Sizes and masses of chondrules and metal-troilite grains in ordinary chondrites: Possible implications for nebular sorting

Intergroup variation in chondrule sizes is commonly attributed to mass or a erodynamic sorting in the solar nebula, a concept that has recently been ex tended to other chondrite constituents (metal chondrules, metal-troilite gr ains). Both sorting mechanisms are dependent on grain density and size. Bec ause metal chondrules and metal-troilite grains have smaller average sizes than their coexisting chondrule populations, the assumption of mass equival ence has been made and invoked in support of nebular sorting. We present th e results of a quantitative comparison of the sizes, masses, and aerodynami c stopping times of chondrules and metal-troilite grains from three ordinar y chondrites: Kelly (LL4), Bjurbole (L/LL4), and Hammond Downs (H4). Chondr ule volumes were determined from corrected thin-section measurements, and m etal-troilite grain volumes were estimated from X-ray tomographic images. C hondrule and metal-troilite grain populations have similar masses in Hammon d Downs but are dissimilar in Bjurbole and Kelly. The difference in the ave rage particle aerodynamic stopping times of the chondrules and metal-troili te grains (15% for Hammond Downs, 16% for Bjurbole, 32% for Kelly) are much smaller than differences in their average masses (28% for Hammond Downs, 7 3% for Bjurbole, 82% for Kelly). The observed ranges in mass of the two pop ulations are relatively narrow in Hammond Downs and are wider in Bjurbole a nd Kelly. Furthermore, in all three meteorites the observed range in mass o f the chondrules is narrower than that of the corresponding metal-troilite grain populations. It appears that the chondrules were sorted more efficien tly than the metal-troilite grains. Our results agree with the idea that ae rodynamic stopping times vary with particle size and density and disagree w ith sorting only by mass. While the average stopping times (expressed as r( p) p(s)) of the two populations correlate better than their average masses, the percent difference in (r(p) p(s)) between the two populations (Hammond Downs, 15%; Bjurbole, 16%; and Kelly, 32%) is greater than that between th e chondrules and metal chondrules previously reported for the Acfer 059 car bonaceous chondrite (W. R. Skinner and J. M. Leenhouts 1993, Proc. Lunar Pl anet. Sci. Conf. 24th, 1315-1316). We attribute this to the small and irreg ular shapes of the metal-troilite grains, although thermal metamorphism may have also affected the metal-troilite grain data. For these reasons, our r esults are at variance with the concept of nebular mass sorting but may be in agreement with aerodynamic sorting models. Our results are consistent wi th the hypothesis that chondrule sorting is related to the phenomena of met al-silicate fractionation. However, these data are only preliminary. Final interpretation should be reserved until more meteorites can be analyzed, th e effects of thermal metamorphism on metal grain sizes quantified, and soft ware capable of true three-dimensional analysis developed. (C) 1999 Academi c Press.