Mineralogy of Yamato-791192, HED breccia and relationship between cumulate eucrites and ordinary eucrites

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Cumulate eucrites and noncumulate (ordinary) eucrites are considered to have come from one (or similar) parent body and to have formed the crust of the Vesta-like asteroid. However, their relationship is not well established as to whether they have crystallized in a different magma or have differentiated within the same magma. We studied mineralogically Yamato-791192,which is a unique HED breccia with abundant cumulate eucrite and a rare ordinary eucrite clasts. The characteristics of pyroxene suggest that the polymict breccia was generated by gathering locally ordinary eucrites and cumulate eucrites. On the other hand, the Fe/Mg distribution shows that the liquid coexisting with most cumulate eucrites was too Fe-rich (mg [=Mg/ (Mg+Fe) atomic ratio]=0.25-0.30) to have crystallized as ordinary eucrites (mg=0.35-0.40). We also applied the liquid trapping model of fractional crystallization and calculate the change of mg of liquid and solid during fractional crystallization. This calculation suggests that if the cumulates include a large amount of residual liquid (40-50%), a cumulate eucrite (mg=0.50-0.55) could crystallize from ordinary eucritic liquid (mg=0.35-0.40). In conclusion, cumulate eucrites probably crystallized directly from slightly evolved liquid, or they are produced by fractional crystallization with a large amount of trapped liquid.
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