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

Abstract

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