Oxygen isotope microanalysis of a large orthopyroxene clast in A-881526 diogenite

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Submillimeter-scale oxygen isotope analysis was conducted on a chemically zoned orthopyroxene clast in Asuka-881526 diogenite for the first time. Major elements, Ca, Mg, and Fe of orthopyroxene clast (∿5 mm) in A-881526 does show nearly constant composition of Wo_<1-4>En_<73-76>Fs_<22-25>, but minor elements, Al_2O_3 and Cr_2O_3 displayed continuous and discontinuous zoning within the crystal grain. Along the same section, oxygen isotope compositions of the large clast showed a very weak heterogeneity from the central (δ^<18>O=3.8-4.1‰) to the marginal zones (3.5-4.1‰). Matrix orthopyroxene represented a wider range of δ^<18>O values (3.4-4.3‰). Nearly constant δ^<18>O (∿4.0‰), Al_2O_3 (∿0.7wt%), and Cr_2O_3 (∿0.6wt%) features of most part of the orthopyroxene clast, and the slight depletion of δ^<18>O (<3.8‰) and enrichment of Al_2O_3 (>1.2 wt%) at the clast margin could be explained by isothermal crystallization from a large volume of orthopyroxenerich asteroidal magma where Rayleigh fractionation had undergone until 85% degree of crystallization. The fluid-absent magmatism may have aided the preservation of weak δ^<18>O heterogeneity in the large orthopyroxene clast.
国立極地研究所の論文