Early crust building enhanced on the Moon's nearside by mantle melting-point depression

Early magmatism on the Moon's nearside may have been enhanced by a geochemical anomaly lowering the melting point of the mantle source region, according to high-temperature experiments and thermal numerical modelling. The Moon's Earth-facing hemisphere hosts a geochemically anomalous region, the Procellarum KREEP Terrane, which is widely thought to have provided radiogenic heat for mantle melting from 3.9 to 1 billion years ago. However, there is no agreement on such a link between this region and the earliest pulse of post-differentiation crust-building magmatism on the Moon at 4.37 billion years ago; whether this early magmatism was global or regional has been debated. Here we present results of high-temperature experiments that show the nearside geochemical anomaly may have caused asymmetric early crust building via mantle melting-point depression. Our results demonstrate that the anomalous enrichment in incompatible elements of this nearside reservoir dramatically lowers the melting temperature of the source rock for these magmas and may have resulted in 4 to 13 times more magma production under the nearside crust, even without any contribution from radioactivity. From thermal numerical modelling, we show that radiogenic heating compounds this effect and may have resulted in an asymmetric concentration of post-magma-ocean crust building on the lunar nearside. Our findings suggest that the nearside geochemical anomaly has influenced the thermal and magmatic evolution of the Moon over its entire post-differentiation history.