diffraction (XRD) and Electron Energy Loss Spectroscopy (EELS) on nanometric inclusions can unambiguously identify various iron oxy-hydroxide phases. In Nickel-laterite and bauxite profiles, iron oxy-hydroxides (e.g. lepidocrocite, ferrihydrite, goethite…) are abundant and may form complex intergrowth with various types of phyllosilicates. Part of it host valuable metals such as Nickel. Combined XRF-XRD and Raman spectroscopy allow phase mapping and differentiation at micron scale of these phases, and even detect solid solutions (e.g. Ni-rich and Ni-poor goethite; El Mendili et al., 2019). Results from coupled laboratory analyses are necessary for building up data bases. They allow calibrating recently developed combined XRF-XRD-Raman benchtop systems. For industrial applications coupled and combined analyses will increase resource efficiency, and ensure a quality control for natural and synthetic iron oxide products. Such systems are recently developed by EU projects, such as SOLSA (www.solsa-mining.com).

El Mendili, Y., Chateigner, D., Orberger, B., Gascoin, S, Bardeau, JF., Petit, S., Le Guen, M., Pillière, H. (2019). Combined XRF, XRD, SEM-EDS, and Raman analyses on serpentinized harzburgite (Nickel Laterite Mine, New Caledonia): Implications for Exploration and Geometallurgy. ACS Earth and Space Chemistry. 3, 10, 2237-2249; DOI: 10.1021/acsearthspacechem.9b00014

How to cite: Orberger, B., Wagner, C., El Mendili, Y., Chateigner, D., Gascoin, S., Pillière, H., Fialin, M., Rividi, N., Boudouma, O., and Wirth, R.: Coupled and combined analyses for unambiguous iron-oxy hydroxides characterization: from laboratory to industrial use , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2019, https://doi.org/10.5194/egusphere-egu2020-2019, 2020.

EGU2020-3941 | Displays | GMPV1.2

Combining SEM, EDS & EBSD: Challenges and considerations in the micro-analysis of rock thin sections

Alexandra Stavropoulou, Matthew Hiscock, Balz Kamber, and Juan-Diego Rodriguez-Blanco

Quantitative modal analysis of rock thin sections or liberation analysis of minerals processing plant materials can be very complex as grain sizes can vary by more than 7 orders of magnitude: Thin sections of rocks may contain extremely coarse grains (mm-sized crystals) down to glassy material with no long-range order (ordered domains <1 nm).

Material characterisation and modal analysis have traditionally been carried out with a combination of solid-state, microscopic and spectroscopic techniques (e.g., optical / scanning electron microscopy, powder X-ray diffraction, X-ray fluorescence spectroscopy). These techniques require different sample preparation routines, data acquisition and evaluation - a time-consuming process that may be considered too complex to implement in mineral processing plants despite requiring the relevant sample preparation equipment. Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS provides an opportunity to carry out this characterisation in a more rigorous and, in certain cases, automated way. This process includes image thresholding (setting of grey levels of present phases by the analyst) and X-ray data collection with EDS. EDS is an ideal analytical technique for this work as it offers high acquisition speeds and the collection of the whole energy spectrum with a single detector, not requiring the selection of a fixed element list prior to data acquisition. Characterisation of coarse-grained rocks requires larger areas to be scanned in order to ensure representativity.

The analytical workflow can be further optimised by combining SEM-based analytical techniques for in situ, non-destructive, and potentially simultaneous bulk analysis. Electron backscatter diffraction (EBSD) is an SEM-based technique which can be used to determine the crystallographic properties and orientation of mineral grains, as well as to perform fabric analyses on polycrystalline materials. EBSD allows for crystallographic data to be collected simultaneously with chemical data and does not require powdered samples. As a result, the texture of the material can be fully preserved. The sample preparation requirements of the technique are similar to those for standard SEM-EDS, with an additional final polishing step, essential for the removal of surface imperfections, as the EBSD signal is generated on the sample surface. The coupling of EDS and EBSD datasets permits the enhanced interpretation of feature analysis data, allowing for a deeper understanding of the compositional, structural and textural properties of the sample. This, highly-efficient, in-situ, bulk material characterisation, is key for the mining industry, as it provides insights for optimising downstream procedures thereby saving time and resources and bolstering throughput and efficiency.

How to cite: Stavropoulou, A., Hiscock, M., Kamber, B., and Rodriguez-Blanco, J.-D.: Combining SEM, EDS & EBSD: Challenges and considerations in the micro-analysis of rock thin sections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3941, https://doi.org/10.5194/egusphere-egu2020-3941, 2020.

EGU2020-4515 | Displays | GMPV1.2

Table-top Cathodoluminescence Microscopy for Geology

Toon Coenen and Albert Polman

Cathodoluminescence (CL) microscopy is a well-known technique for imaging geological specimens, in which the light that is generated with an energetic electron beam is collected and analyzed with a CL detector. CL provides a unique imaging contrast that can be used for visualizing growth zonation, distinguishing cement and granular detrital material, detecting trace elements, and characterizing fractures and deformation features in a large range of rocks, to name a few examples. In its simplest form CL imaging is performed with a static electron beam in an optical microscopy system (optical CL) but for more advanced experiments CL imaging is performed in a scanning electron microscope (SEM). This enables high scan speeds, high spatial resolution (< 100 nm), and correlation with other SEM based techniques such as X-ray imaging (EDS), secondary electron (SE) and backscattered electron (BSE) imaging, and more.

Currently, SEM-based CL work is mostly performed on costly floor model SEMs that require large amounts of space, complex auxiliary support systems, and an experienced operator to run the machine. In contrast, compact, affordable, and user-friendly table-top SEMs have improved substantially in the last years but they typically lack (advanced) CL imaging capabilities. Here, we will present our progress in developing a table-top SEM based CL system that can be used for geological research amongst other applications.

In particular, we have integrated a CL collection and detection system in a Thermo Fisher Scientific/Phenom XL table-top microscope, which already is equipped with SE, BSE, and EDS imaging modalities. In this SEM, electron energies of 5 – 15 keV can be used which is appropriate for most CL imaging experiments. The CL is collected using a multimode fiber optic cable connected with a graded index lens to increase the numerical aperture of the collection. Subsequently, the light is send to a spectrometer where the CL emission spectrum can be measured for every excitation point on the sample; a technique known as hyperspectral CL  imaging. To synchronize electron beam scanning with data acquisition and for data analysis we have developed dedicated software control.

We assess the potential of table-top CL by imaging representative polished zircon and quartz samples for various beam and acquisition parameters. To benchmark the system performance we compare our experimental results with results obtained from a state-of-the-art floor model SEM (Thermo Fisher Scientific Quanta 650 SFEG) system equipped with a high-end Delmic SPARC CL system. In the future, these developments may lower the threshold for using CL imaging through cost reduction and workflow simplification, making it accessible to a larger range of users within the field of geology and beyond.

How to cite: Coenen, T. and Polman, A.: Table-top Cathodoluminescence Microscopy for Geology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4515, https://doi.org/10.5194/egusphere-egu2020-4515, 2020.

EGU2020-4623 | Displays | GMPV1.2

Eliminating Laser Induced Elemental U-Pb Fractionation using low sample volume multi-shot ablation protocols

Eoghan Corbett, Antonio Simonetti, Phil Shaw, Loretta Corcoran, Quentin Crowley, and Brendan Hoare

The interaction of incident laser radiation and sample substrate is complex and difficult to predict. Natural zircons areoften both structurally and chemically heterogeneous in 3-dimensional space. Encountering growth-related, structural micro-heterogeneities, inclusions and chemical complexities is almost inevitable when employing ‘conventional’ static, high-frequency laser sampling protocols often lasting several tens of seconds at a time.

 

A multi-shot approach to laser ablation by contrast implements a minimal sample exposure time to incident laser radiation by applying multiple 1 Hz shots in delayed succession to a single sampling site. This process can be conceptualised as a “slowing down” of a high-frequency (10-20 Hz) static Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis. Each laser pulse applied in this manner, produces signal peak which is distinct albeit transient. The ability to integrate and collate signal pulses for a small number of consecutive laser shots (10-30 shots), as opposed to continuously pulsing the laser, produces highly precise age determinations (<1% reproducibility, 2slevel) on small sample volumes (~695µm3 on 91500 zircon standard). The multi-shot LA-ICP-MS protocol employed here effectively eliminates ‘downhole’ fractionation as the resultant craters are extremely shallow (as shallow as ~553nm on 91500 zircon standard) and maintain an aspect ratio of <<1. Further benefits include a reduced probability of thermally induced effects (e.g., substrate melting), plasma loading, and the potential for signal mixing (with depth) in a heterogeneous sample.

How to cite: Corbett, E., Simonetti, A., Shaw, P., Corcoran, L., Crowley, Q., and Hoare, B.: Eliminating Laser Induced Elemental U-Pb Fractionation using low sample volume multi-shot ablation protocols, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4623, https://doi.org/10.5194/egusphere-egu2020-4623, 2020.

EGU2020-12779 | Displays | GMPV1.2

Nanoscale Pb clustering and multi-domain Pb-mobility in zircon

Tyler Blum, Chloe Bonamici, and John Valley

Uranium-Lead dating of zircon remains one of the most widely utilized and most reliable temporal records throughout Earth history. This stems from the mineral’s widespread occurrence, pristine zircon being both physically and chemically robust, and the ability to evaluate the presence of open system behavior (i.e. “concordance”) through comparison of the independent 238U→206Pb, 235U→207Pb, and 232Th→208Pb decay chains. The phenomenon of discordance is well documented in zircon, and is typically (though not always) associated with radiation damage accumulation and Pb-loss. Despite a long history of research, the nanoscale controls on Pb mobility and Pb loss (i.e. the relative rates of radiation damage, annealing, and Pb diffusion) remain poorly defined. The unique characterization capabilities of atom probe tomography (APT) provide a novel means to study U-Pb systematics on the scale of the radiation damage, annealing and diffusion processes. APT studies have documented nanoscale heterogeneity in trace elements, Pb, and Pb isotope ratios, and correlated the 207Pb/206Pb ratios within clusters to transient thermal episodes in the history of a zircon.

 

This work seeks to provide a foundation for multi-scale U-Pb characterization, including how differential Pb mobility at the nanoscale can influence micron- to- grain-scale U-Pb systematics. Historically, concordia diagrams have used simple Pb-loss models to extract temporal information about the timing of Pb mobility/loss; however, these models assume 207Pb and 206Pb are uniformly disturbed within a grain and lost in equal proportions at the time of Pb loss. Our previous studies suggest that radiogenic Pb can be concentrated and immobilized in nanoscale clusters, leading to differential retention of Pb in clusters vs. matrix domains, and requiring a more complex treatment of isotopic shifts during any post-clustering Pb loss. This “multi-domain element (Pb) mobility” (MDEM or MDPM) influences subsequent Pb-loss trajectories on concordia diagrams, manifesting in systematic offsets for discordia as a function of the zircon crystallization age, the timing of cluster formation, and the timing of Pb mobility. These results highlight that (1) traditional interpretations of discordia in the presence of cryptic nanoscale clustering can lead to inaccuracies, and (2) multi-scale U-Pb characterization offers a means to both study discordance and to extract additional temporal information from zircon with otherwise ambiguous and/or complex Pb-loss patterns.

How to cite: Blum, T., Bonamici, C., and Valley, J.: Nanoscale Pb clustering and multi-domain Pb-mobility in zircon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12779, https://doi.org/10.5194/egusphere-egu2020-12779, 2020.

The laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) is a unique method for local analysis that allows studying mineral grains in situ. The aims of these geochemical researches are to estimate concentrations and distributions of REE, Hf, U, Th, Y, Ti, PGE and other elements in accessory and ore minerals from complex deposits in the Arctic region (Fennoscandian Shield), using the LA-ICP-MS local analysis of trace elements. Accessory minerals of zircon and baddeleyite are much valued to study distributions of rare and rare earth elements (REE). Besides, pyrite, pentlandite, pyrrhotite and other sulfides are important for determining platinum-group elements (PGE), REE, etc.

The electron (LEO-1415) and optic (LEICA OM 2500 P, camera DFC 290) spectroscopy have been applied to study the morphology of the samples. Analytical points have been selected on baddeleyite, zircon crystals and sulfide minerals based on analyses of their BSE, CL and optical images. REE, PGE and other elements have been estimated in situ by ICP-MS, using an ELAN 9000 DRC-e (Perkin Elmer) quadrupole mass spectrometer equipped with UP-266 MAСRO laser (New Wave Research).

More than 19 elements were profiled during each measurement in zircon or baddeleyite. For the first time, LA-ICP-MS techniques have been applied to estimate PGE, REE and other (S, Cr, Fe, Cu, Ni, Co, As, Se, Mo, Cd, Sn, Sb, Re, Te, Tl, Hf, W, Bi, Pb, Th, U) elements in sulfide minerals. NIST 610, NIST 612 and tandem graduation (using solutions), considering sensitivity coefficients of isotopes have been used to check the accuracy of estimations. Fe, Ni and Cu have been used as internal standards, being most evenly distributed elements in minerals, when concentrations of elements in sulphides were calculated. The estimates have been carried out, using inter-laboratory standards of chalcopyrite, pentlandite and pyrrhotite, which had been preliminarily prepared and studied using micro probe analysis (Cameca MS-46).

These techniques had been used to estimate elements in zircon extracted from basic and acidic rocks of the Lapland belt (1.9 Ga), the Keivy zone (2.7 Ga), the Kandalaksha and Kolvitsa zone (2.45 Ga) and from the Cu-Ni deposit (Terrace, Mt. Nyud, 2.5 Ga). Novel techniques have been used to analyze baddeleyite from rocks of layered PGE intrusions of the Monchegorsk ore area (2.5 Ga) and carbonatites of Kovdor and Vuoriyarvi (380 Ma). Elaborated LA-ICP-MS techniques have been applied to provide in situ measurements of PGE, Au, Ag, siderophile and chalcophile elements in sulphide minerals from the Pechenga and Allarechka Cu-Ni deposits (1.98 Ga), Fedorova Tundra and Severny Kamennik PGE deposits (2.5 Ga).

The scientific researches are supported by RFBR Grant No 18-05-70082, scientific themes 0226-2019-0032 and 0226-2019-0053.

How to cite: Drogobuzhskaya, S., Bayanova, T., and Novikov, A.: Geochemical researches in situ (LA-ICP-MS) of accessory and ore minerals from multimetal (PGE, Cu-Ni) deposits in the Arctic zone (Fennoscandian Shield) of the Russian Federation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13123, https://doi.org/10.5194/egusphere-egu2020-13123, 2020.

EGU2020-14000 | Displays | GMPV1.2

Quantitative automated mineralogy to constrain metamorphic processes using ZEISS Mineralogic

Rich Taylor, Eddy Hill, Pierre Lanari, Chris Clark, and Tim Johnson

The Scanning Electron Microscope (SEM) is the most prolific piece of analytical equipment in the Earth Sciences, therefore quantitative mineral chemistry obtained directly from the SEM has the potential to streamline many geological fields. Mineral chemistry provides direct constraints on geological processes that are used in a wide variety of Earth Science disciplines. As a result, major element analysis of rock forming minerals have been one of the major contributors to geochemistry for decades. Electron beam techniques have been the most widely used method of obtaining in situ major element chemistry, dominated by the quantitative Wavelength Dispersive Spectroscopy (WDS) employed by the Electron Probe Micro Analyser (EMPA). More rapid, and typically more qualitative Energy Dispersive Spectroscopy (EDS) major element measurements are often obtained on a standard SEM instrument.

The relative simplicity of the EDS technique saw the growth of automated mineralogy systems beginning in the 1980’s. The peaks of EDS spectra are characteristic of the major elements present, and therefore lookup tables can be used to match the spectra to known mineral compositions and provide a likely mineralogy in both grain mounts and mapped thin sections. The automated mineral analysis technique remained essentially unchanged for decades, with an experienced operator required for many of the analytical tasks, such as creating the files for matching spectra to known minerals, processing the data, and interpreting complex phases and solid solutions (e.g. Fe/Mg-bearing silicates).

The ZEISS Mineralogic automated quantitative mineralogy (AQM) takes a new approach, using EDS detectors, but following an analytical protocol more closely aligned with EPMA. A combination of matrix corrections, peak deconvolution, and standard calibration means that peak intensities are converted directly into wt% element directly at the time of analysis. The result is a data output that can be immediately interpreted, even for minerals not previously analysed, by both new and experienced users.

Here we demonstrate the use of the ZEISS Mineralogic system for mapping thin sections from high grade metamorphic rocks. The bulk chemistry of the entire thin section, as well as individual mineral compositions can be used to constrain P-T conditions directly from the SEM, without the need for an additional step of obtaining mineral chemistry from an EPMA. With quantitative analysis at every pixel, major element profiles can be obtained at any point in the thin section, and P-T can therefore be determined from any domain within the mapped section. This approach makes the use of P-T pseudosections possible with greater speed and flexibility than has previously been possible.

How to cite: Taylor, R., Hill, E., Lanari, P., Clark, C., and Johnson, T.: Quantitative automated mineralogy to constrain metamorphic processes using ZEISS Mineralogic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14000, https://doi.org/10.5194/egusphere-egu2020-14000, 2020.

With this work, we aim at exploring the extent to which valence-to-core X-ray emission spectroscopy (vtc-XES) can provide this first- and second-coordination-shell information from amorphous germanium oxides.

We measured the vtc-XES spectra of germanium oxides at ambient and high pressure. The Kbeta’’ emission line, part of the vtc-XES spectra, is sensitive to coordination and oxygen-germanium bond distance as first coordination shell effects. Furthermore, it reflects the different binding energies of bridging and non-bridging oxygen atoms. The Kb’’ emission line may thus allow for tracking the coordination and the state of polymerization of a germanium oxide glass under pressure in diamond anvil cells or in other confining environments.

 

Spiekermann et al. (2019) Persistent octahedral coordination in amorphous GeO2 up to 100 GPa revealed by Kbeta'' X-ray emission spectroscopy, Physical Review X, 9, 011025.

How to cite: Spiekermann, G.: XPS-like evaluation of valence-to-core X-ray emission spectra of germanates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20142, https://doi.org/10.5194/egusphere-egu2020-20142, 2020.

EGU2020-20802 | Displays | GMPV1.2

U-Pb in situ dating and trace elements profiles in chrysocolla and pseudomalachite : Application to supergene copper mineralization.

Mathieu Leisen, Zia Steven Kahou, Stéphanie Brichau, Stéphanie Duchêne, François-Xavier d’Abzac, and Sandrine Choy

Over the past two decades, laser ablation coupled with the mass spectrometer has become a major analytical tool for the measurement of isotopic ratios and the determination of trace elements. The improvement of the sensitivity has provided new perspectives and permits to study new types of targets. For example, many questions remain open about the formation of supergene mineralization such as: what is exact timing for their deposition? What are the required associated physico-chemical conditions? To answer these questions, we focused on two copper deposits located in Chile (Mina Sur) and Burkina Faso (Gaoua) to develop U-Pb analysis and trace element profiles in pseudomalachite and chrysocolla. The analyses were carried out at the GET Laboratory (Toulouse). Different couplings between a femtosecond laser (fs-LA) or a nanosecond laser (ns-LA) and a HR-ICPMS or a MC-ICPMS were used. Trace elements determination and in situ U-Pb analysis present different challenges. For U-Pb analyses, matrix effects must be taken into account and the contribution of common lead (204Pb) must be subtracted. As there is no chrysocolla or pseudomalachite reference materials, zircon and apatite were used as the primary external standards and fs-LA was used as a matrix independent sampling method. No significant U-Pb fractionation was observed, whatever the structure of the ablated matrix (silicate, phosphate). The bias linked to common lead was calculated from fs-LA-MC-ICPMS measurements. The 206Pb / 204Pb intensity ratio gives a first approximation on the possibility to determine the U-Pb age. Three cases have been distinguished: 1) If 204Pb is low (206Pb / 204Pb ≥ 500) the U-Pb age obtained by this first analyze can be used. 2) If 204Pb is significant and the intensity ratio of 206Pb / 204Pb range between 500 and 5, a second step is necessary. In such a case, 204Pb must be determined more precisely using a MC-ICPMS to retrieve the common lead corrected U-Pb age. 3) If 204Pb is high (206Pb / 204Pb <5), then it is not possible to determine the U-Pb age of the sample. Trace element profiles were also performed on the same chrysocolla and pseudomalachite samples. These analyses have been carried out using a ns-LA coupled to HR-ICPMS and NIST SRM 610 was used as primary standard. The reproducibility and accuracy of the analyses were verified by the ablation of secondary standards (91500 zircon and Durango apatite) and comparison with EMPA analyses. In this study we demonstrate that supergene mineralization can be directly dated and the trace elements in pseudomalachite and chrysocolla can be determined. The combination of these methods provides a new tool to understand the physico-chemical and geological conditions that are required for the formation of supergene mineralization.

How to cite: Leisen, M., Kahou, Z. S., Brichau, S., Duchêne, S., d’Abzac, F.-X., and Choy, S.: U-Pb in situ dating and trace elements profiles in chrysocolla and pseudomalachite : Application to supergene copper mineralization. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20802, https://doi.org/10.5194/egusphere-egu2020-20802, 2020.

EGU2020-21866 | Displays | GMPV1.2

In situ AFM imaging of dissolution and growth of struvite surface

Aikaterini Vavouraki, Helen King, Christine Putnis, and Petros Koutsoukos

Phosphorus removal from wastewaters is a great interest to avoid eutrophication of natural waters (i.e. rivers, lakes). Additionally phosphorus recovery is also important due to increasingly limited nutrient resources. Struvite (MgNH4PO4*6H2O) is of current interest as it is considered an alternative way for water remediation and potential use as a renewable fertilizer. Towards environmental sustainability, phosphorus and nitrogen removal and recovery from domestic, industrial, and/ or agricultural inputs, in the form of struvite may be an attractive alternative for the valorization of wastewaters (Mpountas et al., 2017).

Direct observations of crystal dissolution and growth process at the nano- level are possible through the use of Atomic Force Microscopy (AFM). A considerable number of studies have investigated mineral dissolution and growth by in-situ AFM imaging in a fluid-cell (Vavouraki et al., 2008; 2010). These direct observations and measurements allow the investigation of possible process mechanisms at the mineral-solution interface. Previous study on AFM imaging indicated struvite micro- to nanocrystal morphology. Hövelmann & Putnis (2016) investigated the interactions of ammonium-phosphate solutions with brucite (Mg(OH)2) cleavage surfaces by AFM suggesting coupled brucite dissolution and struvite precipitation at the mineral-fluid interface. To the best of our knowledge, there are no records of nanoscale observations of dissolution and/ or growth of struvite surfaces. The aim of this study is to perform in situ AFM experiments using freshly cleaved struvite surfaces at flow conditions. Step retreat and/ or etch pit spreading were observed. Dissolution rates of struvite using doubly deionized water at different pH values were determined  whereas growth rates at different saturation values and pH using magnesium and ammonium-phosphate bearing solutions were also measured . Raman and SEM analyses were carried out to assess chemical structure and morphology of the obtained struvite crystals before and after AFM experiments.

Acknowledgments: The work has been supported by IKY-DAAD (2018-4) and KRHPIS II (Action PERAN).                                                                                

References: Hövelmann & Putnis, 2016. Env. Sci. Technol. 50, 13032−13041; Mpountas et al., 2017. J. Chem. Technol. Biotechnol. 92, 2075–2082; Vavouraki et al., 2008. Chem. Geol. 253, 243–251; Vavouraki et al., 2010. Cryst. Growth Des. 10, 60–69.

How to cite: Vavouraki, A., King, H., Putnis, C., and Koutsoukos, P.: In situ AFM imaging of dissolution and growth of struvite surface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21866, https://doi.org/10.5194/egusphere-egu2020-21866, 2020.

The marine Fe-Mn polymetallic nodules contain relatively high concentrations of Mn, Cu, Ni, Zn, Co and rare earth elements plus yttrium (REY), with a growing economic potential interest in their exploitation. To determinate the metallogenic processes and occurrence phases of the economic metals in the Fe-Mn nodules, we have undertaken high resolution mineralogical and geochemical studies of Fe-Mn nodules collected from the South China Sea (SCS).
The whole-rock mineralogical and chemical compositions of the SCS Fe-Mn nodules indicate hydrogenetic origin. The Mn mineral phases mainly are composed of nanocrystalline vernadite with interlayered 10 Å and 7 Å phyllomanganates, such as todorokite, birnessite, and buserite. Fe(-Ti) oxides/hydroxides are intergrown and essentially X-ray amorphous feroxyhyte and goethite. But we recognize two main types of internal microlayers in the SCS Fe-Mn nodules: Layer type A of suboxic diagenetic precipitates with extremely high Mn/Fe ratio and concentrations of Cu, Ni, Zn, Ba, Li and Mg; Layer type B of oxic hydrogenetic accretions with low fractionation of Mn and Fe and high contents of Co, REY, Ti, Sr and Pb. Furthermore, the elemental mapping indicates that the enrichment of Co and REY mainly associated with Fe mineral phases rather than Mn mineral phases, which are enriched in Mg, Cu, Ni, Zn, Li and Ba. Two mineralization processes and distributions of metals in the individual microlayers respectively are controlled and occurred by the different mineral phases. The increasing occurrence of 10 Å and 7 Å phyllomanganates present in the Layer type A are typically enriched in trace metals such as Ni, Cu, Zn, Li, Ba, and Mg, whereas the metals associated with the Layer type B include Co, Ti, Pb, Sr, REY, which might be carried by the intergrown of Fe(-Ti) oxyhydroxides and vernadite. Thus, hydrogenesis is more beneficial to the enrichment of Fe, Co, Ti, Sr, Pb and REY, while diagenesis is more favorable for the enrichment of Mn, Ni, Zn, Cu, Li, Ba and Mg during the metallogenic processes of the SCS nodules.

How to cite: Guan, Y.: Fine scale study of major and trace elements in the Fe-Mn nodules from the South China Sea and constraints on their formation processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21817, https://doi.org/10.5194/egusphere-egu2020-21817, 2020.

The recent introduction of a high-brightness RF plasma oxygen ion source (Hyperion H201, Oregon Physics) to large geometry secondary ion mass spectrometers (e.g. CAMECA IMS1280/1300) has increased the range of available primary beam options compared to the several decades old technology of the duoplasmatron it replaces. Notably, the new source provides considerably higher beam density (ca. 10x and 3x for O- and O2- respectively), which in principle allows for higher spatial resolution and/or shorter analysis times, coupled with unprecedented long-term beam stability.

Incorporating the RF plasma into both conventional spot analysis and ion-imaging geochronology routines at the NordSIMS facility has, however, revealed that the source upgrade has consequences for data-acquisition and data reduction strategies, which need to be modified in order to avoid degradation in precision. The most significant difference using the new source for spot analyses is the significant change in aspect ratio (width/depth) of the analysed volume. During a comparable length analysis, a three times brighter O2- primary beam (still favoured for U-Th-Pb geochronology) will sputter a three times deeper crater that is half the width of a comparable intensity duoplasmatron beam, an effective aspect ratio change of six times, introducing “down-hole” inter-element and, to a lesser degree, isotope fractionation effects that SIMS has largely been free of. Depending on the target matrix, this can have a marked effect on the within-run ratio evolution during an analysis, particularly the inter-element ratios Pb/U and UOn/U required for full U-Pb geochronology, with standard error of the mean values several times higher than counting statistics, compared to analyses with the lower beam density of the duoplasmatron where s.e. mean commonly closely approaches Poisson counting statistics during a ca. 10 minute analysis. In line with previous observations [1], some improvements can be made by using a Pb/UO vs. UO2/UO calibration scheme instead of Pb/U vs. UOn/U, but clearly this is not the complete answer. Shortening analyses via fewer cycles in a peak-hopping routine also means smaller √n, affecting s.e. mean; lower integration times can be introduced to permit more cycles, but magnet settling times between peak jumps cannot be reduced in proportion, so the duty cycle is less efficient.

Strategies developed to mitigate this degradation and take full advantage of the new RF source include: 1) rastering of critically focused primary beams to retain high aspect ratio (at the expense of improved spatial resolution); 2) use of a defocused aperture-projected (Köhler-mode) primary beam (effectively lower beam density); 3) modelling of within-run ratio evolution based on standard analyses in a manner similar to that employed by laser ablation methods [2]; and/or 4) introduction of multicollection capabilities [3] to increase duty cycle efficiency in a shorter analysis. Ultimately, the choice of which method(s) to use will depend upon the goal of a specific project.

References: [1] Jeon, H. & Whitehouse, M.J.., Geostds & Geoanal. Res. 2014, 39, 443-452]; [2] Paton, C. et al., Geochem. Geophys. Geosyst., 2010, 11, Q0AA06]; [3] Li et al., J. Anal. At. Spectrom., 2015, 30, 979-985

How to cite: Whitehouse, M. and Jeon, H.: Living up to the Hype(-rion)! – observations on ion microprobe geochronology using a high-brightness oxygen plasma source, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20107, https://doi.org/10.5194/egusphere-egu2020-20107, 2020.

GMPV1.4 – The Big Fat Session of the Year: Microstructures, a Journey into Tiny Things

EGU2020-352 | Displays | GMPV1.4 | Highlight

Microstructural Constraints on Magmatic Mushes under Kīlauea Volcano, Hawai'i

Penny Wieser, Marie Edmonds, John Maclennan, and John Wheeler

Distorted olivines of enigmatic origin are ubiquitous in erupted products from a wide range of volcanic systems (e.g., Hawai'i, Iceland, Andean Southern Volcanic Zone). At Kīlauea volcano, distorted olivines are commonly attributed to ductile creep within dunitic bodies located around the central conduit, or within the deep rift zones (~5–9 km depth). However, a recent suggestion that lattice distortions are produced by an early phase of branching dendritic growth, followed by textural ripening and the merging of misoriented crystal buds, has gained considerable traction.

A quantitative examination of the microstructures in distorted olivines by electron backscatter diffraction (EBSD) reveals striking similarities to microstructures observed in deformed mantle peridotites, but significant differences to the crystallographic signatures of dendritic growth. This suggests that lattice distortions record the application of differential stresses at high temperatures within the magmatic plumbing system, rather than rapid crystal growth. Previous petrological work has suggested that differential stresses are produced by ductile creep within Kīlauea’s deep rift zones. Crucially, this has fuelled suggestions that significant quantities of magma must travel along these rift zones in order to acquire distorted olivines, despite the paucity of geophysical evidence for these magma transport paths. In contrast, we show that the spatial distribution of eruptions containing distorted olivines is consistent with their derivation from the main magma storage reservoir. This model not only aligns petrological and geophysical observations at Kīlauea, but also accounts for the occurrence of distorted olivines in a wide variety of basaltic systems worldwide (which lack deep rift zones).

Application of piezometers developed for mantle peridotites reveals that distorted olivines have experienced differential stresses of ~3–12 MPa. Assuming that mush piles behave as granular materials, and form force chains, these stresses can be generated within cumulate piles of ~180–720 m. Based on available constraints on the magma supply rate and the geometry of Kīlauea’s summit reservoir, these thicknesses accumulate in a few centuries (consistent with residence times inferred from melt inclusion records).

Overall, we demonstrate that microstructural investigations of erupted olivine crystals by EBSD generates rich datasets which provide quantitative insights into crystal storage within mush piles. Under the increasingly prevalent view that crustal magmatic systems are mush-dominated, constraining the geometry and dynamics of crystal storage regions is crucial to further our understanding of magmatic plumbing systems. The presence of distorted olivines in many different volcanic settings highlights the global applicability of the methods developed in this study. Furthermore, assessments of deformation conditions using EBSD need not be restricted to olivine-bearing lavas. Microstructural fabrics types in natural and experimental samples have been established for a wide variety of igneous phases (e.g. diopside, plagioclase, hornblende), so similar approaches may be utilized in more evolved volcanic systems.

How to cite: Wieser, P., Edmonds, M., Maclennan, J., and Wheeler, J.: Microstructural Constraints on Magmatic Mushes under Kīlauea Volcano, Hawai'i, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-352, https://doi.org/10.5194/egusphere-egu2020-352, 2020.

Melt inclusions originate from small juvenile melt droplets trapped at magmatic pressures and temperatures during crystallization of their host mineral. Thus, melt inclusions retained by their host crystals can uniquely preserve evidence for the thermochemical conditions in the magma during crystal growth. Zircon is a resistant mineral even under magmatic conditions, and it is common in many different rock types (igneous, metamorphic, and sedimentary). Moreover, zircon crystallization significantly affects trace element concentrations of the melt during processes such as fractionation, melt separation, and/or retention of accessory phases in the residual melt. Its potential as a host mineral for melt inclusions, however, has not been fully realized, mainly because of the small size of zircon and its inclusions.

Here, we developed a new technique for ion imaging of elemental distributions in melt inclusions in zircon, and applied it to melt-inclusion bearing zircon crystals from selected Miocene ignimbrites of the Central Anatolian Volcanic Province, Turkey. The high-sensitivity ion imaging of zircon provides information about the 2-D distribution of critical elements in the crystal and its inclusions, and element distributions can be directly compared to cathodoluminescence (CL) patterns of the host. High-sensitivity element maps were obtained using a CAMECA 1280-HR IMS at Heidelberg University for areas of 25×25 µm at 2-3 µm lateral resolution. Ion images for each element containing 128×128 pixels raw intensity values were initially processed using instrumental software (WINImage©) to accumulate data from measurement cycles into a single image data. Each element map was then recorded as a grayscale image with intensities encoded in each pixel. The raster images for each element was further processed using ImageJ© and ARCGIS© programs, where each element map was converted to a color scale expressing the appropriate value ranges and the data obtained on the same trace element for each zircon in different units were reduced to the same legend values. The color ion images obtained from the grayscale images of each map were overlaid onto CL images to correlate trace element abundances with growth regions visible in CL images.

Imaging has the important advantage compared to spot analyses of melt inclusions that contamination from the wall of the host mineral can be mitigated. For this, Zr ion images were used as controls for selecting ROIs (Regions of Interest) in order to eliminate pixels with mixed signals at the interface between zircon and the inclusion due to the finite width of the ion beam. High resolution imaging of melt inclusions and zircon allowed re-evaluating zircon-melt partitioning behavior of important trace elements for natural melt compositions. Partitioning values for elements with comparatively low abundances in the melt relative to zircon (Y, Th, U and Dy) are slightly lower than spot analyses and previously published results but they all follow a similar trend with predicted partitioning coefficients. 

This research was financed by The Scientific and Technological Research Council of Turkey within the research program number of 2214/A. 

How to cite: Akin, L., Aydar, E., and Schmitt, A. K.: High Sensitivity Mapping of Melt Inclusions in Miocene Zircons of Central Anatolian Volcanic Province (CAVP), Cappadocia, Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-36, https://doi.org/10.5194/egusphere-egu2020-36, 2020.

EGU2020-5267 | Displays | GMPV1.4 | Highlight

A preliminary assessment of the application of Sr, Nd, Pb, He and N isotope analysis to fluid inclusions in kimberlite olivine: A new approach to trace deep-mantle sources

Andrea Giuliani, Janne M. Koornneef, Peter Barry, Patrizia Will, Henner Busemann, Colin Maden, Roland Maas, Alan Greig, and Gareth R. Davies

Kimberlites are the deepest melts that reach Earth’s surface and, therefore, can provide unique insights into the composition and evolution of the convective mantle through time. Application of isotope geochemistry to trace the composition of kimberlite sources has thus far been hindered by the ubiquitous alteration and incorporation of xenocrystic material in kimberlite rocks. Bulk-kimberlite analyses are typically considered reliable for Nd and Hf isotopes due to their overwhelmingly higher concentrations in kimberlite melts compared to common mantle and crustal contaminants. Conversely, Sr and Pb isotope compositions of bulk kimberlite samples are seldom considered representative of their parental melts thus requiring analysis of robust magmatic phases, primarily perovskite. Addressing the primary (i.e. magmatic) isotopic composition of volatile elements, such as N and noble gases, requires analyses of volatile-rich phases, and fluid inclusions in olivine represent a typical primary target in mantle-derived magmas. However, fluid inclusions in kimberlitic olivine are dominantly secondary in origin. Secondary inclusions can form at any time after crystallisation of their mineral host, which requires assessment of the origin of trapped fluids (i.e. pristine magmatic fluids, crustal fluids of external derivation, or combination thereof) before their isotopic composition can be used to make inferences about kimberlite mantle sources.

Here we present trace-element and Sr-Nd-Pb-He-N isotopic compositions of multiple olivine aliquots representing two different magmatic units of the ~88 Ma Wesselton kimberlite (Kimberley, South Africa). The Sr and Nd isotopic composition of olivine analysed by isotope-dilution (ID) TIMS are within the narrow range of perovskite 87Sr/86Sr (0.7043-0.7046) and whole-rock 143Nd/144Nd (eNdi = 0.4–2.2) for the Kimberley kimberlites. These results indicate that the secondary fluid inclusions, which dominate the incompatible trace-element budget of olivine separates, have a pristine magmatic origin devoid of crustal contribution.

Helium isotope compositions were measured by laser heating of 1.6 to 9.8 mg of olivine using an ultrahigh-sensitivity compressor-source noble gas mass spectrometer. 3He/4He ratios are between 1.6 RA and 3.7 RA (where RA indicates the atmospheric 3He/4He ratio), values more radiogenic than MORBs but comparable to HIMU OIBs. These results indicate a high time-integrated (U+Th)/He ratio in the source of the Kimberley kimberlites, which is consistent with the moderately high (i.e. HIMU-like) time-integrated U/Pb ratio implied by elevated initial 206Pb/204Pb in Wesselton olivine (19.1-19.5), Kimberley kimberlites (up to 19.9) and megacrysts in southern African Cretaceous kimberlites (up to 20.5). The combination of low 3He/4He, moderately radiogenic 87Sr/86Sr, and negative d34S values (-2.6‰ to -5.7‰) require a contribution from subducted recycled material in the source of the Kimberley kimberlites. Conversely, a preliminary N isotope analysis of Wesselton olivine by in-vacuo crushing using a noble gas mass spectrometer returned a mantle-like d15N of -2.9‰, which might suggest limited recycling of surface N (d15N >0‰) in the source of these kimberlites. We conclude that the combination of Sr-Nd-Pb and He-N isotope tracing of fluid inclusions in olivine can provide a robust new approach to address the composition of kimberlite sources and, therefore, the evolution of the deep mantle through time.

How to cite: Giuliani, A., Koornneef, J. M., Barry, P., Will, P., Busemann, H., Maden, C., Maas, R., Greig, A., and Davies, G. R.: A preliminary assessment of the application of Sr, Nd, Pb, He and N isotope analysis to fluid inclusions in kimberlite olivine: A new approach to trace deep-mantle sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5267, https://doi.org/10.5194/egusphere-egu2020-5267, 2020.

EGU2020-16794 | Displays | GMPV1.4 | Highlight

Geochemistry of basic magmatism of Western Antarctic Rift: implications for volatiles storage and recycling in the mantle

Pier Paolo Giacomoni, Carmelo Ferlito, Costanza Bonadiman, Federico Casetta, Luisa Ottolini, Alberto Zanetti, and Massimo Coltorti

The petrologic study of olivine-hosted melt inclusions (MIs) from alkaline primary Cenozoic basalts of Northern Victoria Land (Antarctica) provide new insights on the role of volatiles in the onset of rift-related magmatism. The concentration of volatile species (H2O, CO2, F, Cl) have been determined by Secondary Ion Mass Spectrometry (SIMS) on a selection of MIs which have been previously re-homogenized at high pressure and temperature conditions in order to avoid any heterogeneity and reducing the H diffusion. The least differentiated MIs vary in composition from basanitic to alkaline basalts, analogously to what is found in McMurdo volcanics, while their volatile concentrations reach up to 2.64 wt% H2O, 3900 ppm CO2, 1377 ppm F and 1336 Cl. Taking into account the most undegassed MIs a H2O/(H2O+CO2) ratio equal to 0.88 was determined, which in turn brings the CO2 content in the basanitic melt with the highest water content up to 8800 ppm.

Major and trace element melting modelling indicate that basanite and alkali basalt composition can be reproduced by 3 and 7% of partial melting of an amphibole-bearing spinel lherzolite respectively. Assuming a perfect incompatible behavior for H2O and CO2 these melting proportions allow to constrain the water and CO2 contents in the mantle source in the range 780-840 and 264-273 ppm respectively. The resulting CO2/Nb, CO2/Ba and H2O/Ce ratio are lower than those estimated for Depleted MORB Mantle (DMM), suggesting that the NVL Cenozoic alkaline magmatism could be originated by an enriched mantle source composed by a range from 70% to 60% of Enriched Mantle (EM) and from 30% to 40% of Depleted Morb Mantle (DMM).

A global comparison of fluid-related, highly incompatible and immobile/low incompatible elements such as Li, K, Cl, Ba, Nb, Dy and Yb allow to put forward that the prolonged (~500 to 100 Ma) Ross subduction event played a fundamental role in  providing the volatile budget into the lithospheric mantle before the onset of the Cenozoic continental rifting.

How to cite: Giacomoni, P. P., Ferlito, C., Bonadiman, C., Casetta, F., Ottolini, L., Zanetti, A., and Coltorti, M.: Geochemistry of basic magmatism of Western Antarctic Rift: implications for volatiles storage and recycling in the mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16794, https://doi.org/10.5194/egusphere-egu2020-16794, 2020.

Magnetoplumbite (yimengite-hawthorneite, HAWYIM), crichtonite (lindsleyite-mathiasite, LIMA) and hollandite (priderite) minerals are exotic titanate phases, which formed during metasomatism at the conditions of high alkali activity, especially K, in the fluids in the upper mantle peridotites. The paper presents data on experiments on formation of K-end-members priderite, yimengite and mathiasite, as the result of the interaction of chromite, chromite+rutile and chromite+ilmenite assemblages in the presence of a small amount of silicate material with H2O-CO2-K2CO3 fluids at 3.5 and 5 GPa and 1200°C. The experiments demonstrated the principal possibility of the formation of the titanates in the reactions of chromite with alkaline aqueous-carbonic fluids and melts. However, the formation of these phases does not proceed directly on chromite, but requires additional titanium source. The relationship between titanates is found to be a function of the activity of the potassium component in the fluid/melt. Priderite is an indicator of the highest potassium activity in the mineral-forming medium. Titanates in the run products are constantly associated with phlogopite. Experiments prove that the formation of titanates manifests the most advanced or repeated stages of metasomatism in mantle peridotites. Association of titanates with phlogopite characterizes a higher activity of the potassium component in the fluid/melt than the formation of phlogopite alone. The examples from natural associations, reviewed in the paper, well illustrate these conclusions. Experiments revealed the following features of crystallization of these phases and allowed interpretation of the titanate associations in metasomatized mantle peridotites.

(1) The principal possibility of the formation of minerals of crichtonite and magnetoplumbite groups and priderite in the reactions of chromite with alkaline aqueous-carbonic fluids and melts is confirmed. Such substances are considered as main agents of potassium metasomatism, leading to the formation of titanates in the upper mantle (Konzett et al., 2013; Rezvukhin et al., 2018).

(2) The formation of these phases does not proceed directly on chromite (e.g. Haggerty et al. 1983; Haggerty, 1983; Nixon, Condliffe, 1989), and requires additional titanium source. They are rutile and ilmenite, which are themselves usually are products of modal metasomatism of peridotites. This experimental fact demonstrates that the formation of titanates marks probably the most advanced or repeated stages of metasomatism in mantle peridotites.

(3) This is also proved by the relationships of titanates with phlogopite. Association of titanates with phlogopite characterized by a higher activity of the potassium component in the fluid/melt than the formation of phlogopite alone. Such conditions can again be created at the most advanced or repeated stages of mantle metasomatism.

(4) The relationship between titanates is also a function of the activity of the potassium component in the fluid/melt. Priderite is an indicator of the highest potassium activity in the mineral-forming medium. The above examples from natural associations (Zhou, 1986; Konzett et al., 2013; Almeida et al., 2014) well illustrate this conclusion.

How to cite: Vorobey, S., Butvina, V., and Safonov, O.: Syntheses of rare K-titanates (yimengite, mathiasite and priderite) at high TP conditions: application to modal mantle metasomatism., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-203, https://doi.org/10.5194/egusphere-egu2020-203, 2020.

EGU2020-17010 | Displays | GMPV1.4

Geochemistry of noble gases and CO2 in mantle xenoliths and arc lavas from central Mexico

Andres Libardo Sandoval-Velasquez, Alessandro Aiuppa, Andrea Rizzo, Maria Luce Frezzotti, Susanne Straub, Arturo Gomez-Tuena, and Ramon Espinasa-Perena

The Ventura Espiritu Santo Volcanic Field (VESVF) and the Sierra Chichinautzin (SCN) are two monogenetic volcanic fields originated in different tectonic environments in the central portion of Mexico (continental rift and subduction). The VESVF is located 35 km NE of the city of San Luis Potosí in the south of the Basin and Range extensional province. This volcanic field was formed by the eruption of alkaline magmas of mafic composition transporting mantle xenoliths described as spinel lherzolites and pyroxenites (Luhr et al., 1989; Aranda -Gómez and Luhr, 1996). The SCN is a Quaternary volcanic field located in the Trans-Mexican Volcanic Belt (TMVB) between two Quaternary arc-volcanoes (Popocatepetl and Nevado de Toluca[AR1] ). Some authors believe that its origin has been related to the subduction of the Cocos plate beneath the North American plate (Marquez et al., 1999; Meriggi et al., 2008); however, the basalts present in the SCN are geochemically similar to OIBs.

New isotopic data of noble gases and CO2 in fluid inclusions from the VESVF and SCN are presented in this work, since these two areas offer a great opportunity to study the local lithospheric mantle features and related processes (e.g., metasomatism, partial melting) occurring beneath Mexico. Twelve fresh xenoliths from the VESVF and two aliquots of olivine phenocrysts of andesites from SCN were selected. Based on the petrographic analysis, it was determined that the set of xenoliths exhibit same paragenesis: Ol> Opx>> Cpx> Spinel; all samples are plagioclase-free and are classified as spinel-lherzolites and harzburgites. Both the boundaries and the fractures of the crystals develop veins composed of yellowish glass and tiny crystals of carbonates. Lavas from SCVF were previously described as olivine andesites mainly aphanitic and porphyritic with few (<10%) phenocrysts of olivine and orthopyroxene (Marquez et al., 1999; Straub et al., 2011).

The mantle xenoliths and the olivine phenocrysts have comparable Rc/Ra values (where Rc/Ra is the 3He/4He corrected for air contamination and normalized to air He). We find Rc/Ra compositions of 6.9-7.7 and 7.2-7.3, respectively, which are within the MORB-like upper-mantle range (Graham, 2002). The highest CO2 concentrations are observed in olivine phenocrysts from SCN (9.2·10-7 mol/g and 1.3·10-6 mol/g), while the xenoliths cover a wide range of concentrations with values as high as 3.9·10-7 mol/g in Cpx. The isotopic composition of CO2 (d13C vs PDB) in the olivine phenocrysts is around -6.2‰ with CO2/3He ratios of 3.3·109, which are comparable to MORB-like range (-8‰<d13C<-4‰); the mantle xenoliths in contrast, although displaying similar CO2/3He ratios (2.8·109), exhibit more positive d13C signature between -1.0 and -2.7%. We propose that these differences testify for isotopic heterogeneity in the mantle beneath the two areas, with and reflect mantle metasomatism underneath VESVF driven by interaction with carbonate rich-melts (likely consequence of carbonate recycling during the subduction process), as also evidenced by the petrographic analysis.

