Achondrites as witnesses of partial melting processes in planetesimals and planetary embryos: experimental constraints and new meteorite discoveries

Abstract

Achondrite meteorites are highly variable in composition: some are ultramafic (primitive achondrites) while others are basaltic (e.g. eucrites and angrites), to trachy-andesitic (e.g. GRA 06128 and Erg Chech 002). Those different groups correspond to the mantle and crust of planetary building blocks, respectively. They represent a unique opportunity to constrain the early melting processes that affected planetesimals and could have influenced the final composition of planets. I will describe two sets of low pressure (20-120 bars) melting experiments on chondritic compositions, simulating batch and fractional melting (0-30 wt.% melting). They are the first experiments in which the alkali elements were retained successfully during melting at low pressure. By comparing the experimental results with the meteorite record, I will argue that the melting processes, and the efficiency of melt extraction, varied greatly in achondrite parent bodies but that all planetesimals from the inner solar system (i.e. the non-carbonaceous reservoir) were initial rich in alkalis and other moderately volatile elements. The efficient extraction of silicate melts from the ureilite parent body suggest that highly volatile elements were also present during partial melting.