KREEPY MOON ROCKS: KREEP BASALT PETROGENESIS

KREEP is a chemical component found on the Moon and is thought to be derived from the remains of the last liquid dregs of the lunar magma ocean. Named for the abundance of Potassium (K), Rare Earth Elements (REE), and Phosphorus (P), it is enriched in all incompatible trace elements (ITE) and has been found at every sample return mission location on the Moon. ‘Pure’ KREEP is not found in the sample collection, and the least diluted form of KREEP is found in the KREEP basalts. Two hypotheses purport to explain how KREEP basalt formed, one proposes that they are pristine endogenous melts of the lunar interior, and the other proposes that they formed by the impact-induced mixing of KREEP rich lithologies with Mg-rich ones. By using a variety of tools, (petrographic microscopy, crystal size distributions, X-ray element mapping, and in situ techniques such as electron probe microanalysis, and Laser Ablation Inductively Coupled Plasma Spectrometry) we evaluate the petrographic history of lunar KREEP basalts. Using trace element analyses of pyroxenes, three distinct groups of equilibrium liquids are calculated for KREEP basalts. 1) highly enriched or “Uber”-KREEP, is enriched in the LREE relative to high-K KREEP and possesses super-chondritic Zr/Hf ratios and the sub chondritic HFSE/LREE ratios, and are likely to have formed first. 2) High-K KREEP-like pyroxene-equilibrium liquids, possess chondritic to super-chondritic Zr/Hf and HFSE/LREE ratios, and derived from a variety of pyroxene compositions. 3) LREE-depleted pyroxene-equilibrium liquids, possess sub-chondritic to super-chondritic Zr/Hf ratios and sub- chondritic to super-chondritic HFSE/LREE ratios. Several models were produced to model the ITE elements within different samples. An orthopyroxenite source region underwent several stages of partial melting producing melts of decreasing ITE enrichment. These melts then mixed with each other, in varying amounts to reproduce the trace element signatures seen in the KREEP basalts. No one melting and mixing regime could explain all the observed KREEP basalt samples, implying that KREEP basalts may come from a hybrid source or from a variety of sources through magma mixing.