How to cite: Sandoval-Velasquez, A. L., Aiuppa, A., Rizzo, A., Frezzotti, M. L., Straub, S., Gomez-Tuena, A., and Espinasa-Perena, R.: Geochemistry of noble gases and CO2 in mantle xenoliths and arc lavas from central Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17010, https://doi.org/10.5194/egusphere-egu2020-17010, 2020.

EGU2020-15965 | Displays | GMPV1.4

High volumes of mineral dissolution by localized fluid pulses in UHPM metasediments of Lago di Cignana, Western Alps

Hugo van Schrojenstein Lantman, David Wallis, Marco Scambelluri, and Matteo Alvaro

Fluids play key roles in many geological processes across wide ranges of spatial and temporal scales. A major challenge in establishing the impacts of fluids is that partial replacement of minerals by dissolution-precipitation produces gaps in the rock record. Finding the records of such processes can help in understanding and reconstructing the processes of fluid flow, mineral dissolution and related volume changes.

The ultra-high pressure metamorphic (UHPM) Lago di Cignana Unit (Zermatt-Saas Zone, Western Alps) has been intensively studied because it is a piece of exhumed coesite- and diamond-bearing oceanic lithosphere. In this unit, schistose quartzite hosts coesite-bearing garnet and contains lenses of garnetite, which previously have been attributed to local bulk-compositional differences. Almost the entire quartzite consists of a retrograde mineral assemblage, and therefore processes occurring during subduction are best recorded in garnet.

A combined microstructural and petrological study of the garnetite lenses and their host rock reveals evidence for compaction by dissolution during subduction, partially driven by intergranular pressure solution. As the host rock matrix is removed, garnet is preferentially preserved and concentrated into garnetite. Garnet-garnet contacts then result in pressure solution and grain boundary migration. Different garnet densities and microstructures in the garnetite, alongside dissolution-reprecipitation structures in host rock garnet, suggest a complex process driven by fluid pulses. Linking garnet composition and structures to P-T through barometry on inclusions reveals an evolving fluid pathway during prograde to peak metamorphism, resulting in significant mass removal by pressure solution in metasediments subducted to UHPM conditions.

The occurrence of pressure solution and mass removal at UHPM conditions in combination with the large amounts of fluids produced by slab dehydration suggests that dissolution may play a significant role in metasediments during subduction.

 

Acknowledgements: This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro)

How to cite: van Schrojenstein Lantman, H., Wallis, D., Scambelluri, M., and Alvaro, M.: High volumes of mineral dissolution by localized fluid pulses in UHPM metasediments of Lago di Cignana, Western Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15965, https://doi.org/10.5194/egusphere-egu2020-15965, 2020.

EGU2020-2674 | Displays | GMPV1.4 | Highlight

How to disclose local equilibrium in subducted metapelites from the Cima Lunga Unit (Central Alps)

Francesca Piccoli, Pierre Lanari, Jörg Hermann, and Thomas Pettke

Subducted metapelites are more prone to re-equilibrate during exhumation than mafic or ultramafic rocks to the point that recognizing high-pressure (HP) relicts is often very challenging. Geologic evidence from the Cima Lunga Unit (Central Alps) show this apparent discrepancy between high to ultra-high pressure metamorphism (28 kbar and 780 °C) recorded in mafic/ultramafic lenses, and Barrovian metamorphism (<10 kbar, 650°C) in the adjacent metapelitic rocks. We collected a white mica – garnet – biotite – plagioclase – kyanite (+ quartz, + zircon, + rutile) bearing metapelite adjacent to the garnet metaperidotite lens that displays an apparently well equilibrated Barrovian mineral assemblage (garnet + plagioclase + biotite), with no macroscopic or microtextural indication of a HP and/or HT metamorphic event (e.g. omphacite crystals; migmatitic texture; polyphase inclusions). Nevertheless, microstructures like atoll-like garnet or large white mica flakes surrounded by biotite and ilmenite replacing rutile suggest incomplete re-equilibration. We investigated garnet and phengite crystals by electron probe and laser ablation-ICP-MS mapping. Major and trace element mapping reveals very complex mineral zoning in both minerals. In particular, high Ti content in phengite and increasing P and Zr contents in pyrope-rich garnet indicate that the studied rock underwent a HP-HT event. This is also supported by Zr in rutile thermometry that indicates temperatures well above the Barrovian metamorphism (T > 700 °C). We combined detailed textural analysis with petrological-geochemical data and thermodynamic modelling to reconstruct the metamorphic evolution of the studied rock. We show that, thank to incomplete re-equilibration, the rock documents an evolution from prograde to UHP-HT peak (27 kbar and 800 °C) to retrograde (Barrovian) conditions (10 kbar and 620 °C). Noteworthy, peak metamorphic conditions of metapelite coincide with peak metamorphic conditions of the garnet metaperidotite. Lastly, geochemical evidence for minor wet melting of the studied metapelite at HP-HT conditions was recognized and is likely linked to the dehydration of chlorite to form garnet peridotite in the adjacent ultramafic body. We propose that metapelites and ultramafic rocks were coupled before subduction or at least in its early stage. This finding opens new scenarios for the geodynamic interpretation of the Cima Lunga unit. We propose that the ultramafic lenses at Cima di Gagnone were parts of the exhumed and serpentinised mantle emplaced at the hyper-extended European continental margin of the Piemont-Ligurian ocean. Slices of the margin were detached and tectonically mixed in the subduction channel. These new constraints call for re-evaluation of the paleogeographic position of the Adula-Cima Lunga nappe.

How to cite: Piccoli, F., Lanari, P., Hermann, J., and Pettke, T.: How to disclose local equilibrium in subducted metapelites from the Cima Lunga Unit (Central Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2674, https://doi.org/10.5194/egusphere-egu2020-2674, 2020.

EGU2020-9085 | Displays | GMPV1.4

Strange polymorphs and where to find them: a melt inclusion story

Silvio Ferrero, Ross J. Angel, Alessia Borghini, Iris Wannhoff, Rico Fuchs, Mahyra Tedeschi, Omar Gianola, Patrick J. O'Brien, Bernd Wunder, and Martin A. Ziemann

Small portions of pristine melt with diameters of 2 to 50µm are increasingly recognized as a rather common occurrence in high grade metamorphic terranes which experienced melting. Their study delivers crucial chemical information on partial melts at depth. But they are also unique "natural experimental charges" where the behaviour of the silicate melt can be investigated, directly in the natural rocks, under P-T-t conditions which cannot be completely reproduced in the laboratory.

Each nanogranitoid case study has consistently shown H2O-bearing, silica and alkali-rich melt. However, rather than a classic granitoid assemblage consisting mainly of quartz and feldspar(s), on cooling these isolated melt droplets produce a plethora of mineral phases identified via microRaman spectroscopy that are rarely –or never- observed as rock-forming minerals. Cristobalite (tetragonal) and tridymite (orthorhombic) are often present as SiO2 polymorphs, and hexagonal kokchetavite as a polymorph of KAlSi3O8. NaAlSi3O8 occurs as orthorhombic kumdykolite, whereas CaAl2Si2O8 may occur either as monoclinic svyatoslavite or trigonal dmisteinbergite. Two presently unidentified phases have been also recognized via Raman and analysed via electron microprobe. One has the main peak at 426-430 cm-1 and has the composition of a granitic glass, whereas the second has a main peak at 412 cm-1 and a variable composition depending on the inclusion in which it occurs. As their main peaks occur in the same region of most tectosilicates, it is likely that they are two new polymorphs of feldspar, to the best of our knowledge never reported before. These polymorphs have been so far identified in inclusions mainly hosted in garnet, zircon and, in one case, sapphirine and trapped under an extremely variable range of metamorphic conditions (from low P migmatites to UHP eclogites) in very different rock types (metagranitoids, metasediments, mafic and ultramafic rocks).

Microstructures confirm that all of these phases crystallize directly from the trapped melt on cooling, independently of the internal P of the inclusions or the original conditions of melt entrapment. They appear to be the result of metastability in the inclusions, possibly during rapid crystallization of a melt, not caused by rapid cooling but by the peculiar undercooled and supersaturated conditions achieved on cooling by a melt confined in a small cavity (Ferrero & Angel, 2018). According to this possibility, these polymorphs can be regarded as kinetically stabilized, yet possibly thermodynamically metastable, phases as recently proposed by Zolotarev et al. (2019) for dmisteinbergite. A preliminary crystallization experiment on a haplogranitic melt at undercooled conditions however failed to reproduce such phases. Another possibility is that under natural cooling the confined inclusions experience underpressurization, and the system (i.e. the trapped melt) reacts crystallizing phases, i.e. the polymorphs, less dense than their common counterparts. This would result in the decreasing of the P gradient between inclusions and surrounding rock, equivalent to reducing the free energy of the system.

References

Ferrero, S. & Angel, R. 2018. JPet 59, 1671–1700.

Zolotarev, A.A. et al. 2019. Minerals  9, 570.

How to cite: Ferrero, S., Angel, R. J., Borghini, A., Wannhoff, I., Fuchs, R., Tedeschi, M., Gianola, O., O'Brien, P. J., Wunder, B., and Ziemann, M. A.: Strange polymorphs and where to find them: a melt inclusion story , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9085, https://doi.org/10.5194/egusphere-egu2020-9085, 2020.

Over the recent years, Raman elastic barometry has been developed as an additional method to calculate metamorphic conditions in natural systems. A major advantage of Raman elastic barometry is that it does not depend on thermodynamic databases and classic geobarometry methods but relies on mechanical calculations. As a consequence, Raman elastic barometry offers an independent method for estimating the pressure conditions that prevailed at the time of entrapment of minerals during growth of their hosts.

The difference between the pressure calculated using elastic geobarometry and that calculated by phase equilibria methods has recently been employed to estimate the extent of metamorphic reaction overstepping in natural systems. Quantification of the latter however implicitly assumes that the rheology of the inclusion-host system is perfectly elastic. This assumption may not hold at high temperatures, where viscous creep of minerals takes place.

The amount of viscous relaxation of a host-inclusion system is a path-dependent quantity which mostly depends on the temperature-time (T-t) path followed. Here, we present examples of visco-elastic relaxation of mineral inclusions and calculate the apparent reaction overstepping which results by assuming that the mechanical system is purely elastic. Our modelling shows that host-inclusion systems that experienced large peak temperatures for long periods of time will retain inclusion residual pressures that cannot be simply related to the growth of their hosts and should therefore not be used for reaction overstepping calculations.

How to cite: Moulas, E., Zhong, X., and Tajcmanova, L.: Viscous relaxation of mineral Inclusions and its implications for reaction overstepping calculations in metamorphic rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7568, https://doi.org/10.5194/egusphere-egu2020-7568, 2020.

EGU2020-4143 | Displays | GMPV1.4 | Highlight

Quartz inclusions in garnet from high-temperature metamorphic rocks change their shape

Bernardo Cesare, Matteo Parisatto, Lucia Mancini, Luca Peruzzo, Matteo Franceschi, Tommaso Tacchetto, Steven M. Reddy, Richard Spiess, and Federica Marone Welford

Trapped and sheltered inside other crystals, mineral inclusions preserve fundamental and otherwise lost information on the geological history of our planet. In the last decade, quartz inclusions in garnet have become a fundamental tool to estimate pressure and temperature of metamorphic rocks at the time of inclusion entrapment. In these approaches, as well as in all other applications, inclusions are regarded as immutable objects and the possibility of a change in their shape has never been considered.

With a detailed characterization of samples from greenschist and granulite facies, performed by optical and electron microscopy, EBSD, X-ray tomographic microscopy, laser Raman spectroscopy and FIB serial slicing, we show that after being trapped with irregular (“scalloped”) shape in low-temperature rocks, quartz inclusions in garnet from granulites formed at 750-900 °C and various pressures acquired a polyhedral “negative crystal” shape imposed by the host garnet, and almost exclusively defined by the facets of dodecahedron and icositetrahedron. A similar behaviour is also observed in biotite inclusions. The 3-fold and 4-fold morphological symmetry axes of the polyhedral negative crystals are parallel to corresponding crystallographic axes in the host garnet.

The systematic presence of a fluid film at the quartz-garnet boundary is not supported by Raman and FIB investigation.

Strengthened by microstructures indicating the process of “necking down” of polycrystalline quartz inclusions, our data support that - like in fluid inclusions changing shape to negative crystals - shape maturation of mineral inclusions occurs by temperature-assisted dissolution-precipitation via grain boundary diffusion. This process tends to minimize the surface free energy of the host-inclusion system by forming energetically favored facets and by decreasing the inclusion surface/volume and aspect ratios.

Optical investigation of numerous samples of worldwide provenance suggests that the negative crystal shape of quartz inclusions in garnet from granulites is a widespread microstructure that underpins a systematic phenomenon so far overlooked.

How to cite: Cesare, B., Parisatto, M., Mancini, L., Peruzzo, L., Franceschi, M., Tacchetto, T., Reddy, S. M., Spiess, R., and Marone Welford, F.: Quartz inclusions in garnet from high-temperature metamorphic rocks change their shape, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4143, https://doi.org/10.5194/egusphere-egu2020-4143, 2020.

With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that 40Ar/39Ar geochronology using this mineral, produces cooling ages alone.

Further, we show that high-definition ultra-high-vacuum (UHV) 39Ar diffusion experiments using phengitic white mica routinely allow the extraction of muscovite sub-spectra in the first 10-30% of 39Ar gas release during 40Ar/39Ar geochronology. The muscovite sub-spectrum is distinct and separate to the main spectrum which is dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the cooling history of the eclogite-blueschist belt. Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones.

This study thus illustrates a new method for the quantitative determination of the timing of movement in mylonites and/or in strongly stretched metamorphic tectonites. Shear bands formed in such structures are rarely coarsely crystalline enough to allow mineral grains that can be individually dated using laser spot analysis. Where phengitic white mica is involved, interlaying is usually so fine as to preclude the application of laser methods. In any case, laser methods do not have the capability of extracting exact and detailed age-temperature spectra, and can never achieve the definition of the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments.

Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses. In consequence, we discover a robust correlation between microstructure and age, even down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections.

A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of 39Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. To apply percentage-release formula for the estimation of diffusivity, there must be a sharp rise to the temperature in question, then that temperature must be maintained at a constant value, then dropped sharply to relatively low values.

How to cite: Forster, M., Nie, R., Yeung, S., and Lister, G.: Microstructural shear bands in mylonites dated using muscovite sub-spectra from high-definition ultra-high-vacuum (UHV) argon diffusion experiments with phengitic white mica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1813, https://doi.org/10.5194/egusphere-egu2020-1813, 2020.

EGU2020-2869 | Displays | GMPV1.4 | Highlight

Measuring stress and strain in rocks by spectroscopy

Ross Angel, Mara Murri, Nicola Campomenosi, Boriana Mihailova, Mauro Prencipe, and Matteo Alvaro

Microstructures and the different thermoelastic properties of minerals ensure that no rock is ever under perfect hydrostatic stress at the grain level. If deviatoric stresses and strains significantly modify thermodynamic properties of minerals so that the equilibrium assemblage and compositions are different from that predicted from hydrostatic conditions, it is crucial to be able to measure the stress state of minerals in-situ in rocks. Forty years ago it was considered that ‘Analysis of residual stresses at the scale of mineral grains within a polycrystalline aggregate such as a rock is virtually intractable’ [1]. This is no longer true.

Confocal Raman spectroscopy allows spectra to be collected from small volumes of mineral grains within a section. The positions of Raman peaks depends on the elastic strains in the minerals through the phonon-mode Grüneisen tensors [2]. The development of precise DFT simulations of crystal structures and their Raman spectra now allows the components of the phonon-mode Grüneisen tensors to be calculated [3]. With these tensors it is possible to determine the strains from measured Raman peak positions, to thereby map the strain, and hence the stress state, of individual mineral grains. We have now extended the DFT simulations to show that the Raman shifts of crystals subject to symmetry-breaking stresses (e.g. around inclusions) are, as expected, not solely determined by the phonon-mode Grüneisen tensors of the ideal crystal. We have also recently developed the measurement of the change in peak intensities in cross-polarised Raman spectra to determine the stress [4] in these cases. For minerals such as garnets, this effect is stronger and therefore more sensitive to stress than the shifts in peak positions and offers at the moment the possibility to quickly visualise stress and strain fields in minerals in-situ in rocks. Quantitative stress values from this method await the determination of the piezo-phonon tensors for garnets, but comparison of peak positions and intensities show that the two methods return consistent results.

