Earth-Moon refractory element similarity constrains a thoroughly-mixed Moon-forming disk

Abstract

The canonical Moon-forming giant-impact models allow for substantial chemical differences between the bulk silicate Moon and Earth due to incomplete mixing of the impactor and the proto-Earth. In comparison, the emerging high-energy giant-impact (Synestia) model requires the refractory element compositions of the Earth and Moon to be nearly identical, owing to extensive chemical homogenization of the Moon-forming disk in a vigorously mixed silicate fluid. These distinct chemical predictions make the lunar refractory element composition crucial for testing Moon-formation hypotheses, yet it remains highly controversial and necessitates new approaches to resolve. In this study, we develop a novel method using the composition of pristine lunar anorthosite samples to constrain the Moon's refractory lithophile element compositions. We obtained a very close match of refractory major and trace element compositions for the lunar magma ocean model, suggesting indistinguishable refractory element abundances between the bulk silicate Moon and Earth. This striking refractory element similarity is difficult to reconcile with the relatively poor mixing conditions of the canonical giant-impact models. The compatibility of this result with disk equilibration models other than the Synestia has yet to be quantitatively verified. Our results further constrain that the formation of the Earth-Moon system requires a thoroughly-mixed protolunar disk of chemical and isotopic homogenization with an initially fully-molten Moon, as enabled by emerging models like the Synestia.