Compositional diversity of ordinary chondrites inferred from petrology, bulk chemical, and oxygen isotopic compositions of the lowest FeO ordinary chondrite, Yamato 982717

摘要

We performed a petrologic, geochemical, and oxygen isotopic study of the lowest FeO ordinary chondrite (OC), Yamato (Y) 982717. Y 982717 shows a chondritic texture composed of chondrules and chondrule fragments, and mineral fragments set in a finer grained, clastic matrix, similar to H4 chondrites. The composition of olivine (Fa(11.17 +/- 0.48 (1 sigma))) and low-Ca pyroxene (Fs(11.07 +/- 0.98 (1 sigma))Wo(0.90 +/- 0.71(1 sigma))) is significantly more magnesian than those of typical H chondrites (Fa(16.0-20), Fs(14.5-18.0)), as well as other known low-FeO OCs (Fa(12.8-16.7); Fs(13-16)). However, the bulk chemical composition of Y 982717, in particular lithophile and moderately volatile elements, is within the range of OCs. The bulk siderophile element composition (Ni, Co) is within the range of H chondrites and distinguishable from L chondrites. The O-isotopic composition is also within the range of H chondrites. The lack of reduction textures indicates that the low olivine Fa content and low-Ca pyroxene Fs content are characteristics of the precursor materials, rather than the result of reduction during thermal metamorphism. We suggest that the H chondrites are more compositionally diverse than has been previously recognized.