This work was supported by ERC-StG TRUE DEPTHS grant (number 714936) to M. Alvaro. N. Campomenosi was also supported by the University of Genova.

 

[1] Holzhausen & Johnson (1979) Tectonophysics 58, 237.

[2] Angel et al. (2019) Zeitschrift für Kristallographie, 234, 129.

[3] Murri et al. (2018) American Mineralogist, 103, 1869.

[4] Campomenosi et al. (2020) Contributions to Mineralogy and Petrology, accepted.

How to cite: Angel, R., Murri, M., Campomenosi, N., Mihailova, B., Prencipe, M., and Alvaro, M.: Measuring stress and strain in rocks by spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2869, https://doi.org/10.5194/egusphere-egu2020-2869, 2020.

EGU2020-18344 | Displays | GMPV1.4

Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and Piemontais and Briançonnais Zones

Emmanuelle Ricchi, Edwin Gnos, Daniela Rubatto, and Thomas Pettke

Ion probe 208Pb/232Th fissure monazite ages from high pressure regions of the Western Alps and from the Argentera Massif provide new insights on the tectonic evolution of the Western Alps during Cenozoic times. Fissure monazite is a hydrothermal mineral crystallizing during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Fissure monazite ages directly record chemical disequilibrium occurring in a fissure environment, but growing evidences indicate that fissure monazite commonly register tectonic activity. Fissure monazite age domains from this study show that monazite crystallization occurred between ~32-30.5 Ma and ~31.5-30 Ma in the Piémontais and Briançonnais Zone of the High Pressure regions, and between ~17-15 Ma and in the north-eastern border of the Argentera Massif. So far, monazite ages were recorded between ~32-23 Ma and at ~20.5 Ma in the Briançonnais Zone and in the south-western border of the Argentera Massif respectively. Thus the presented dataset corroborate and complement already reported fissure monazite 208Pb/232Th ages from the Western Alps. This new fissure monazite ages compilation supports that Late Oligocene thrusting affected the High Pressure regions of the Western Alps, and that Early and Middle Miocene dextral strike-slips movements respectively affected the south-western and north-eastern margins of the Argentera Massif. Chemical observations provide new hints on fissure monazite growth conditions (e.g. leached host-rock minerals, oxidation conditions) encouraging to pursue chemical studies with a larger dataset on natural fissure monazite to better understand growth conditions under cleft environment.

How to cite: Ricchi, E., Gnos, E., Rubatto, D., and Pettke, T.: Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and Piemontais and Briançonnais Zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18344, https://doi.org/10.5194/egusphere-egu2020-18344, 2020.

Phlogopite is accepted as a major mineral indicator of the modal metasomatism in the upper mantle within a very wide P-T range and fluid/melt compositions. It extensively forms in mantle peridotites transforming initial harzburgites and lherzolites to phlogopite wehrlites both in garnet and spinel-facies. A reaction 5En + Grt + [K2O + 2H2O in fluid] = Phl + Di (Grt – pyrope-grossular garnet CaMg2Al2Si3O12) is considered as the major mechanism for phlogopite formation in garnet-facies peridotites. This reaction is commonly accompanied by regular compositional changes of primary garnet and pyroxenes. In order to illustrate the regularities, we report result of experimental study of the phlogopite-forming reactions in the model systems pyrope-enstatite, grossular-pyrope-enstatite and knorringite-pyrope-enstatite systems in presence of a H2O-KCl fluid at pressure 3 and 5 GPa and temperatures of 900 and 1000°C. The experiments were aimed at the tracing of variations of grossular and knorringite contents in garnet, as well as Al content of pyroxenes, with variations of the KCl content in the fluid.

The increase of XKCl in the fluid is accompanied by gradual decomposition of garnet and Al-bearing enstatite in all systems. The Al2O3 content in orthopyroxene decreases in the pyrope – enstatite system at 5 GPa and 900°C. In the system enstatite-pyrope-grossular at 5 GPa and 1000°C phlogopite forms at the KCl content 10 mol. % in the fluid. Further increase of the KCl content in the fluid results in gradual disappearance of garnet and orthopyroxene and stronger domination of phlogopite and clinopyroxene. Grossular content in garnet increases with the KCl concetration in the fluid up to 10 mol. %, but further increase of the KCl concentration to 20 mol. % results in decrease of the grossular content in garnet. In the system enstatite-pyrope-knorringite at the KCl content in the fluid 0 – 10 mol. %, garnet contains 8-9 mol. % of knorringite. Cr-bearing phlogopite (about 2 wt. % Cr2O3) appears in this system at 10 mol. % KCl in the fluid, and its formation results in a slight increase of the knorringite content in garnet. Because of relatively high SiO2 bulk content in comparison to the typical peridotite, Cr-bearing kyanite (not spinel) forms at 20 mol. % KCl in the fluid resulting in a decrease of the knorringite content in garnet down to 3-5 mol. %. The Cr2O3 content in the coexisting phlogopite concomitantly decreases by about 1 wt. %.

The above experiments reproduced some characteristic regularities in variations of garnet and pyroxene compositions in the course of phlogopite formation in mantle peridotites. The applicability of the experimental results is illustrated by examples from peridotite xenoliths from kimberlites. These effects can be applied for the quantitative and qualitative estimates of variations in K activity during the modal mantle metasomatism.

How to cite: Limanov, E., Butvina, V., and Safonov, O.: Experimental study of phlogopite-forming reactions in the system orthopyroxene+garnet in presence of the H2O-KCl fluids., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-228, https://doi.org/10.5194/egusphere-egu2020-228, 2020.

EGU2020-1192 | Displays | GMPV1.4

Amphibole lamellae formation in the upper mantle due to interaction of fluid inclusions and host minerals: a case study from Persani Mountains Volcanic Field, Transylvania

Thomas Pieter Lange, Zsófia Pálos, Levente Patkó, Márta Berkesi, Nóra Liptai, László Előd Aradi, Ábel Szabó, Csaba Szabó, and István János Kovács

Amphibole is one of the most abundant ’water’-bearing minerals in the Earth’s upper mantle. Amphiboles occur as interstitial grains, lamellae within pyroxenes or as daughter minerals within fluid inclusions.  Most commonly amphibole formation is related to mantle metasomatism, where the agent has a subducted slab (e.g. Manning 2004) or an asthenospheric origin (e.g. Berkesi et al. 2019).  After the formation of fluid inclusions, a subsolidus interaction can take place where the H2O content of fluid inclusions may crystallize pargasite (e.g. Plank et al. 2016).

Here we present amphibole lamellae formation in mantle xenoliths from the Persani Mountains Volcanic Field that is interrelated to a reaction between fluid inclusions and host clinopyroxene.  Newly formed amphibole lamellae occur only in the surroundings of the fluid inclusions and grow within the host clinopyroxene in a preferred crystallographic direction.  Studied lamellae do not reach the rim of the host mineral implying that components needed for formation of amphibole lamellae in clinopyroxene could have only originated from the fluid inclusion itself.  We measured the major element composition of amphibole lamellae and host clinopyroxene (1) and used Raman spectroscopy and FIB-SEM on fluid inclusion study situated next to the lamellae (2).  Results support the hypothesis that chemical components (dominantly H+) migrated sub-solidus from the fluid inclusion into the host mineral after fluid entrapment via subsolidus interaction.  Beyond the clinopyroxene-hosted fluid inclusions, fluid inclusions in orthopyroxenes were also studied as a reference.  Our study shows that post-entrapment diffusion from a fluid inclusion into the host mineral changes the solid/fluid ratio of the mantle  which could modify the rheology of the lithospheric mantle.

Berkesi, M. et al. 2019. Chemical Geology, 508, 182-196.

Kovács et al. (2017) Acta Geodaetica et Geophysica, 52(2), 183-204.

Manning C. E. 2004. Earth and Planetary Science Letters, 223, 1-16.

Plank, T. A. et al. 2016. In AGU Fall Meeting Abstracts.

How to cite: Lange, T. P., Pálos, Z., Patkó, L., Berkesi, M., Liptai, N., Aradi, L. E., Szabó, Á., Szabó, C., and Kovács, I. J.: Amphibole lamellae formation in the upper mantle due to interaction of fluid inclusions and host minerals: a case study from Persani Mountains Volcanic Field, Transylvania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1192, https://doi.org/10.5194/egusphere-egu2020-1192, 2020.

EGU2020-3047 | Displays | GMPV1.4

Volumetric response of crushed dunite during carbonation reaction under controlled σ-P-T conditions

Jinfeng Liu, Timotheus Wolterbeek, and Christopher Spiers

Naturally, olivine reacts with CO2-rich fluids, producing carbonates and silica. If in completion, this reaction will cause a large increase in the solid volume (~85%), which can generate a significant stress/force when it occurs in a confined space. This may be used to fracture the surrounding rocks in the context of the injection of industrially captured CO2 into peridotites for permanent sequestration. Contrarily, this volume-increasing reaction may also clog transport paths and thus inhibit CO2 access, leading to little or no volumetric increase at industrial time scales. Although observations from natural systems suggest that reaction-induced fracturing during peridotite carbonation can occur, the fracturing mechanism has not been experimentally reproduced under in-situ stress-temperature-chemical conditions. Here, we report 9 flow-through experiments performed on pre-compacted Åheim dunite (containing ~85% olivine) powders (grain size 36-50 µm) during carbonation reaction under controlled σ-P-T conditions. This was done using a purpose-built apparatus, consisting of a flow-through system accommodated with a uniaxial servo-controlled loading system. Before experiments, the dunite powders were compacted stepwise up to 250MPa to form a disc-shape sample with starting porosity of ~25%. The sample was covered by a thin Teflon sleeve plus Vaseline to reduce the friction against the vessel wall. The experiments were performed at a constant temperature of 150℃ and constant (Terzaghi) effective stress of 1, 5, 15MPa, respectively. The sample was first exposed to deionized (DI) water at a pore fluid pressure of 10MPa, and then the DI water was replaced, maintaining constant pore pressure of 10MPa, by flow-through of a certain chemical fluid, such as CO2 saturated brine (containing 1M NaCl plus 0.64M NaHCO3, pH~3), CO2 saturated water (pH~3), NaHCO3 saturated solution (pH~9) and NaHSO4 solution (pH~3). The permeability was measured for all experiments using the flow-through system by means of the steady-state method, and each experiment took 2-4 weeks. The experiments show that the samples exhibited 0-0.37% compaction strain when CO2 saturated brine, CO2 saturated water, and NaHCO3 saturated solution flow through, independently of poroelastic effects, and the sample permeability drops in the order from 10-17 to 10-20 m2. By contrast, for the NaHSO4 flow-through experiment where no carbonation reaction occurred, the sample permeability increased from 2*10-17 to 7*10-17 m2, associated with 0.05% compaction. The sample mass after the NaHSO4 flow-through experiment reduced ~5%, suggesting that magnesium and silica may be partly leached out from the sample. Microstructure observations and XRD analysis on these samples demonstrate a drastic reduction in porosity of the reaction zone where CO2 was integrated into the crystal structure of the product carbonates by means of carbonation reactions. The mechanism responsible for the observed behavior seems to be that the dissolution of olivine that occurred first at the grain contact surface leads to compaction, followed by precipitation of carbonates at porous that clogs the transport paths and thus reduces the permeability, though the detailed chemical analysis is still performing. As a result, our current findings suggest that the volume-increasing precipitation produced via the carbonation reaction under in-situ subsurface conditions will clog transport paths.

How to cite: Liu, J., Wolterbeek, T., and Spiers, C.: Volumetric response of crushed dunite during carbonation reaction under controlled σ-P-T conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3047, https://doi.org/10.5194/egusphere-egu2020-3047, 2020.

In the eclogite facies shear zone of Bottarello, Monte Rosa, Western Alps [1], fault recrystallization around 600 °C gives concordant Lu-Hf (garnet) and 39Ar-40Ar (white mica, WM) 47 Ma ages whereas <100 m from the fault the unsheared rock at the same T preserves Mesozoic inheritance. The Ar retentivity of WM is not accurately predicted by hydrothermal laboratory experiments, because the latter are plagued by massive dissolution artefacts [2]. Independent field observations confirm that WM only starts losing Ar in dry rocks above 600 °C [3-8], but when retrograde reactions occur, WM can recrystallize and be totally reset below 230 °C [9]. The Bottarello fault obliterated all relict WM from the protolith; the neoformed WM records its own formation age.

The island of Naxos (Cyclades, Greece) is the classic example of multiple, coexisting WM generations [10]: relict pre-eclogitic basement WM, and eclogitic phengite retrograded to muscovite. Electron microprobe element maps demonstrate intergrowths at a scale <5 µm, which makes laser microprobe dating useless. Bulk mica dissolution for Rb-Sr gives Eocene ages [11], which agree with bulk K-Ar ages. This is paradoxical, as Ar diffusivity is c. 4 orders of magnitude higher than that of Sr [12]; the only explanation is that both chronometric systems record formation ages around 500-600 °C. The WM generations can be unravelled by their Ca/Cl/K signatures; coarse and fine sieve fractions are never isomineralic. Ages of pure mica generations are obtained by extrapolating Ca/Cl/K-vs-age trends.

The in-sequence thrusts of the Garhwal Himalaya add one complication: thrusting was long-lived. Microstructures combined with chemical microanalysis distinguish three monazite generations (dated by U-Pb) and three WM generations: relicts in microlithons, foliation-defining mica, and static coronas. As in the previous examples, intergrowths are <<10 µm and only combining Ca/Cl/K systematics with the observed differences in structural breakdown temperatures can assign the different WM ages in the same sample to chemically distinct generations [13]. WM formation ages overlap with Mnz ages and date the onset of faulting, the kinematic peak, and the post-faulting corona formation.

There is no free lunch: dating deformation is extremely labor-intensive and requires, always, establishing the context between microtextural, microchemical, petrological and multichronometric analyses. Whenever one of these four is missing, the tectonic reconstruction is invariably faulty [14].

 

[1] Villa &al, J Petrol 55 (2014) 803-830

[2] Villa, Geol Soc London Spec Pub 332 (2010) 1-15

[3] Di Vincenzo &al, J Petrol 45 (2004) 1013-1043

[4] Itaya &al, Island Arc 18 (2009) 293-305

[5] Heri &al, Geol Soc London Spec Pub 378 (2014) 69-78

[6] Laurent &al, Lithos, 272-273 (2017) 315-335

[7] Airaghi &al, J Metam Geol 36 (2018) 933-958

[8] Imayama &al, Geol Soc London Spec Pub 481 (2019) 147-173

[9] Maineri &al, Mineralium Deposita 38 (2003) 67-86

[10 Wijbrans & McDougall, Contrib Min Petr 93 (1986) 187-194

[11] Peillod &al, J Metam Geol 35 (2017) 805-830

[12] Cherniak & Watson, EPSL 113 (1992) 411-425

[13] Montemagni &al, Geol Soc London Spec Pub 481 (2019) 127-146

[14] Bosse & Villa, Gondwana Res 71 (2019) 76-90

How to cite: Villa, I. M.: Dating deformation: multichronometric examples from the Western Alps, Naxos, and the Garhwal Himalaya, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4133, https://doi.org/10.5194/egusphere-egu2020-4133, 2020.

EGU2020-4543 | Displays | GMPV1.4

Melt inclusions in metamorphic rocks: how localized melting promoted the formation of the Gore Mountains mega garnets (Adirondacks, US)

Silvio Ferrero, Iris Wannhoff, Robert Darling, Bernd Wunder, Laurent Oscar, Patrick J. O' Brien, Martin A. Ziemann, and Christina Günter

Melt inclusions have been for almost 150 years an exclusive feature of magmatic rocks. However, intensive research activity in the last decade has shown that melt inclusions, or nanogranitoids, are also a widespread feature of high grade metamorphic rocks. Such inclusions rapidly became fundamental tools to unravel partial melting and melt-related processes taking place during orogenesis.

