KREEP petrogenesis revisited

Abstract

An experimentally determined crystallization sequence for a composition similar to the LKFM breccias and experimentally determined crystal/liquid distribution coefficients are used to calculate the evolution of the abundances of several key elements during the crystallization of LKFM liquids. Results indicate that the LKFM breccias are not themselves precursors to Apollo 15 igneous KREEP, but that compositions slightly poorer in Ti and Fe might be. Consideration of the evolution of Ti and Sm abundances during the differentiation of proposed whole-moon compositions suggests that it may be necessary to appeal to 'unorthodox' mechanisms such as magma hybridization or interaction of magma with genetically unrelated minerals within the lunar crust to produce KREEP-rich liquids having the observed Ti/Sm. It is concluded that the compositions of samples with Sm contents greater than those in LKFM are dominated by igneous processes, while samples with lower Sm contents may be mixtures of an anorthositic component, a Mg-rich Sm-poor component, and a component similar in composition to LKFM.