Mineralogy of Yamato 791073 with reference to crystal fractionation of the Howardite parent body

Abstract

Shock textures and mineral chemistry indicate that Yamato 791073 is a type of meteorite intermediate between diogenite and cumulate eucrite and is similar to the Yamato 75032‐type achondrites. The presence of vesicles in shock melt glass and swirly glasses of plagioclase composition as in the suevite in the matrix indicates that it is the most intensely shocked sample among this group of achondrites. Pyroxene compositions in Y791073 fall in a limited range from Mg/(Mg + Fe) = 0.7 to 0.6 and are compatible with the continuum of rock types produced presumably by differentiation on the howardite parent body. The chemical variation of pyroxene is wider than that of the monomict Y75032 group. In addition to Fe‐rich diogenite‐like pyroxene (Ca2Mg66Fe31) common in Y75032, Y791073 contains clasts of pyroxene texturally like those in Moore County. The plagioclase glass with An89 may originally have been associated with this pyroxene as a cumulate eucrite. Such clasts are uncommon in the Y75032‐type achondrites, Y791073 also contains a very differentiated Si‐ and K‐rich clast. Y791073 shows a more polymict nature than other Y75032‐type achondrites, but the lithic variation more limited than normal howardites suggests that it sampled two or three layers of the layered crust or pluton produced by the crystal fractionation.