The mineralogy and alteration history of the Yamato-type (CY) carbonaceous chondrites

• DOI: 10.1016/j.gca.2023.09.024
• Publication Date: 2023-09-30T00:46:26Z
• Authors: M.D. Suttle; A.J. King; C.S. Harrison; Q.H.S. Chan; A. Greshake; R. Bartoschewitz; A.G. Tomkins; T. Salge; P.F. Schofield; S.S. Russell
• Keywords: Chondrule; Parent body; Breccia; Pyroxene

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