Thermal metamorphism of primitive meteorites—VII. Mineralogy-petrology of heated Murchison (C2) and alteration of C30 and other chondrites

Abstract

Alterations caused by week-long heating of Murchison in a low-pressure environment at 400–1400°C are of two types: thermodynamically-favored, kinetically-controlled or thermodynamicallycontrolled, rapid processes. Kinetically-controlled changes pertinent to chondritic evolution and which vary progressively with temperature in heated Murchison include: chondrule blurring; matrix coarsening; increasing mean Fa and Fs contents of ferromagnesian silicates; equilibration of olivine; increasing Mg/Si, Ca/Si, Ai/Si and Cr/Si and decreasing Fe/Si, Ni/Si and S/Si in matrix; Cr loss from kamacite; homogenization and Ni-zoning in taenite at high temperatures. Low-temperature thermodynamicallycontrolled changes include: transformation of high-Ni troilite to low-Ni and formation of Ni- and Co-rich metal from pentlandite. High-temperature changes include formation of Cr-rich magnetite and formation of a Ni-rich sulfide similar to that found in highly-altered chondrites. Trends resulting from processes of both kinds in Murchison are consistent with characteristics of a postulated C30 metamorphic suite while those changes causes by reactions of the second kind are similar to those in heavily shock-heated, ordinary chondrites and the heavily-metamorphosed C5–6 chondrite. Mulga West. Either our simulations support the metamorphic origin of the C30 suite and other thermally-induced changes or the natural alterations support the utility of laboratory simulations in studying meteoritic evolution.