¹Molly C. McCanta,^2,3M. Darby Dyar,^4,5Stephen R. Sutton,⁶Sarah E. Roberts,^7,8Cai R. Ytsma
American Mineralogist 110, 1597-1613 Link to Article [https://doi.org/10.2138/am-2024-9602]
¹Department of Earth, Environment, and Planetary Sciences, University of Tennessee, 1621 Cumberland Avenue, Knoxville, Tennessee 37996, U.S.A.
²Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, Arizona 85719, U.S.A.
³Department of Astronomy, Mount Holyoke College, 50 College Street, South Hadley, Massachusetts 01075, U.S.A.
⁴Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁵Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁶Corning Glass, Corning, New York 14830, U.S.A.
⁷Cai Consulting, Glasgow, Scotland, U.K.
⁸Institute of Health Informatics, University College London, Gower Street, London WC1E 6BT, U.K.
Copyright: The Mineralogical Society of America

Iron redox works well for constraining oxygen fugacity (⁠⁠) in terrestrial igneous materials due to the relatively high  of the Earth’s atmosphere, crust, and upper mantle ⁠, where there are large changes in Fe²⁺/Fe³⁺ with relatively small changes in ⁠. At  values <QFM, Fe redox becomes less sensitive, and analytical uncertainties may make it difficult to determine  differences between samples. The valence change between Cr²⁺ and Cr³⁺ occurs at lower  values than for iron, potentially making it a more sensitive oxybarometer for materials equilibrated under reducing conditions. The current approach to measuring  from X-ray absorption (XAS) measurements derives Cr valence first from the 1s → 4s transition and then uses that redox couple to estimate  as a function of temperature and composition. Here, that method is compared to an alternate approach of predicting  directly from the spectra of experimentally homogenized glasses of geological relevance without an intermediate step of attempting to discern Cr²⁺/Cr³⁺. In this study, partial least-squares (PLS) multivariate (MVA) regression models were trained on the whole XAS energy spectral range, and accuracy was quantified using root mean square error (RMSE). MVA results showed significantly higher accuracy (RMSE-C of ±0.75 log units) for predicting  ΔIW relative to known experimental conditions relative to the two-step method, which yielded RMSE-C of ±2.75 to ±7.65 log units for our data set vs. those of Berry and O’Neill (2004) and Berry et al. (2006), respectively. The MVA results calibrate a new Cr oxybarometer for use in low- glasses with a cross-validated (RMSE-CV) accuracy of ±0.84 log units  relative to a standard oxygen buffer. Finally, the new Cr oxybarometer was applied to lunar glasses, both volcanic and impact metamorphosed, to assess the range in oxidation conditions the materials experienced during formation. Lunar volcanic glasses cluster ∼IW±1, close to that of previous studies while agglutinates and lunar impact melts record a wide range of  values using Cr oxybarometry.

¹Dwijesh Ray,¹Aditya Das,¹Subham Sarkar,²Satadru Bhattacharya,³Chandrani Nayak
American Mineralogist 110, 1516-1526 Link to Article [https://doi.org/10.2138/am-2024-9360]
¹Physical Research Laboratory, Ahmedabad 380 009, India
²Space Applications Centre (ISRO), Ahmedabad 380 015, India
³Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
Copyright: The Mineralogical Society of America

We report mineralogy, chemical composition, visible near-infrared (VNIR) spectroscopy, and X-ray near-edge spectroscopy of phyllosilicate-sulfate mineral assemblages, including natrojarosite, from the Matanomadh Formation (Palaeocene) of the Kutch region in Gujarat, India. The unit-cell parameters of sulfate minerals and VNIR spectral properties are consistent with natrojarosite and other minerals. The mineralogy and unit-cell parameters indicate supergene or hydrothermal alteration conditions within the Kutch region. The Fe oxidation record (Fe3+/total Fe ∼0.83) is quantified using Fe K-edge XANES. Additionally, the Fe-S bond length for natrojarosite is determined to be 3.158 Å, and the experimental result fits well with the modeled iron sulfate data curve. The occurrences of natrojarosite and associated natroalunite and kaolinite in the post-Deccan volcanic province are consistent with an oxidizing environment and periodic shift of humid to arid environments that appear to be similar to the early geologic history of Mars. The oxidative alteration of pyrite further enhances the formation of natrojarosite. Our results, in concert with earlier studies, highlight the formation process of natrojarosite and provide insights into detailed mineralogy at the Kutch region and the magnitude of the redox state. The natrojarosite and the associated minerals (including anhydrite and other sulfates) argue for acidic and water-limited conditions analogous to the Noachian-Hesperian epoch of Mars, with implications for past weathering processes.