Petrology and mineralogy of the Y-793605 martian meteorite

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Yamato-793605 is a lherzolitic shergottite and very similar in texture and mineralogy to other two lherzolitic shergottites, ALHA77005 and LEW88516,but is not a pair with the latter two. It consists mainly of olivine, pyroxene, maskelynite, chromite, ilmenite, and pyrrhotite. Small silicate inclusions occur in olivine and pyroxene. Olivine has a wide compositional range from Fo_<25> to Fo_<36>, that included in pyroxene is more magnesian, and that in contact with maskelynite is more ferroan. Chromite shows remarkable compositional zoning, and the ranges change according to the occurrence of chromite; chromite included in olivine is Cr-Ti-poor, that in pyroxene is Cr-rich and Ti-poor, and that in contact with maskelynite is Ti-rich. There is a compositional gap between chromites in olivine and pyroxene. Silicate inclusions in olivine consist mainly of aluminous pyroxene, fine-grained intergrowth of plagioclase and silica, and rhyolitic glass. Plagioclase of the intergrowth is more calcic with An_<63-83> than maskelynite of the host lithology (An_<30-58>). The rhyolitic glass saturates with silica components and is peraluminous, whereas the host lithology does not include any silica mineral and is not peraluminous. These differences between the silicate inclusions and the host lithology, as well as the compositional gap between chromites in olivine and pyroxene, suggest that magma mixing took place in the Y-793605 magma reservoir between olivine and pyroxene crystallization stages. Y-793605 experienced secondary alteration of olivine, nonrhyolitic glass, chromite, ilmenite, pyrrhotite, and phosphate by introduction of Si and K, and loss of Ti, Cr, Mg, Ca, and Na probably at Antarctica.
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Ikeda Yukio
Department Of Earth Science Faculty Of Science Ibaraki University

Petrology and mineralogy of the Y-793605 martian meteorite

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