Origin of the high-temperature fraction of C2 chondrites |
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| Authors: | Lawrence Grossman Edward Olsen |
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| Institution: | Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A.;Department of Geology, Field Museum of Natural History, Chicago, Illinois 60605, U.S.A. |
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| Abstract: | The coarse-grained fraction of C2 chondrites is composed mostly of single crystals and aggregates of crystals of Mg-rich olivine and pyroxene. They do not possess compelling textural evidence of being the solidification products of rapidly-quenched molten droplets. Metal inclusions in the silicates contain 3·82–8·88 mole% Ni, 0·16–0·70 per cent Co, 0·17–1·07 per cent Cr and up to 5·70 per cent P. Thermodynamic calculations show that alloys of these compositions may be condensates from the solar nebula. The implication is that the high-temperature fraction of C2 chondrites consists mostly of high-temperature condensates. Chemical data show that the high-temperature fraction has an Fe/Mg atomic ratio of ? 0·31 compared to 1·3 in the matrix, indicating that much of the iron has been lost from the high-temperature fraction and converted to the troilito and oxidized iron of the low-temperature fraction. The presence of low-Ni metal grains in the aggregates and high Ni/Fe and Co/Fe ratios in the matrix of some C2's indicates preferential loss of early NiCo-rich metal from the high-temperature fraction during condensation. |
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