Mineralogical study of angrite Asuka-881371: Its possible relation to angrite LEW87051

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Antarctic angrites LEW87051 (LEW) and Asuka-881371 (Asuka) have distinctive textural and mineralogical affinities. LEW is porphyritic in texture with zoned olivines (∿0.5mm) widely distributed in a fine-grained groundmass. Several olivines are known to contain Mg, Cr-rich cores which are out of equilibrium with groundmass olivines and are considered to be xenocrysts. Asuka is ophitic in texture with large olivine xenocrysts (some larger than 2mm across) included in a groundmass. These olivines reach up to Fo_<89>, and are as rich in Cr and poor in Ca as LEW olivine cores. Despite these similarities, the variability of the olivine compositions differs between the two. Olivine cores of LEW are almost homogeneous in composition from one grain to another, while the large olivines in Asuka show significant grain-to-grain variation. Each large olivine xenocryst in Asuka is nearly homogeneous except for the remarkably zoned edge indicating reaction with the surrounding melt. Some small olivine xenocrysts in Asuka (∿0.5mm across) have zoning profiles similar to the two-stage zoning profile observed in LEW olivines. However they are interpreted to have the same origin as the other xenocrysts, with the difference being in the zoning profiles, caused by the effect of off-center cuts through the grains. We propose that the difference between LEW and Asuka olivine xenocrysts is produced by differing degrees of melting. The cores of LEW olivines are more clearly zoned than Asuka xenocrysts possibly due to a higher degree of atomic diffusion in LEW olivine. We previously calculated the cooling rate of LEW olivines and estimated that their burial depth was shallower than 2m. Our model for the formation of the LEW and Asuka angrites is as follows. Homogeneous olivine crystals with extensive chemical variation from one crystal to another were somehow incorporated into a melt and reheated. In the case of LEW, almost all the olivines except for the extremely Mg-rich ones were totally melted, while in Asuka even the Fo_<70> olivines did not melt. After the melting stage, the groundmass of both LEW and Asuka crystallized. Asuka groundmass minerals have ∿2x larger grain sizes than LEW. This difference will reflect differences in burial depth. LEW crystallized near the surface (∿2m) where the degree of melting was larger, whereas Asuka crystallized at a deeper burial depth and yet experienced less heating and melting. More intense melting at a shallower depth suggests an external heat source, possibly impact melting.
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Mineralogical study of angrite Asuka-881371: Its possible relation to angrite LEW87051

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