One of the latest discoveries in this field has been the identification of nanogranitoids and glass inside the mega almandine-pyrope garnets of Barton Mine (Gore Mountain, NY State, US). These crystals are arguably the world’s largest garnets and occur within garnet hornblendite. Their size is ca. 35 cm in average, while garnet diameters up to 1 m were reported in historical record. Fluid is often invoked in the formation of large crystals, but so far no study has identified clear witnesses for the presence of fluid during garnet formation, e.g. primary fluid inclusions.

Polycrystalline inclusions of primary nature were instead reported by Darling et al. (1997) to occur inside the garnet: such inclusions are the main target of our study. Their shape ranges from tubular (2-100 µm in length) to negative crystal shape (2-50 µm). They mainly contain cristobalite/quartz, kumdykolite and amphibole. Minor phases such as biotite/phlogopite, enstatite, rutile, ilmenite and a second, Ca-richer plagioclase (or its rare polymorphs dmisteinbergite and svyatoslavite) may be also present. The inclusions were re-homogenized to a silicate-rich glass via piston cylinder experiments at 1.0-1.5 GPa and 925-940°C. Experimental results prove that such inclusions are former droplets of melt, in agreement with the finding of preserved residual glass in one single inclusion before the experimental runs. The melt composition measured in situ via electron microprobe is tonalitic-trondhjemitic with 5-6 wt% H2O.

The identification of melt inclusions points toward a melt rather than a fluid as the medium which favored extreme garnet growth under low nucleation rate conditions. The elements necessary to grow garnets – mainly Fe, Al, Si, Mg- are indeed far more effectively transported by a silicate melt rather than simple aqueous fluid, at least at the limited depth envisioned for this process. In conclusion, the finding of melt inclusions in metamorphic rocks brought us forward along the path toward the solution of the enigma represented by the formation of these giant garnets.

References
Darling, R.S., Chou, I.M., Bodnar, R.J., 1997. An Occurrence of Metastable Cristobalite in High-Pressure Garnet Granulite. Science 276, 91.

How to cite: Ferrero, S., Wannhoff, I., Darling, R., Wunder, B., Oscar, L., O' Brien, P. J., Ziemann, M. A., and Günter, C.: Melt inclusions in metamorphic rocks: how localized melting promoted the formation of the Gore Mountains mega garnets (Adirondacks, US), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4543, https://doi.org/10.5194/egusphere-egu2020-4543, 2020.

EGU2020-5045 | Displays | GMPV1.4

Beef veins record fluid overpressure during oil primary migration in source rocks

Miao Wang, Yong Chen, and Matthew Steele-MacInnis

Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression, Bohai Bay Basin, East China. Based on the study of vein petrography and fluid inclusions features, we demonstrate the vein was the product of hydrocarbon generation and expulsion from organic-rich shales. Consequently, the primary inclusions in the fibrous calcites recorded the fluid conditions during maturation of these source rocks. In most cases, the calcite-hosted primary inclusion assemblages are composed of the two-phase (oil + gas) hydrocarbon inclusions, with or without coexisting aqueous inclusions. Less commonly, the assemblages are made up of inclusions with only liquid hydrocarbon (i.e., monophase, high-density petroleum inclusions). In addition, many bitumen-bearing oil inclusions could also be observed in the fibrous calcite veins. By modelling the isochores of two-phase oil inclusions and coexisting aqueous inclusions, in light of the burial history for the basin, we conclude the fluid overpressure up to approximately twice (2x) the hydrostatic value (i.e., ~0.5–0.6x lithostatic) are the most common during the hydrocarbon generation and primary migration. The highest degrees of overpressure are recorded by the rare monophase petroleum inclusions. The resulting isochores of these highest density inclusions project to pressures that overlap with the lithostatic gradient. Thus, the monophase inclusions indicate pressures approaching and in some cases exceeding lithostatic. Our results indicate that fluids present during hydrocarbon generation and expulsion in organic-rich shales were indeed overpressured, but that lithostatic pressures were not the norm and evidently not a prerequisite for vein dilation, which means the fluid pressures during dilation of horizontal veins are not necessarily equal to the overburden throughout the history of the opening. This suggests that at least some of the vein dilation is accommodated and offset by concomitant narrowing of the adjacent wall rock laminae, likely by scavenging (dissolution/reprecipitation) of CaCO3 from the adjacent wall rock, owing to the positive pressure dependence of calcite solubility, and presence of organic acids as byproducts of hydrocarbon generation.

How to cite: Wang, M., Chen, Y., and Steele-MacInnis, M.: Beef veins record fluid overpressure during oil primary migration in source rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5045, https://doi.org/10.5194/egusphere-egu2020-5045, 2020.

EGU2020-5725 | Displays | GMPV1.4

Elastic geobarometry of multiphase inclusions

Kira Musiyachenko, Mara Murri, Ross John Angel, Mauro Prencipe, Matteo Alvaro, and Hugo van Schrojenstein Lantman

Elastic geobarometry allows one to recover the PT entrapment conditions of a host-inclusion pair from measurements of the residual pressure of the inclusion which develops upon exhumation due to differences of its thermo-elastic properties from the host (Angel et al., 2015). At the present, calculations assume that the inclusion is a single phase. For a soft inclusion in a stiffer host, the volume change of a free inclusion crystal would be greater than that of the host, which leads to the inclusions being compressed into a smaller volume than expected and thus positive inclusion pressures. Conversely, an inclusion stiffer than the host should develop a negative pressure.

Rutile-in-garnet would be a good candidate for elastic geobarometry because of its common occurrence in high-pressure high-temperature (HP-HT) metamorphic rocks, its simple structure and chemistry and its broad PT stability field. However, recent work by Zaffiro et al. (2019) demonstrated that rutile trapped in garnet should always exhibit negative pressure upon exhumation because rutile is stiffer than garnet, making this pair unsuitable for elastic geobarometry.

Nevertheless, rutile inclusions in garnets from the Pohorje eclogite seem to challenge this thermodynamic prediction. Rutile inclusions show no Raman peak shifts relative to free crystals within the measurement error, despite there being strain birefringence in the garnet host around the rutile which indicates the relaxation of stressed inclusions. High resolution 3D Raman mapping on one of these rutile inclusions revealed the presence of tiny (2-3 µm thick) amphibole crystals located between the garnet and rutile, with the amphibole occupying about 25-30% of the volume of the inclusion. The presence of this amount of amphibole lowers the bulk modulus of the composite inclusion (rutile + amphibole) to less than the bulk modulus of the garnet, hence leading to pressurization of the inclusions upon exhumation. This study shows that careful characterization of host inclusion systems, linked to thermodynamic modelling, is thus necessary to interpret residual pressures (Pinc) in terms of entrapment conditions.

This project has received funding from the European Research Council under the H2020 research and innovation program (N. 714936 TRUE DEPTHS to M. Alvaro).

References:

Angel R.J. et al. 2015. J. Metamorph. Geol., 33(8), 801-813.

Zaffiro G. et al. 2019.  Mineralogical Magazine, 83(3), 339-347.

How to cite: Musiyachenko, K., Murri, M., Angel, R. J., Prencipe, M., Alvaro, M., and van Schrojenstein Lantman, H.: Elastic geobarometry of multiphase inclusions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5725, https://doi.org/10.5194/egusphere-egu2020-5725, 2020.

EGU2020-6060 | Displays | GMPV1.4 | Highlight

Low-temperature thermochronology of fault zones

Takahiro Tagami

Thermal signatures as well as timing of fault motions can be constrained by thermochronological analyses of fault-zone rocks (e.g., Tagami, 2012, 2019).  Fault-zone materials suitable for such analyses are produced by tectocic and geochemical processes, such as (1) mechanical fragmentation of host rocks, grain-size reduction of fragments and recrystallization of grains to form mica and clay minerals, (2) secondary heating/melting of host rocks by frictional fault motions, and (3) mineral vein formation as a consequence of fluid advection associated with fault motions.  The geothermal structure of fault zones are primarily controlled by the following three factors: (a) regional geothermal structure around the fault zone that reflect background thermo-tectonic history of studied province, (b) frictional heating of wall rocks by fault motions and resultant heat transfer into surrounding rocks, and (c) thermal influences by hot fluid advection in and around the fault zone.  Geochronological/thermochronological methods widely applied in fault zones are K-Ar (40Ar/39Ar), fission-track (FT), and U-Th methods.  In addition, (U-Th)/He, OSL, TL and ESR methods are applied in some fault zones, in order to extract temporal information related to low temperature and/or recent fault activities.  Here I briefly review the thermal sensitivity of individual thermochronological systems, which basically controls the response of each method against faulting processes.  Then, the thermal sensitivity of FTs is highlighted, with a particular focus on the thermal processes characteristic to fault zones, i.e., flash and hydrothermal heating.  On these basis, representative examples as well as key issues, including sampling strategy, are presented to make thermochronological analysis of fault-zone materials, such as fault gouges, pseudotachylytes and mylonites, along with geological, geomorphological and seismological implications.  Finally, the thermochronological analyses of the Nojima fault are overviewed, as an example of multidisciplinary investigations of an active seismogenic fault system.

 

References:

  1. Tagami, 2012. Thermochronological investigation of fault zones. Tectonophys., 538-540, 67-85, doi:10.1016/j.tecto.2012.01.032.
  2. Tagami, 2019. Application of fission track thermochronology to analyze fault zone activity. Eds. M. G. Malusa, P. G. Fitzgerald, Fission track thermochronology and its application to geology, 393pp, 221-233, doi: 10.1007/978-3-319-89421-8_12.

How to cite: Tagami, T.: Low-temperature thermochronology of fault zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6060, https://doi.org/10.5194/egusphere-egu2020-6060, 2020.

EGU2020-6962 | Displays | GMPV1.4

First-principle partitioning and disequilibrium of chromium in garnet – clinopyroxene assemblage

Benoit Dubacq, Sarah Figowy, Yves Noël, and Philippe D'Arco

Using partition coefficients is extremely useful to model melting processes and fluid-rock interactions. However, partition coefficients values remain scarce in regard of their sensitivity to mineral composition and to the variability of mineral composition. In addition, the inferred equilibrium between phases is not necessarily reached, even in high-grade metamorphic conditions associated to melting. Disequilibrium may dramatically hamper the effective mobility of species and lead to element distribution far from the predicted values.

This contribution aims at estimating partition coefficients for chromium (Cr) between garnet and clinopyroxene, and testing them in natural rocks of various metamorphic grades. As a poorly mobile trivalent element, Cr is chosen as a proxy to rare earth elements.

Theoretical partition coefficients for Cr between garnet and clinopyroxene are calculated ab initio from structures where Cr3+ is modelled as a defect in Al3+ sites using CRYSTAL17 (Dovesi et al., 2014) and the thermodynamic description of Dubacq and Plunder (2018). Results are compared to electron microprobe measurements in mineral assemblages containing tens to thousands of ppm of Cr, where element mapping brings much information.

Results of ab initio computations highlight the role of crystal-chemistry over the strain field around point defects, controlling the dynamics of the Cr3+ = Al3+ exchange between clinopyroxene and garnet. As expected, the partitioning of Cr between garnet and clinopyroxene depends strongly on the grossular and pyrope content: Cr incorporates grossular preferentially to jadeite, but jadeite incorporates Cr preferentially to pyrope.

Comparison between predicted and measured partition coefficients allowed to estimate the deviation from equilibrium. Disequilibrium is evidenced even for samples metamorphosed around 850°C, as shown by the distribution of Cr-rich and Cr-depleted domains. Disequilibrium is attributed to slow diffusivity of Cr in fluid and at grain boundaries during crystal growth, leading to interface-coupled dissolution-precipitation.

Dovesi, R., Orlando, R., Erba, A., Zicovich‐Wilson, C. M., Civalleri, B., Casassa, S., ... & Noël, Y. (2014). CRYSTAL14: A program for the ab initio investigation of crystalline solids. International Journal of Quantum Chemistry114(19), 1287-1317.

Dubacq, B., & Plunder, A. (2018). Controls on trace element distribution in oxides and silicates. Journal of Petrology59(2), 233-256.

How to cite: Dubacq, B., Figowy, S., Noël, Y., and D'Arco, P.: First-principle partitioning and disequilibrium of chromium in garnet – clinopyroxene assemblage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6962, https://doi.org/10.5194/egusphere-egu2020-6962, 2020.

EGU2020-7963 | Displays | GMPV1.4

How trustworthy are absolute age data from reprecipitated mineral grains?

Matthias Konrad-Schmolke and Ralf Halama

Absolute dating of rock deformation is often hampered by the observation that the affected minerals are only partly re-equilibrated with respect to their isotopic composition.

Generally, three different processes enable mineral grains to adjust isotopically during the deformation event, i.e. volume diffusion, recrystallisation as well as dissolution and reprecipitation.

The degree to which the crystal structure is affected by these processes is different and thus the extent of isotopic equilibration during these processes generally differs in a way that diffusive element exchange is believed to be the most ineffective and slowest process, whereas re- and neo-crystallization seem to be fast and thorough.

Fluid-induced dissolution and reprecipitation is a very common mineral reaction mechanism in the solid Earth and as the crystal lattice is intensively reworked during this process, elemental and isotopic exchange between matrix and the newly formed crystal should be facilitated.   

Commonly, element and isotopic exchange during such mineral reactions is thought to occur via aqueous solutions, but new experimental as well as natural data show that the element transfer during mineral dissolution and reprecipitation can also occur in an amorphous material that forms directly by depolymerization of the crystal lattice.

Furthermore, precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface, hence the entire element and isotopic transfer between reactant and product mineral might not involve equilibration with the intercrystalline transport medium with important consequences for the interpretation of age data from re- and neo-crystallized grains.

How to cite: Konrad-Schmolke, M. and Halama, R.: How trustworthy are absolute age data from reprecipitated mineral grains?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7963, https://doi.org/10.5194/egusphere-egu2020-7963, 2020.

EGU2020-9268 | Displays | GMPV1.4

Olivine dis-equilibrium growth in the Val Malenco contact aureole (Northern Italy), sparks before the fireworks?

José Alberto Padrón-Navarta, Maxime Clément, and Andréa Tommasi

Dehydration reactions are metamorphic reactions that release water (aqueous fluids). They are crucial for the dynamics and chemical recycling in subduction zones. The dehydration of serpentinites is of particular importance as it represents the main source of volatiles in subduction zones and it occurs in a narrow temperature interval. It is generally assumed that dehydration reactions proceed at near-equilibrium. In such situation, the pressure of the fluid produced by the reaction approaches the lithostatic pressure. Deviations from the equilibrium model have been invoked for antigorite dehydration based on natural observations of anisotropic olivine grain growth at high-pressure conditions (1.5-2.0 GPa) [1,2]. Here we show another occurrence of texturally non-equilibrated olivine growth at low pressure (0.4 GPa) in olivine-talc veins crosscutting serpentinites in the Bergell intrusion contact aureole at Alpe Zocca (Malenco Unit, Northern Italy) [3].

The dehydration reactions, which resulted in the replacement of serpentinites by olivine-talc metaperidotites, occurred under quasi-static conditions. The main reaction front, which defines the equilibrium ol + tlc isograd, is a 150 m wide zone outcropping at ~750 m from the Bergell intrusion. In the metaperidotites, olivine has a crystallographic preferred orientation (CPO) correlated with the precursor antigorite CPO, with [010]Ol axes parallel to [001]Atg. These CPOs are accompanied by shape-preferred orientations (SPO) that mark the foliation in both rock types. Talc crystals are also oriented and often bent around olivine crystals, suggesting local compaction. We interpret the foliated metaperidotites as formed at near equilibrium conditions, with pervasive fluid extraction from the metaperidotite by viscous metamorphic compaction.

