¹Devin L.Schrader,¹Jemma Davidson,²Timothy J.McCoy,³Thomas J.Zega,⁴Sara S.Russell,³Kenneth J.Domanik,⁴Ashley J.King
Geochimica et Cosmochimica Acta (in Press) Link to Articel [https://doi.org/10.1016/j.gca.2021.03.019]
¹Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution Avenue NW, Washington, DC 20560-0119, USA
³Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA
⁴Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK
Copyright Elsevier

Determining compositional trends among individual minerals is key to understanding the thermodynamic conditions under which they formed and altered, and is also essential to maximizing the scientific value of small extraterrestrial samples, including returned samples and meteorites. Here we report the chemical compositions of Fe-sulfides, focusing on the pyrrhotite-group sulfides, which are ubiquitous in chondrites and are sensitive indicators of formation and alteration conditions in the protoplanetary disk and in small Solar System bodies. Our data show that while there are trends with the at.% Fe/S ratio of pyrrhotite with thermal and aqueous alteration in some meteorite groups, there is a universal trend between the Fe/S ratio and degree of oxidation. Relatively reducing conditions led to the formation of troilite during: (1) chondrule formation in the protoplanetary disk (i.e., pristine chondrites) and (2) parent body thermal alteration (i.e., LL4 to LL6, CR1, CM, and CY chondrites). Oxidizing and sulfidizing conditions led to the formation of Fe-depleted pyrrhotite with low Fe/S ratios during: (1) aqueous alteration (i.e., CM and CI chondrites), and (2) thermal alteration (i.e., CK and R chondrites). The presence of troilite in highly aqueously altered carbonaceous chondrites (e.g., CY, CR1, and some CM chondrites) indicates they were heated after aqueous alteration. The presence of troilite, Fe-depleted pyrrhotite, or pyrite in a chondrite can provide an estimate of the oxygen and sulfur fugacities at which it was formed or altered. The data reported here can be used to estimate the oxygen fugacity of formation and potentially the aqueous and/or thermal histories of sulfides in extraterrestrial samples, including those returned by the Hayabusa2 mission and due to be returned by the OSIRIS-REx mission in the near future.

¹T.J.Barrett,¹A.Černok,¹G.Degli-Alessandrini,¹X.Zhao,^1,2M.Anand,¹I.A.Franchi,³J.R.Darling
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.018]
¹The Open University, School of Physical Sciences, Walton Hall, Milton Keynes, MK7 6AA, UK
²Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
¹University of Portsmouth, School of the Environment, Geography and Geosciences, Burnaby Road Portsmouth, PO1 3QL, UK
Copyright Elsevier

The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion.

In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported volatile data for S1 and S2 eucrites were integrated with EBSD findings in this study.

Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries.

The Cl content and δ37Cl values of highly-shocked apatite grains range from ∼ 940–1410 ppm and – 3.38 to + 7.70 ‰, respectively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to ∼ 4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured δD values range from – 157 to + 163 ‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (δD − 122 ± 20 ‰, δ37Cl + 1.76 ± 0.66 ‰) are consistent with the range displayed in other inner Solar System bodies.

NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with deformation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock.