Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration

作者: ALEXANDER N. KROT , MICHAEL I. PETAEV , EDWARD R. D. SCOTT , BYEON-GAK CHOI , MICHAEL E. ZOLENSKY

DOI: 10.1111/J.1945-5100.1998.TB01713.X

关键词: GeochemistryForsteriteGeologyFayaliteMineralogyAllende meteoriteAndraditeChondriteChondruleOlivinePhlogopite

摘要: Abstract— The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali-like (CV3oxB) and Allende-like (CV3oxA), in which chondrules, calcium-aluminum-rich inclusions (CAIs) matrices show characteristic alteration features (Weisberg et al, 1997; Krot 1997d; Kimura Ikeda, 1997). CV3oxB lithology is present Bali, Kaba, parts of the Mokoia breccia and, possibly, Grosnaja Allan Hills (ALH) 85006. It characterized by presence secondary low-Ca phyllosilicates (saponite sodium phlogopite), magnetite, Ni-rich sulfides, fayalite (Fa>90), Ca-Fe-rich pyroxenes (Fs10–50Wo45–50) andradite. Phyllosilicates replace primary Ca-rich minerals chondrules CAIs, suggests mobilization Ca during aqueous alteration. Magnetite nodules are replaced to various degrees fayalite, minor Fayalite veins crosscut fine-grained rims around extend matrix. Thermodynamic analysis observed reactions indicates that they could have occurred at relatively low temperatures (<300 °C) solutions. Oxygen isotopic compositions coexisting magnetite plot close terrestrial fractionation line with large Δ18Ofayalite-magnetite (∼20%). We infer phyllosilicates, andradite formed fluid-rock interaction an asteroidal environment. Secondary virtually absent CAIs CV3oxA lithology, Allende its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, Vigarano breccias. Instead nepheline, sodalite, fayalitic olivine common. Fayalitic replaces forsterite grains; it also forms coarse lath-shaped grains Secondary abundant. experienced higher than lithology. reduced lithologies CV3oXB all came from one heterogeneously altered asteroid. metamorphosed clasts (Krot Hutcheon, 1997) may rare samples hotter interior CV conclude resulted asteroid progressive metamorphism a heterogeneous mixture ices anhydrous materials mineralogically similar chondrites.

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