^1,2M.D. Suttle,²A.J. King,²C.S. Harrison,^1,3Q.H.S. Chan,⁴A. Greshake,⁵R. Bartoschewitz,⁶A.G. Tomkins,⁷T. Salge,²P.F. Schofield,²S.S. Russell
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2023.09.024]
¹School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
²Planetary Materials Group, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
³Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK
⁴Museum für Naturkunde, Leibniz-Institut für Evolutions und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany
⁵Bartoschewitz Meteorite Laboratory, Weiland 37, D-38518 Gifhorn, Germany
⁶School of Earth, Atmosphere and Environment, Melbourne, Victoria, Australia
⁷Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Copyright Elsevier

The CY chondrites are a group of thermally metamorphosed carbonaceous chondrites. Although they share similarities with the CM and CI chondrites, their primary properties argue for a distinct classification. Previous studies have highlighted their isotopically heavy bulk compositions (δ17O=10 ‰, δ18O=21 ‰, Δ17O=0 ‰) and exceptionally high sulphide abundances (10-30 vol%). In this work we explore their petrography and alteration history. The CYs accreted low abundances of chondrules (15-20 area%) with average apparent diameters slightly larger (∼320-340 µm) than the CM chondrites. In contrast to the CMs, the CYs record an early episode of brecciation prior to the main window of aqueous alteration. Subsequent fluid activity produced a range of alteration extents with both CY2 and CY1 chondrites documented. Phyllosilicate minerals in the CYs were a mix of serpentine and saponite (including occurrences of Na-saponite) with minor quantities of chlorite (within chondrules). An initial generation of Fe-sulphides formed by sulfidation of metal, and by precipitation from S-rich fluids. Three generations of carbonates are recognized, an early generation that infilled voids left by brecciation and co-precipitated with sulphide, a later generation that co-precipitated with magnetite and a final Fe-Mg-bearing generation which formed large (>100 µm) clasts. Only the first-generation carbonates are found in the CY2s, while the CY1s preserve all three generations. Phosphates occur as Ca-apatite or rarely as Mg-apatite and have hydroxylapatite compositions, indicating low halogen activities in the alteration fluids. Refractory oxides (ilmenite and Cr-spinel) occur as precipitates adhering to the margins of phyllosilicates. They formed late in the alteration sequence and attest to oxidizing conditions. During the late-stages of aqueous alteration Fe-sulphides were replaced by magnetite. Thermal metamorphism (Stage II-IV: ∼300-750 °C) overprinted aqueous alteration leading to dehydration and recrystallization of the phyllosilicate matrix and the decomposition of some carbonate phases. Most Fe-sulphide grains survived heating without decomposition as initial partial decomposition from pyrrhotite to troilite under closed system conditions led to elevated ƒS2 gas and resulted in a stabilizing effect. Retrograde reactions between trapped S2 gas and metal/magnetite formed a final generation of Fe-sulphides. The survival of Fe-sulphides and their stochiometric troilite compositions are evidence for near-closed system heating. Analysis of organic matter by Raman spectroscopy supports an interpretation of short-duration heating (on the scale of minutes to days), at peak temperatures between 750-900 °C. Thus, an impact event was the most likely cause of metamorphic heating.

^1,2S. S. Rout,³J. Storz,⁴A. Davydok,³A. Bischoff,⁵T. John,⁴C. Krywka,⁶M. Ritter
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14082]
¹School of Earth and Planetary Sciences, National Institute of Science Education and Research, Khorda, India
²Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai, India
³Institut für Planetologie, University of Münster, Münster, Germany
⁴Institute of Materials Physics, Helmholtz-Zentrum Hereon, Outstation DESY, Hamburg, Germany
⁵Freie Universität Berlin, Institut für Geologische Wissenschaften, Berlin, Germany
⁶Electron Microscopy Unit, Hamburg University of Technology, Hamburg, Germany
Published by arrangement with John Wiley & Sons

The origin of diamond in ureilites has been frequently debated. We investigated carbon phase assemblages (CPAs) in five ureilitic samples of the brecciated asteroid 2008 TC3, found within the Almahata Sitta (AHS) strewn field, by transmission electron microscopy, Raman spectroscopy, synchrotron X-ray diffraction, and cathodoluminescence. Samples MS-MU 006, MS-187, and MS-170, are of low to moderate shock degree (U-S2 and U-S3), and samples MS-MU 027 (U-S4) and MS-MU 045 (U-S5) have a higher shock degree. In MS-MU 006 and MS-187, we did not find any diamond grains. MS-170 contains disordered and distorted graphite with diamond grains up to 12 μm in size and containing inclusions of Fe,Ni-metal, FeS, Fe-phosphide, and Cr,Fe-oxide. These diamond grains formed under relatively low (5–15 GPa) shock pressures through a catalytic process in the presence of a Fe,Ni,Cr,S,P-rich melt. The highly shocked and fine-grained ureilites MS-MU 027 and MS-MU 045 have three different types of CPAs, namely a nanopolycrystalline assemblage of diamond and defect-rich diamond/lonsdaleite, disordered and distorted graphite, and polycrystalline diamond with abundant Fe-rich mineral inclusions. The CPAs that have only diamond and planar defect-rich diamond (e.g., MS-MU 027) most likely formed through martensitic transformation of graphite to diamond and lonsdaleite at >15 GPa and >2000 K. The assemblage of diamond, defect-rich diamond, and disordered and distorted graphite (e.g., MS-MU 045) formed by martensitic transformation of graphite to diamond and lonsdaleite, followed by back-transformation to disordered graphite. We did not find any conclusive evidence to support the formation of diamond grains under high static pressure.