Simple Models for Trace Element Fractionation during Melting and Melt Migration in an Upwelling Heterogeneous Lunar Mantle

Introduction: It has long been recognized that lunar mantle is chemically, isotopically, and lithologically heterogeneous. The heterogeneities are ancient and were formed during large-scale differentiation events, such as the solidification of a globally distributed lunar magma ocean (LMO) and subsequent lunar cumulate mantle overturn [1-3]. Mixing of distinct early and late LMO cumulates results in a hybrid cumulate mantle [4]. Evidence for a heterogeneous lunar mantle is overwhelming, including major element data (low-Ti to high-Ti basaltic samples), incompatible trace element patterns (e.g., REE), selected incompatible trace element ratios (e.g., La/Sm, La/Yb, Fig. 1), and isotopic variations and co-variations (Sr, Nd, Hf, and Pb isotopes) [1-3]. At least three distinct components in the (overturned) lunar mantle have been identified: early harzburgitic cumulate, late ilmenite-bearing cumulate, and KREEP, although the lithologies of the latter two and the length scale and spatial distribution of these mantle components are not known.