Thorium anomaly on the lunar surface and its indicative meaning

Abstract

The Moon has been divided into three terranes: Procellarum KREEP Terrane (PKT), Feldspathic Highland Terrane (FHT), and South Pole-Aitken Terrane (SPAT), using globally measured Th and FeO. Many lunar evolution models have predicted that a lunar magma ocean will produce a residual layer enriched in incompatible elements such as K, REE, and P (i.e., KREEP) in the late age of crystallization; and that the distribution of thorium can be used as a proxy for determining the global distribution of KREEP. The thorium distribution in these three terranes is inhomogeneous. The highest concentration of thorium is in PKT, the medium concentration of thorium is in SPAT, and almost none in FHT. Then what is the specific distribution in each of the terrane and what enlightenment can it tell us? Here we present and describe the detailed thorium distribution in PKT, SPAT, and FHT and provide some information for the origin of asymmetries on the lunar surface.