Other
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..141...96k&link_type=abstract
Icarus, Volume 141, Issue 1, p. 96-106.
Other
25
Scientific paper
Intergroup variation in chondrule sizes is commonly attributed to mass or aerodynamic sorting in the solar nebula, a concept that has recently been extended to other chondrite constituents (metal chondrules, metal-troilite grains). Both sorting mechanisms are dependent on grain density and size. Because metal chondrules and metal-troilite grains have smaller average sizes than their coexisting chondrule populations, the assumption of mass equivalence has been made and invoked in support of nebular sorting. We present the results of a quantitative comparison of the sizes, masses, and aerodynamic stopping times of chondrules and metal-troilite grains from three ordinary chondrites: Kelly (LL4), Bjurböle (L/LL4), and Hammond Downs (H4). Chondrule volumes were determined from corrected thin-section measurements, and metal-troilite grain volumes were estimated from X-ray tomographic images. Chondrule and metal-troilite grain populations have similar masses in Hammond Downs but are dissimilar in Bjurböle and Kelly. The difference in the average particle aerodynamic stopping times of the chondrules and metal-troilite grains (15% for Hammond Downs, 16% for Bjurböle, 32% for Kelly) are much smaller than differences in their average masses (28% for Hammond Downs, 73% for Bjurböle, 82% for Kelly). The observed ranges in mass of the two populations are relatively narrow in Hammond Downs and are wider in Bjurböle and Kelly. Furthermore, in all three meteorites the observed range in mass of the chondrules is narrower than that of the corresponding metal-troilite grain populations. It appears that the chondrules were sorted more efficiently than the metal-troilite grains. Our results agree with the idea that aerodynamic stopping times vary with particle size and density and disagree with sorting only by mass. While the average stopping times (expressed as rp ρs) of the two populations correlate better than their average masses, the percent difference in (rp ρs) between the two populations (Hammond Downs, 15%; Bjurböle, 16%; and Kelly, 32%) is greater than that between the chondrules and metal chondrules previously reported for the Acfer 059 carbonaceous chondrite (W. R. Skinner and J. M. Leenhouts 1993, Proc. Lunar Planet. Sci. Conf. 24th, 1315-1316). We attribute this to the small and irregular shapes of the metal-troilite grains, although thermal metamorphism may have also affected the metal-troilite grain data. For these reasons, our results are at variance with the concept of nebular mass sorting but may be in agreement with aerodynamic sorting models. Our results are consistent with the hypothesis that chondrule sorting is related to the phenomena of metal-silicate fractionation. However, these data are only preliminary. Final interpretation should be reserved until more meteorites can be analyzed, the effects of thermal metamorphism on metal grain sizes quantified, and software capable of true three-dimensional analysis developed.
Carlson William D.
Hirsch David
Kuebler Karla E.
McSween Harry Y.
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