Eucrite-type achondrites: Petrology and oxygen isotope compositions(dagger)

We report petrologic studies and oxygen isotope analyses of normal and anomalous eucrites, termed eucrite-type achondrites. Petrologically anomalous eucrite-type achondrites can have normal oxygen isotope compositions, and vice versa. Two basaltic eucrites with normal oxygen isotope compositions contain pyroxenes with anomalous Fe/Mn engendered by parent body processes acting on normal eucrites: solid-state reduction by S gas in EET 87542, and reduction during crystallization by magmatic S in QUE 94484. Cataclastic basaltic breccias PCA 82502 and PCA 91007 are paired (petrology, anomalous oxygen). Although isotopically like Pasamonte, they are petrologically distinct. We confirm the petrological and isotopic anomalies of cumulate gabbro EET 92023; likely formed by impact melting of mixed cumulate and basaltic materials. Many main group eucrites include plagioclases that retain near-liquidus compositions despite metamorphic overprinting. Stannern group eucrites contain more sodic plagioclase, which is consistent with the melt hybridization hypothesis for Stannern group magma formation. The lack of more calcic plagioclase suggests reactive exchange of the anorthite component of the primary melt with the albitic component of the crust. Asteroids that are modestly different in composition can produce virtually indistinguishable basalts, providing a ready explanation for the eucrite-type achondrite suite. Small stochastic variations in petrologic evolution can cause substantial differences in rocks produced on an asteroid.