The polymict eucrites

Abstract

The polymict eucrites are a suite of about 14 polymict achondritic meteorites that contain at least 90% (vol) of eucritic material and are found both in Antarctica and elsewhere. They are part of a continuum of polymict breccias that includes howardites and polymict diogenites. Rare gas data suggest that some are gas‐rich regolith breccias. The clasts contained in polymict eucrites sample a variety of lithologies from their parent body and include eucritic clasts, alkali‐enriched eucrites, pyroxene‐cumulate eucrites, two‐pyroxene mafic rocks, possibly feldspar‐cumulate eucrites, diogenites, fayalitic lithologies, and a variety of other mineral clasts. The eucritic clasts are probably samples of near surface lithologies on the parent body, but the presence of two‐pyroxene mafic rocks is considered to be evidence of magmatic activity at pressures in excess of 1 kbar in the parent body. Partial melt models may explain some of the diversity among the clasts, but cumulus processes are believed to have been important. If pressures greater than 1 kbar are confirmed, then the basaltic achondrite parent body (BAP) must have a diameter in excess of 200 km. The variety of lithologies suggests that not all basaltic achondrites are samples of igneous rocks formed by single‐stage melting of a chondritic asteroid but are instead samples of an evolved and diverse crust produced by serial magmatism.