Geochemistry and origin of achondritic inclusions in Yamato-75097, -793241 and -794046 chondrites

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We have performed instrumental and radiochemical neutron activation analyses of the host L6 chondrite and five splits of the Yamato (Y)-75097 achondritic inclusion, and achondritic inclusions from the Y-793241 (L6) and Y-794046 (H5) chondrites. The troctolitic inclusions from Y-75097 and Y-793241 show numerous compositional similarities, including low Sc contents, fractionated siderophile element patterns at low abundances, and commonly (though not always) depleted trivalent REE abundances but with Eu at near chondritic levels. The REE, however, are highly variable in the Y-75097 inclusion due to variations in modal merrillite. The lack of pyroxene in these inclusions is best explained by assuming the protolith was non-chondritic in bulk composition. However, it is not clear that nebular processes could produce a pyroxene-poor protolith. The siderophile element patterns in these inclusions can be modeled as residual metal left after partial melting of the Fe-FeS system. The Y-794046 inclusion is compositionally and mineralogically different from the other inclusions. Abundances of refractory through moderately volatile lithophile elements are unfractionated relative to mean H chondrites. Similarly, abundances of refractory through moderately volatile siderophile elements are in H chondritic relative proportions, but at ∿10% the abundance of mean H chondrites. This cosmochemical signature is most simply modeled as a result of impact melting of an H chondrite target, with loss of ∿90% of the immiscible Fe-FeS melt.
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Geochemistry and origin of achondritic inclusions in Yamato-75097, -793241 and -794046 chondrites

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