SIZE‐FREQUENCY‐DISTRIBUTIONS OF EH3 CHONDRULES

Abstract

The size‐frequency‐distributions of different chondrule types in the Qingzhen, Kota‐Kota and Allan Hills A77156 EH3 chondrites were determined by petrographic analysis of thin sections and, in the case of Qingzhen, by examination of large separated chondrules. EH chondrules are considerably smaller than L and LL chondrules and are probably slightly smaller than H, CM and CO chondrules. In the EH3 chondrites, radial pyroxene (RP) chondrules are somewhat (85% confidence level) larger than cryptocrystalline (C) chondrules, nonporphyritic chondrules have a broader size‐frequency‐distribution than porphyritic chondrules, and porphyritic olivine‐pyroxene (POP) chondrules are considerably (98% confidence level) larger than porphyritic pyroxene (PP) chondrules.The larger size of RP chondrules relative to C chondrules in EH3 chondrites may be due to a tendency of the chondrule‐forming mechanism not to have heated large precursor aggregates above the liquidus. Consequent retention of numerous relict grains would have caused these objects to develop RP rather than C textures upon cooling. The large proportion (≥50%) of nonporphyritic EH3 chondrules among the smaller chondrule size‐fractions may have been caused by preferential disruption of large nonporphyritic chondrule droplets. The large proportion (≥50%) of nonporphyritic EH3 chondrules among the larger chondrule size‐fractions is problematic. The larger size of POP relative to PP chondrules is due to reaction of fine‐grained olivine with free silica to form pyroxene during mild thermal metamorphism of the whole‐rocks.