Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust

Abstract

From correlations of Sm concentration and Sm Eu ratio with Th concentration for a large number of polymict samples from various locations in the lunar highlands and the value of 0.91 μg/g for the mean Th concentration of the highlands surface crust obtained by the orbiting gamma-ray experiments, we estimate the mean concentrations of Sm and Eu in the lunar surface crust to be between 2 and 3 μg/g Sm and 0.7 and 1.2 μg/g Eu. The compositional trends indicate that there is no significant enrichment or depletion of Eu, on the average, compared to Sm relative to chondritic abundances; i.e., there is no significant “Eu anomaly” in average upper crust. Although rich in plagioclase (~70%), the upper crust does not offer evidence for a gross vertical separation of plagioclase from the final liquid from which it crystallized. This and the chondritic ratio of Eu Al in average highlands material imply that the net effect of the processes that led to formation of the lunar crust was to put most of the Al and incompatible elements in the crust. The concentration of Eu in most polymict samples from the highlands exceeds that which can be supplied by the fraction of KREEP necessary to supply the incompatible trace elements plus the fraction of plagioclase necessary to supply the Al, if the plagioclase has the composition of that in ferroan anorthosite. Among plutonic rocks, only plagioclase in rocks from the magnesian suite can supply the “excess” Eu in the polymict rocks. Owing to the intermediate value of the mean Mg Fe ratio of the crust, a significant fraction of the mafic rocks of the lunar highlands must have lower Mg Fe ratios than the norites and troctolites of the magnesian-suite of plutonic rocks. A large fraction of the plagioclase in the lunar crust is associated not with ferroan anorthosite, but with more mafic rocks ( e.g., norite, anorthositic norite). There is little evidence in the Eu data that the lunar crust ever consisted of a thick shell of nearly pure plagioclase, as envisioned in some formulations of the “magma ocean” model of its formation.