However, downstream (<100 m) of the equilibrium dehydration reaction front, ol + tlc bearing veins with variable width and shapes are common. These veins are often surrounded by centimeter- to decimeter-scale dehydration reaction zones that propagate into the serpentinite wall-rock. Olivine crystals in dehydration veins also have a strong SPO and CPO that define a jackstraw texture within the plane of the vein. They are elongated (reaching up to 50 cm) parallel to [001] within the vein plane and have their [010] axes normal to the plane of the vein. We interpret the olivine–talc assemblage in the veins and in the nearby reaction zones as resulting from premature dehydration reactions at lower temperature than the equilibrium conditions owing to effective fluid extraction from the wall-rock into the veins. The jackstraw texture indicates fast kinetics, with the crystal orientation controlled by anisotropic growth under a fluid pressure gradient. These textures record local displacement of the reactions towards lower temperature conditions owing to the formation of extensional veins, which acted as high permeability channels allowing for effective draining of the system. These brittle ‘sparkles’ during or before the massive pervasive dehydration seem to have little impact on the overall reaction. 

[1] Padrón-Navarta et al. (2011). Journal of Petrology 52, 2047-2078. [2] Dilissen et al. (2018). Lithos 320-321, 470-489. [3] Clément, M., Padrόn-Navarta, J. A. & Tommasi, A. (2019). Interplay between fluid extraction mechanisms and antigorite dehydration reactions (Val Malenco, Italian Alps). Journal of Petrology, in press.

How to cite: Padrón-Navarta, J. A., Clément, M., and Tommasi, A.: Olivine dis-equilibrium growth in the Val Malenco contact aureole (Northern Italy), sparks before the fireworks?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9268, https://doi.org/10.5194/egusphere-egu2020-9268, 2020.

EGU2020-10711 | Displays | GMPV1.4

Fossil Subduction Recorded By Quartz From The Coesite Stability Field

Matteo Alvaro, Mattia L. Mazzucchelli, Ross J. Angel, Mara Murri, Nicola Campomenosi, Marco Scambelluri, Fabrizio Nestola, Andrey Korsakov, Anatoly Tomilenko, Federica Marone, Marta Morana, and Frederico Alabarse

Investigation of mantle xenoliths can provide information on the architecture and evolution of subcontinental lithospheric mantle through time. These reconstructions rely also on correct estimates of the pressures and temperatures (P-T) experienced by these rocks over time. Unlike chemical geothermobarometers, elastic geobarometry does not rely on chemical equilibrium between minerals, so it has the potential to provide information on over-stepping of reaction boundaries and to identify other examples of non-equilibrium behaviour in rocks. Here we introduce a method that exploits the elastic anisotropy of minerals to determine the unique P and T of equilibration from a measurements of single-crystal mineral inclusions trapped in a crystalline host from an eclogite xenolith [1]. We apply it to perfectly preserved quartz inclusions in garnet from eclogite xenoliths in kimberlites. We show that the elastic strains of inclusions calculated from in-house Raman spectroscopy measurements of the inclusions are in perfect agreement with those determined from in-situ X-ray diffraction measurements performed both in-house and at the synchrotron. Calculations based on these measured strains demonstrate that quartz trapped in garnet can be preserved even when the rock passes into the stability field of coesite (high pressure and temperature polymorph of quartz). This supports a metamorphic origin for these xenoliths that provides constraints on mechanisms of craton accretion from a subducted crustal protolith. Furthermore, we show that some key inclusion minerals do not always indicate the P and T attained during subduction and metamorphism.

 

This project has received funding from the European Research Council under the H2020 research and innovation programme (N. 714936 TRUE DEPTHS to M. Alvaro)

 

[1] M Alvaro, ML Mazzucchelli, RJ Angel, M Murri, N Campomenosi, M Scambelluri, F Nestola, A Korsakov, AA Tomilenko, F Marone, M Morana (2020) Fossil subduction recorded by quartz from the coesite stability field, Geology, 48, 24-28

How to cite: Alvaro, M., Mazzucchelli, M. L., Angel, R. J., Murri, M., Campomenosi, N., Scambelluri, M., Nestola, F., Korsakov, A., Tomilenko, A., Marone, F., Morana, M., and Alabarse, F.: Fossil Subduction Recorded By Quartz From The Coesite Stability Field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10711, https://doi.org/10.5194/egusphere-egu2020-10711, 2020.

EGU2020-12611 | Displays | GMPV1.4

Melt inclusions in olivines from phoscorites and olivinites of the Kovdor massif.

Anna Redina, Cora Wohlgemuth-Ueberwasser, Julia Mikhailova, and Gregory Ivanyuk

The Kovdor massif is a part of the Paleozoic Kola alkaline province and located in the eastern part of the Baltic Shield. Kovdor carbonatites host a unique complex baddeleyite-apatite-magnetite deposit from which iron ores and zirconium have been mined. New data on melt inclusions in olivine crystals from phoscorites and olivinites of the ore complex are presented in this contribution. Daughter minerals in crystallized melt inclusions were identified by Raman spectroscopy and scanning electron microscopy. The trace element composition of inclusions was determined using LA-ICP-MS.

Melt inclusions in olivine from Kovdor phoscorites are negative crystal or round in shape, with sizes ranging from 5 to 50 microns. They form groups or line up. According to the mineral composition, two types of melt inclusions can be distinguished: carbonate and silicate-carbonate. In the first type, Ca-Na-Mg- (Sr?) - REE carbonates are dominant among daughter phases. In the second one, silicate phases (phlogopite, monticellite, diopside), Ca-Na-Mg carbonates and magnetite are found together. Melt inclusions in olivine from olivinites are isometric or elongated, 5–25 μm in size. They form groups or occur as isolated inclusions. Benstoneite, geylussit, ankerite, calcite and hydroxyl-bastnesite along with phyllosilicates (phlogopite, paragonite?) were identified among daughter minerals.

The rare earth elements composition of melt inclusions from both types of rocks is characterized by the predominance of light REE. The content of REE, especially light ones, in inclusions from phoscorites is higher. Strontium and barium contents in most melt inclusions have negative correlations with niobium and zirconium concentrations.

Melt inclusions from phoscorites and olivinites contain carbonate and silicate mineral phases in various proportions, which may imply heterogeneous trapping of crystalline phases and two immiscible melts, silicate and carbonatite. Inclusions from phoscorite represent a more evolved magma with higher concentrations of rare metals.

This work was supported by the Russian Science Foundation, grant No 19-17-00013.

How to cite: Redina, A., Wohlgemuth-Ueberwasser, C., Mikhailova, J., and Ivanyuk, G.: Melt inclusions in olivines from phoscorites and olivinites of the Kovdor massif., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12611, https://doi.org/10.5194/egusphere-egu2020-12611, 2020.

EGU2020-13888 | Displays | GMPV1.4

K-Ar dating of recent thrusts: an application to the Tertiary clay gouges in the Northern Apennines of Italy

Filippo Carboni, Giulio VIola, Luca Aldega, Roelant van der Lelij, Francesco Brozzetti, and Massimiliano R. Barchi

The Northern Apennines (NA) are a characteristic example of foreland fold-and-thrust belt (FTB) migrating towards its foreland. The progressive and quite regular eastward migration of the NA has been classically constrained in time by relying on the age of the syn-orogenic foreland basins, mainly determined by means of foraminifera and nannofossil biostratigraphy.

The well-known age of deformation makes the NA a perfect area where to test the reliability of the K-Ar illite dating applied to Cenozoic deformation involving siliciclatic deposits. In particular, we present the results of the first attempt to directly date, by K-Ar on illite separated from fault rocks, Neogene thrusts within the Trasimeno Tectonic Wedge (TTW), an imbricate thrust complex mainly made up of Tertiary siliciclastic rocks, located in the inner-central part of the NA, which represents the external front of the so-called Tuscan Nappe.

We sampled two WSW-dipping thrust faults, whose fault cores are composed of scaly gouge formed at the expense of the pelitic component of the host rocks. X-ray diffraction (XRD) and K-Ar isotopic analysis of multiple grain-sizes (from < 0.1 to 10 µm) allowed us to discriminate between syn-kinematic illite crystals formed during thrusting and detrital illite crystals inherited from the host rock. XRD data show a mineralogical association composed of quartz, calcite, albite, K-feldspar, chlorite, kaolinite and two populations of illite polytypes (1Md and 2M1). After the X-ray semiquantitative analysis, the results of the K-Ar dating of the two samples were regressed by the Illite Age Analysis (IAA) approach to assessing the effects of potential host rock contamination. Fault slip along the thrusts is then constrained to 15.2 ± 7.6 Ma and 15.4 ± 16.6 Ma.

Despite the large errors, the obtained dates are in excellent agreement with the timing of deformation along the base of the TTW, bracketed between the late Aquitanian and the latest Burdigalian – earliest Langhian and, more in general, with the proposed time evolution of the Northern Apennines.

Even if based on a limited dataset, our results suggest that the application of K-Ar dating of fault gouge can be extended to tectonic settings where independent constraints are not available and thus it becomes a valuable tool to study and constrain the time-space evolution of FTBs in recent and even active orogens.

How to cite: Carboni, F., VIola, G., Aldega, L., van der Lelij, R., Brozzetti, F., and Barchi, M. R.: K-Ar dating of recent thrusts: an application to the Tertiary clay gouges in the Northern Apennines of Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13888, https://doi.org/10.5194/egusphere-egu2020-13888, 2020.

EGU2020-14005 | Displays | GMPV1.4 | Highlight

Geochemistry of noble gas and CO2 in fluid inclusions from lithospheric mantle beneath Eifel and Siebengebirge (Germany)

Andrea Luca Rizzo, Massimo Coltorti, Barbara Faccini, Federico Casetta, Theodoros Ntaflos, and Francesco Italiano

The study of fluid inclusions (FI) composition (He, Ne, Ar, CO2) integrated with the petrography and mineral chemistry of mantle xenoliths representative of the Sub Continental Lithospheric Mantle (SCLM) is a unique opportunity for constraining its geochemical features and evaluating the processes and the evolution that modified its original composition. An additional benefit of this type of studies is the possibility of better constraining the composition of fluids rising through the crust and used for volcanic or seismic monitoring.  

In this respect, the volcanic areas of Eifel and Siebengebirge in Germany represent a great opportunity to test this scientific approach for three main reasons. First, these volcanic centers developed in the core of the Central European Volcanic Province where it is debated whether the continental rift was triggered by a plume (Ritter, 2007 and references therein). Second, Eifel and Siebengebirge formed in Quaternary (0.5-0.01 Ma) and Tertiary (30-6 Ma), respectively, thus spanning a wide range of age. Third, Eifel is characterized by the presence of CO2-dominated gas emissions and weak earthquakes that testify that local magmatic activity is nowadays dormant, but not ended (e.g., Bräuer et al., 2013). It is thus important to better constrain the noble gas signature expected in surface gases in case of magmatic unrest.

This work focuses on the petrological and geochemical study of mantle xenoliths sampled in the West Eifel and Siebengebirge volcanic areas (Germany) and aims at enlarging the knowledge of the local SCLM. Gautheron et al. (2005) carried out the first characterization of noble gases in FI of crystals analyzed by crushing technique (as in our study) but limited to olivines and to West Eifel eruptive centers. Here, we integrate that study by analyzing olivines, orthopyroxenes and clinopyroxenes from a new suite of samples and by including two eruptive centers from Siebengebirge volcanic field (Siebengebirge and Eulenberg quarries).

Xenoliths from the Siebengebirge localities are characterized by the highest Mg# for olivine, clinopyroxene and Cr# for spinel, together with the lowest Al2O3 contents for both pyroxenes, suggesting  that the mantle beneath Siebengebirge experienced high degree of melt extraction (up to 30%) while metasomatic/refertilization events were more efficient in the mantle beneath West Eifel.

In terms of CO2 and noble gas concentration, clinopyroxene and most of the orthopyroxene show the highest gas content, while olivine are gas-poor. The 3He/4He varies between 5.5 and 6.9 Ra. These values are comparable to previous measurements in West Eifel, mostly within the range proposed for European SCLM (6.3±0.4 Ra), and slightly below that of MORB (Mid-Ocean Ridge Basalts; 8±1Ra). The Ne and Ar isotope ratios fall along a binary mixing trend between air and MORB-like mantle. He/Ar* in FI and Mg# and Al2O3 content in minerals confirm that the mantle beneath Siebengebirge experienced the highest degree of melting, while the metasomatic/refertilization events largely affected the Eifel area.

References

Bräuer, K., et al. 2013. Chem. Geol. 356, 193–208.

Gautheron, C., et al. 2005. Chem. Geol. 217, 97–112.

Ritter, J.R.R., 2007. In: Ritter, J.R.R., Christensen, U.R. (Eds.), Mantle Plumes: A Multidisciplinary Approach. Springer-Verlag, Berlin Heidelberg, pp. 379–404.

How to cite: Rizzo, A. L., Coltorti, M., Faccini, B., Casetta, F., Ntaflos, T., and Italiano, F.: Geochemistry of noble gas and CO2 in fluid inclusions from lithospheric mantle beneath Eifel and Siebengebirge (Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14005, https://doi.org/10.5194/egusphere-egu2020-14005, 2020.

EGU2020-19696 | Displays | GMPV1.4

Kinetic aspects of major and trace elements in olivines from variably cooled basaltic melts

Sarah Lang, Silvio Mollo, Lyderic France, Manuela Nazzari, Valeria Misiti, Andrey A. Gurenko, and Jean-Luc Devidal

Olivine is an important mineral phase in naturally cooled basaltic rocks. The texture and composition of olivine are strictly related to the interplay between the degree of magma undercooling and crystal growth rate. Crystals formed at low undercoolings and growth rates generally show polyhedral-hopper textures and quite homogeneous compositions, while skeletal-dendritic textures and evident crystal zonations occur at high undercoolings and growth rates. In this context, we have performed equilibrium and disequilibrium (i.e., cooling rate) experiments to better understand, by a comparatively approach, the effects of crystallization kinetics on the incorporation of major and trace cations in olivine lattice. The experiments were carried out in a 1 atm vertical tube CO-CO2 gas-mixing furnace to perform experiment at atmospheric pressure and oxygen fugacity of QFM-2 using a basaltic glass (i.e., OIB) as starting materials. The equilibrium experiment was performed at 1175 °C. These target temperatures were kept constant for 240 h and then quenched. Conversely, the disequilibrium experiments were performed at the superliquidus temperature of 1250, and 1300 °C, which was kept constant for 2 h before cooling. The final target temperatures of 1150 (undercooling -ΔT = 50 °C), and 1175 °C (-ΔT = 25 °C) were attained by applying cooling rates of 2 °C/h, 20 °C/h, and 60 °C/h. Then the experimental charges were quenched. Results show that the olivine texture shifts from euhedral (i.e., equilibrium) to anhedral (i.e., disequilibrium) under the effect of cooling rate and rapid crystal growth. In equilibrium experiments, the composition of olivine is homogeneous and non chemical gradients are found in the melt next to the crystal surface. In contrast, a diffusive boundary layer develops in the melt surrounding the olivine crystals growing rapidly under the effect of cooling rate and degree of undercooling. The compositional gradient in the melt increases with increasing cooling rate and undercooling, causing the diffusive boundary layer to expand towards the far field melt. Because of the effects of crystallization kinetics, skeletal-dendritic olivines incorporates higher proportions of major and trace elements that are generally incompatible within their crystal lattice under equilibrium conditions.

How to cite: Lang, S., Mollo, S., France, L., Nazzari, M., Misiti, V., Gurenko, A. A., and Devidal, J.-L.: Kinetic aspects of major and trace elements in olivines from variably cooled basaltic melts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19696, https://doi.org/10.5194/egusphere-egu2020-19696, 2020.

We present various examples of age variations in Potassium-bearing minerals of tectonites obtained by 40Ar/39Ar in situ dating. Age variations in pre-kinematic clasts and syn-kinematic blasts both span large age ranges of significantly different ages values at the cores compared to the rims, calling for petrologic interpretation. Some of the synkinematic grains are overgrown by late- to post-kinematic blast that show consequently the youngest ages values within the samples. The concurrence of textural relation and age value in the case of late- to post-kinematic growth seem to be a robust tool to date the termination of deformation.  

