Auto-metasomatism of the western lunar highlands: Result of closed system fractionation and mobilization of a KREEPy trapped liquid

The discovery of REE-rich phosphates (dominantly whitlockite) in pristine, non-mare rocks of the western lunar nearside (Apollo 14, Apollo 12, and most recently, Apollo 17) has created a paradox for lunar petrologists. These phases are found in feldspar-rich cumulates of both the Mg-suite and the Alkali suite, which differ significantly in their mineral chemistries and major element compositions. Despite the differences in host rock compositions, whitlockites in both suites have similar compositions, with LREE concentrations around 21,000 to 37,000 x chondrite. Simple modeling of possible parent magma compositions using the experimental whitlockite/liquid partition coefficients of Dickinson and Hess show that these REE concentrations are too high to form from normal lunar magmas, even those characterized as 'urKREEP.'