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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. 相似文献
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We determined Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn in the 6 ureilite achondrites by neutron activation analysis. All 11 elements are depleted below Cl levels and their characteristic abundance pattern differs substantially from those of chondritic groups. Thus ureilites do not represent a simple mixture of volatile-rich chondrites with achondritic material but perhaps cosmochemically-fractionated achondritic material and a late ‘distillate’ of mobile elements. 相似文献
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