Trace element analyses of plagioclase from troctolite 76535 and implications for the petrogenesis of the lunar highlands Mg‐suite

Abstract We used trace element analyses of plagioclase from Mg‐suite troctolite 76535 to estimate the Rare Earth Element (REE) concentrations of its parental liquid and assess the feasibility of an urKREEP contribution to the Mg‐suite parental liquid. We measured 33 trace elements in 76535 plagioclase separates. Our measurements revealed enrichments in incompatible elements consistent with previous analyses. Using the measured REE concentrations, we estimated the REE concentrations of the unfractionated Mg‐suite parental liquid using a RhyoliteMELTS‐based forward model. Compared to chondritic concentrations, the Mg‐suite parental liquid is ~100 times more enriched in light REEs and ~10 times more enriched in heavy REEs. We sought to explore the feasibility of reproducing these enrichments in the parental liquid through assimilation of urKREEP by a partial melt of rising LMO cumulates during cumulate mantle overturn. We show that these enrichments can be reproduced by a 30%–50% addition of fully molten urKREEP to the LMO cumulate melt, if the LMO cumulate melt and urKREEP are in thermal equilibrium with each other. However, the Mg# of these mixtures (57–68) is too low to produce the most Mg‐rich olivine (Fo 91) observed in Mg‐suite troctolites. Alternatively, assuming that the LMO cumulate melt and urKREEP are in thermal disequilibrium, we reproduced both the REE abundances and Mg# of the Mg‐suite parental liquid with only a 10% addition of the urKREEP partial melt. These results support the feasibility of urKREEP assimilation as a mechanism for generating the incompatible element enrichments in Mg‐suite magmas while preserving their major element chemistry.