THE ORIGIN AND NEBULAR HISTORY OF THE METAL PHASE OF ORDINARY CHONDRITES

Abstract

We present new INAA results for bulk metal from H and LL chondrites. Both have characteristic elemental patterns similar to L chondrite metal, in particular increasing abundances of W and Ga from unequilibrated to equilibrated chondrites but a reverse variation of V and Cr abundances. These characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material. The similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor, similar to CI or CM material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites. Co/Ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups. This suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions. The chondrites that accreted earliest have preserved their melting characteristics, while those that accreted later reequilibrated with the ambient gas at different temperatures. The differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies. The initial metal composition established during the melting stage (13 wt% Ni; Co/Ni = 0.031) is inferred from the metal in highly unequilibrated chondrites. The accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction. H chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups. LL chondrites were accreted at the lowest temperature and are most oxidized. Compositionally, the metal component that would account for the Fe/Si fractionation among H, L, and LL chondrites is different from the metal formed by melting, indicating that the different Fe/Si ratios of H, L, and LL chondrites were established before their accretion.