Petrology of the unbrecciated eucrites

Abstract

Twenty-nine unbrecciated eucrites have been thoroughly characterized in terms of the petrologic factors that affect their spectra, such as mineral chemistry, modal adundances, grain sizes, and textures. We have conducted a combined petrologic and spectral study designed to provide insight into the petrogenesis of the basaltic crust of Vesta and the variety of rock-types that exist within it, as well as aid in the petrologic interpretation of spectra to be collected by the Dawn orbiting spacecraft. This paper details the petrology part of the study. Unbrecciated eucrite samples were selected to avoid the complications of lithologic mixing in the accompanying spectral study. A wide variety of textural types are seen within the basaltic eucrites, encompassing quenched, coarse-grained, and granoblastic samples. Zoned pyroxenes in eucrites and those that preserve a history of initial rapid cooling are rare. Nearly all eucrite samples have been thermally metamorphosed and would commonly be classified as equilibrated; however, this term reflects only the quadrilateral (Mg, Fe, and Ca) compositions of pyroxenes, and considerable variations are seen within the minor elements (Al, Ti, and Cr) in pyroxenes as well as plagioclase compositions. Determination of both pyroxene and plagioclase compositions together with pyroxene geothermometry provides a better estimate for the relative degree of thermal metamorphism a eucrite has experienced. The petrologic differences observed here might allow different eucrites to be distinguished spectrally. This is especially true for the varying pyroxene compositions as the spectra of eucrites are dominated by absorption features attributed to pyroxene.