Mathematics – Logic
Scientific paper
Jun 1967
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1967gecoa..31..921d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 31, Issue 6, pp.921-934
Mathematics
Logic
50
Scientific paper
The iron contents of olivines and low-calcium pyroxenes in thirty-one ordinary (H-, L- and LL-group) chondrites were determined by microprobe analysis. Twenty-five of these contain inhomogeneous silicates and are, in this respect, unequilibrated. Petrographic study of these chondrites reveals that those of each chemical group whose silicates are most heterogeneous possess many characteristics which suggest that they are nearly unmetamorphosed. These are textural (absence of chondrule-chondrule and chondrule-matrix intergrowth; presence of a fine-grained, dark matrix; presence in some chondrules of silicic glass), mineralogical (near-absence of visible plagioclase; predominance of monoclinic low calcium pyroxenes), and chemical (high abundances of water, carbon, argon, and other volatile constituents). A complete gradation of properties exists between these meteorites and the equilibrated ordinary chondrites. We believe the least-metamorphosed chondrites of each chemical group closely approximate the unmetamorphosed precursors of the equilibrated ordinary chondrites. That the three chemical groups were distinct at the earliest stages of metamorphism indicates that their different contents of the siderophile elements were established during or before agglomeration of the parent materials. Close similarities in the mean compositions and proportions of olivines and pyroxenes in the least metamorphosed members of the three groups suggest that the siderophile fractionation among these groups may have resulted from mechanical mixture of a common silicate fraction with different proportions of metal rich material. There appears to be no comparably simple chemical or genetic relationship between the ordinary chondrites and the carbonaceous or enstatite chondrites. The origin of chondrules and chondrites remains, in the light of this study, an open question, for our results are consistent with any disequilibrium process entailing the production of chondrules through rapid cooling of molten and semi-molten droplets.
Dodd Robert T. Jr.
Koffman D. M.
van Schmus Randall W.
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