Additional examples where break down reactions lead to dissolution of the prekinematic texture and crystallization of new minerals as coronas, within fractures of strain shadows also yield partly reset age values with larger scatter. The interpretation of those age values is more challenging and might be obscured by the 3D textural geometry of the analysed volume as well as disequilibrium between reactants and products.      

Commonly based on the percentage errors age values are interpreted as being geological significantly different, when errors do not overlap, or as natural geologic scatter, in the case errors overlap. This interpretation is biased by the absolute age value itself and might lead to over- and underestimations of geological events in geologic history. There is a strong need of error calculation that enables geological interpretation of tectonic event independent from their absolute age.

How to cite: Schneider, S.: Inter- and Intragranular age variations: Diffusion or mineral growth? – And what is wrong with the error? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21192, https://doi.org/10.5194/egusphere-egu2020-21192, 2020.

EGU2020-21425 | Displays | GMPV1.4

Dating extensional deformation to unravel exhumation patterns in the Internal Dinarides

Georg Löwe, Susanne Schneider, Jörg A. Pfänder, and Kamil Ustaszewski

Ar/Ar-in-situ geochronology by laser ablation NGMS (noble gas mass spectrometry) provides a powerful tool to determine inter- and intra-granular age variations of potassium-bearing minerals while maintaining the structural integrity of a sample. This makes it an excellent method in targeting the understanding of the post-collisional evolution of an orogen by dating different mica generations. In order to investigate the timing of exhumation related to extensional deformation in the Internal Dinarides, we sampled paragneisses from the upper greenschist- to amphibolite-grade mylonitic detachment zones of two metamorphic core complexes (MCC’s). The MCC’s are located at the distal Adriatic passive margin (Cer MCC, central western Serbia) and within the Late Cretaceous suturing accretionary wedge complex (Motajica MCC, northern Bosnia and Herzegovina) that separates Adria-derived units from blocks of European affinity.

Mica grains were assigned to either pre-kinematic or syn-kinematic growth, according to their structural context, texture and grainsize. Pre-kinematic growth is characterized by large, deformed minerals of up to 3.5 mm in size, while rather fine-grained, recrystallized mineral aggregates that usually formed in the strain shadow of larger clasts represent syn-kinematic growth.

The ages of pre-kinematic white mica from paragneisses of the Motajica detachment range from approx. 80 to 25 Ma. They partly show a large intra-granular age spread characterized by significantly older core ages becoming progressively younger towards the rim. This pattern likely suggests diffusive loss of radiogenic Ar. Ages between 80-55 Ma in the central parts of the grains, associated with a top-W transport direction, are interpreted as the time interval of mineral growth and subsequent deformation in an accretionary wedge during E-ward subduction of the Adriatic passive margin underneath European units.

Syn-kinematic white mica from Motajica yielded ages between 22 and 16 Ma, which are interpreted as the time of peak activity of extension. This also corresponds with the time of crustal extension in the Pannonian Basin to the north. At Cer MCC, located roughly 150 km ENE of Motajica MCC and structurally below the accretionary wedge complex, ages of deformed white mica indicate exhumation between 19 and 15 Ma with a top-N directed transport.  

Our results suggest that the opening of the Pannonian Basin in response to slab-retreat underneath the Carpathian orogen resulted in the extensional reactivation of suturing thrusts that separated Adriatic from European units, leading to exhumation of parts of the accretionary wedge (Motajica MCC). This event was followed by the progressive exhumation of the passive Adriatic margin (Cer MCC) that occupied a structural position below the suturing accretionary wedge.

How to cite: Löwe, G., Schneider, S., Pfänder, J. A., and Ustaszewski, K.: Dating extensional deformation to unravel exhumation patterns in the Internal Dinarides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21425, https://doi.org/10.5194/egusphere-egu2020-21425, 2020.

GMPV1.6 – Geochemical monitoring in volcanic and seismically active regions: new advances

EGU2020-8485 | Displays | GMPV1.6

Using wehrlites to monitor the passage of CO2-bearing melts in the shallow lithosphere

Sonja Aulbach, A-Bing Lin, Yaakov Weiss, and Gregory Yaxley

Continental rifting has been linked to the thinning and destruction of cratonic lithospheres and to the release of enough CO2 to impact the global climate [1]. This fundamental plate tectonic process facilitates the infiltration and/or mobilisation of small-volume carbonated melts, which interact with mantle peridotite to form wehrlite through the reaction: enstatite + CO32- (melt) = forsterite + diopside + CO2 (vapour) [2]. An analysis of the literature reveals that wehrlites are common in shallow mantle lithosphere in disrupted craton settings affected by either extension or subduction, and they have been linked to agents ranging from carbonatites to basanites [e.g. 3,4]. Conversely, the low abundance of wehrlitic diamond and garnet in cratonic mantle xenoliths (as opposed to lherzolitic or harzburgitic) indicate that wehrlitisation is not an important process at depths >~120 km. This may be due to the presence of a dominantly reducing lithosphere, which favours diamond precipitation or dissolution during reaction with carbonated silicate melts, depending on carbon saturation. Based on the relationship between wehrlite and small-volume carbonated melts, we suggest that wehrlite-bearing xenoliths can be used to monitor CO2 mobility through the shallow continental lithosphere. Assuming a depleted protolith, typically harzburgite at shallow lithospheric depth, the amount of newly-added cpx can be estimated and related to CO2 in the melt volume based on the above reaction. Considering the proportion of wehrlite in the xenolith population, an estimate of the total CO2 transported out of the shallow lithosphere can be made. For example, peridotite xenoliths from Liaoyuan in the reactivated northeastern North China craton sample the mantle beneath the mid-Cretaceous Tan Lu Fault Belt (TLFB), which is even vaster in size (5000 x 800-1000 km) than the EAR, the latter linked to degassing of 28 to 34 Mt C/yr over 40 Ma [1]. If wehrlitisation affected only a 10 km depth interval over a similar time, 23 Mt C/yr has passed through the TLFB, possibly contributing to the mid-Cretaceous greenhouse climate. Thus, wehrlites reveal the hidden carbon cycle in lithospheric provinces where CO2-rich melts are not necessarily observed at the surface.

 

[1] Foley and Fischer 2017 Nature Geosci 10; [2] Yaxley et al. 1998 J Petrol 39; [3] Aulbach et al. J Petrol 2017 58; [4] Lin et al. subm. J Geophys Res

How to cite: Aulbach, S., Lin, A.-B., Weiss, Y., and Yaxley, G.: Using wehrlites to monitor the passage of CO2-bearing melts in the shallow lithosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8485, https://doi.org/10.5194/egusphere-egu2020-8485, 2020.

EGU2020-21937 | Displays | GMPV1.6

Continuous monitoring of fault-controlled CO2 degassing in the Los Humeros Volcanic Complex, Mexico

Anna Jentsch, Walter Düsing, and Egbert Jolie

Over the last few decades, monitoring of soil CO2 efflux has been widely used in different scientific disciplines like volcanic and seismic hazard assessment, carbon capture and sequestration, geothermal well integrity, and others. We installed a comprehensive LICOR LI-8150 monitoring system on the Los Humeros normal fault, one of the major structures in the geothermal production field. Over a five-months period, seven accumulation chambers measured CO2 efflux every hour in combination with an on-site meteorological station recording air temperature, air humidity, barometric pressure, precipitation, wind speed, and wind direction. Seismic activity was recorded simultaneously by a seismic array of 42 stations distributed across the volcanic complex, which identified both, high frequency, volcano tectonic (>10 Hz) and low frequency, long-period events (1-8 Hz). Furthermore, monthly geothermal production and re-injection data are available. Our study aims to (1) characterize significant temporal variations of soil CO2 efflux, (2) assess the effect of environmental parameters, (3) analyze the possible influence of natural seismicity and geothermal exploitation (production/re-injection) on CO2 degassing rates. The latter aspect plays an important role to better understand the hydraulic connection and communication between subsurface and surface along structural discontinuities in the volcanic-geothermal system.

How to cite: Jentsch, A., Düsing, W., and Jolie, E.: Continuous monitoring of fault-controlled CO2 degassing in the Los Humeros Volcanic Complex, Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21937, https://doi.org/10.5194/egusphere-egu2020-21937, 2020.

EGU2020-869 | Displays | GMPV1.6

Noble gas monitoring at the seemingly inactive Ciomadul volcano, Eastern Carpathians, Romania

Boglarka-Mercedesz Kis, Szabolcs Harangi, László Palcsu, and Botond Hegyeli

The Ciomadul volcano is the youngest volcano (32 ka) built by the Neogene volcanism in the Carpathian-Pannonian Region. This volcanic area is characterized by intense gas emissions (Kis et al., 2017) (CO2, CH4, H2S) in the form of bubbling pools, mofettes and mineral water springs. The isotopic compositions of carbon, 13CCO2 up to -3‰ VPDB and helium up to 3.1 Ra suggest magmatic origin of the gas up to 80% (Kis et al., 2019).

Although the volcano seems to be inactive, several features, petrologic and geophysical studies suggest that melt-bearing magmatic body could still exist beneath the volcano (Harangi et al., 2015). Moreover the geodynamic system is characterized by frequent earthquakes with magnitude up to 7 at Vrancea area, close to the CO2-rich gas emissions of Ciomadul and the neighbouring areas.

In 2015 we started the monitoring of the helium isotopic ratios of Ciomadul to chech the possible relationship with seismicity. Our results show that in several cases the helium isotopic ratios increase at a seismic event with magnitude between 4 and 5.8 suggesting a relationship between the two phenomena.

 

Harangi, Sz., Lukács, R., Schmitt, A.K., Dunkl, I., Molnár, K., Kiss, B., Seghedi, I., Á. Novothny, Molnár, M. 2015, Constraints on the timing of Quaternary volcanism and duration of magma residence at Ciomadul volcano, east-central Europe, from combined U-Th/He and U-Th zircon geochronology, Journal of Volcanology and Geothermal Research, 301, 66-80

Kis, B.M., Ionescu, A., Cardellini, C., Harangi, Sz., Baciu, C., Caracausi, A; Viveiros, F. 2017, Quantification of carbon dioxide emissions of Ciomadul, the youngest volcano of the Carpathian-Pannonian Region (Eastern-Central Europe, Romania), Journal of Volcanology and Geothermal Research, 341, 119–130

Kis, B.M., Caracausi, A., Palcsu, L., Baciu, C., Ionescu, A., Futó, I., Sciarra, A., Harangi, Sz. 2019, Noble gas and carbon isotope systematic at the seemingly inactive Ciomadul volcano (Eastern-Central Europe, Romania, Geochemistry, Geophysics, Geosystems, 20, 6, 3019–3043

This research belongs to the scientific project supported by the OTKA, K116528 (Hungarian National Research Fund), the EU and Hungary, co-financed by the European Regional Development Fund in the project GINOP-2.3.2-15-2016-00009 ‘ICER’ and the Deep Carbon Observatory.

How to cite: Kis, B.-M., Harangi, S., Palcsu, L., and Hegyeli, B.: Noble gas monitoring at the seemingly inactive Ciomadul volcano, Eastern Carpathians, Romania, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-869, https://doi.org/10.5194/egusphere-egu2020-869, 2020.

EGU2020-11646 | Displays | GMPV1.6

Diffuse He degassing monitoring of the Tenerife North-western Rift Zone (NWRZ) volcano, Canary Islands

Guillermo Recio, Eleanor Dunn, Yasmin McInally, Nemesio M. Pérez, Cecilia Amonte, Mar Alonso, Eleazar Padrón, María Asensio-Ramos, and Francisco A. Morales

Tenerife (2034 km2), the largest island of the Canarian archipelago, is characterized by three volcanic rifts oriented NW-SE, NE-SW and N-S with a central volcanic complex, Las Cañadas Caldera, hosting Teide-Pico Viejo volcanoes. The North West volcanic Rift Zone (NWRZ, 72 km2) of Tenerife is one of the youngest and most active volcanic systems of the island, where two historical eruptions have occurred: Arenas Negras in 1706 and Chinyero in 1909. Diffuse degassing studies has become an important volcanic surveillance tool at those volcanic areas where visible manifestations of volcanic gases are absent, as in the case of NWRZ. Mapping soil gas emission along volcanic structures can provide a better understanding of the processes occurring at depth and allows monitoring the spatial distribution, magnitude and temporal evolution of the surface gas emissions. The geochemical properties of He, minimize the interaction of this noble gas on its movement toward the earth’s surface, and make this gas an almost ideal geochemical indicator of changes occurring in the magmatic plumbing system of the volcano (Padrón et al., 2013, Geology 41(5):539–542). Since 2014, surface He emission surveys have been performed once a year as an additional geochemical tool to monitor the volcanic activity of NWRZ. At 345 sampling sites soil gas samples were collected at 40 cm depth and analyzed for He concentration within 24 hours by means of QMS, model Pfeiffer Omnistar 422. The soil helium concentration data were used to estimate the diffusive helium flux at each point, to construct spatial distribution maps by sequential Gaussian simulation and then to estimate the total helium emission in the NWRZ. Helium emission ranged between non-detected values up to 7.2 mgm-2d-1, and the emission rate of the entire area was in the range ~1 – 45 kg d-1. An increasing trend was observed in the period 2016-2018, showing a good temporal coincidence with a significant increase in seismic activity recorded in Tenerife. The promising results observed in the NWRZ and in other volcanic systems (Padrón et al., 2013) indicate that soil helium emission monitoring could be an excellent early warning geochemical precursory signal for future volcanic unrest.

How to cite: Recio, G., Dunn, E., McInally, Y., Pérez, N. M., Amonte, C., Alonso, M., Padrón, E., Asensio-Ramos, M., and Morales, F. A.: Diffuse He degassing monitoring of the Tenerife North-western Rift Zone (NWRZ) volcano, Canary Islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11646, https://doi.org/10.5194/egusphere-egu2020-11646, 2020.

EGU2020-12242 | Displays | GMPV1.6

Ultra-acid volcanic waters: origin and response on volcanic activity. A review

Yuri Taran and Elena Kalacheva

Some active volcanoes host thermal springs with ultra- (1<pH<2) and even hyper- (pH < 1) acidic waters with composition corresponding to a mixture of HCl and H2SO4 acids and with cations where Al and Fe are often the major components. Such springs sometimes are known as inferred drainages from active crater lakes (e.g., Rios Agrio at Poas and Copahue volcanoes). However, there are a number of acidic volcano-hydrothermal systems of Cl-SO4 composition at volcanoes without crater lakes.  At least ten groups of manifestation of this type are known for Kuril Islands. Several groups of acid volcanic springs including the famous Tamagawa springs are described in Japan.  Most of the acid Cl-SO4 volcano-hydrothermal systems are characteristic for island volcanoes, probably due to specific hydrological conditions of small volcanic islands. Maybe most known are coastal acid springs at Satsuma Iwojima volcano, Ryukyu arc, Japan. The accepted idea about the origin of such systems is scrubbing (dissolution) of magmatic HCl, HF and SO2 by groundwaters above magmatic conduits.  If so, the composition of acid springs must reflect the state of activity of a volcano. This review describes case histories that are known from the literature and from authors’ studies. Most of the volcanoes hosting acid systems show frequent phreatic activity. We show that  in contrast to crater lakes (Poas, Ruapehu, Copahue, White Island), acid springs on slopes of active volcanoes generally do not response on the preparing or ongoing volcanic eruptions. The aquifers and flow paths of the acid waters in volcanic edifices can be not associated with active conduits but with other degassing magmatic bodies and/or with deeper aquifers. One of the examples of such a complicated system is Ebeko volcano with Yuryevskye springs in Kuril Islands. These springs have a hydrochemical record since 1950s, and during this period Ebeko volcano had at least 10 strong phreatic eruptions.

How to cite: Taran, Y. and Kalacheva, E.: Ultra-acid volcanic waters: origin and response on volcanic activity. A review, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12242, https://doi.org/10.5194/egusphere-egu2020-12242, 2020.