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

We present new INAA results for bulk metal from H and LL chondrites. B oth have characteristic elemental patterns similar to L chondrite meta l, in particular increasing abundances of W and Ga from unequilibrated to equilibrated chondrites but a reverse variation of V and Cr abunda nces. These characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material. The simi larities of melting features and the complementary nature of compositi ons between metal and chondrules suggest that these two components wer e derived from a common precursor, similar to CI or CM material in red ox state and compositionally related to the matrix of highly unequilib rated ordinary chondrites. Co/Ni abundance ratios are similarly low fo r bulk metal in the least metamorphosed ordinary chondrites of all thr ee chemical soups. This suggests that metal in ordinary chondrites ini tially had the same composition and formed under the same melting cond itions. The chondrites that accreted earliest have preserved their mel ting characteristics, while those that accreted later reequilibrated w ith the ambient gas at different temperatures. The differences in redo x state between equilibrated and unequilibrated chondrites show that f ormation 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 accret ed at the highest temperature and are most reduced among the three ord inary chondrite groups. LL chondrites were accreted at the lowest temp erature and are most oxidized. Compositionally, the metal component th at would account for the Fe/Si fractionation among H, L, and LL chondr ites is different from the metal formed by melting, indicating that th e different Fe/Si ratios of H, L, and LL chondrites were established b efore their accretion. Copyright (C) 1997 Elsevier Science Ltd