Trace Element Analyses of Plagioclase from Troctolite 76535 and Implications for Mg-suite Petrogenesis

Certain Mg-suite samples display enrichment in incompatible elements, likely resulting from the assimilation of the material that crystallized at the very late stages of magma ocean (ur-KREEP). This study uses trace element analyses of plagioclase separates from sample 76535 to estimate the Rare Earth Element (REE) concentration of the Mg-suite parental liquid and assess the extent of contribution from ur-KREEP. Thirty-three trace elements, including REEs, were measured in the separates and the measured REEs reflect magmatic conditions being free from subsolidus alteration. The Mg-suite parental liquid was estimated using these REE data as targets for a Python-based forward model which employs a RhyoliteMELTS-defined liquid line of descent. The estimated parental liquid shows REE enrichments of 200 times chondritic levels for Light REEs and 20 times for Heavy REEs. Mixing models between the REE compositions of a potential Mg-suite primary liquid and modeled ur-KREEP indicate that 30-50% assimilation of ur-KREEP is required to reproduce the observed REE concentrations in the Mg-suite parental liquid. We demonstrate an approach to determine the petrogenesis of a sample by characterizing a very limited quantity of grains, in an effort to maximize the scientific output from current and future returned samples such as Artemis.