WAS THE ANGRITE PARENT BODY A LARGE PROTOPLANET IN THE EARLY SOLAR SYSTEM? NEW INSIGHTS FROM HIGH-PRESSURE CLINOPYROXENE PHENOCRYSTS IN ANGRITE METEORITES

Angrites are a group of critically silica-undersaturated, igneous achondrites. Meteorites belonging to the angrite suite have crystallization ages as old as 4564.2 Ma and are generally regarded as samples from a body that accreted and differentiated within inner solar system during the first several million years of solar system evolution. Although models for the petrogenesis of angritic magmas are still a matter of debate, it is generally accepted that the angrites represent crystallized silicate liquids that originated from an ancient, differentiated parent body that contained a sizeable metallic core . Of the many unresolved scientific questions about angrites, the problem of obtaining robust estimates for the size of the APB has perhaps been the most vexing. This difficulty is compounded by the possibility that the APB was likely fragmented or partially destroyed in the earliest stages of its history. The unusually Al-enriched clinopyroxene phenocrysts present in two porphyritic angrite meteorites (NWA 12774 and NWA 7812) suggest that these meteorites experienced an episode of high-pressure crystallization prior to eruption or emplacement into the shallow crust. Results from phase equilibrium modeling with MELTS software package and a thermodynamic analysis of the pressure-sensitive clinopyroxene-liquid equilibrium that governs the uptake of Al in the pyroxene M1-site collectively suggest that the phenocryst assemblages in these two angrites require equilibration pressures greater than 9 kbar. This result indicates that the APB was much larger than differentiated asteriods such as 4-vesta and may have had a radius that exceeded 1000 km.