Paper No. 239-6
Presentation Time: 9:50 AM
KREEP BASALTS: ORIGIN(S) AND EVOLUTION
Derived from the last liquid dregs of the lunar magma ocean, the KREEP chemical component has been recorded in samples from each Apollo sample return mission location. Named for its abundance of [K] Potassium, Rare Earth Elements and Phosphorous, it is enriched in all ITEs KREEP basalts, the least diluted form of KREEP, are hypothesized to have formed two different ways 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. Through a variety of tools, (petrographic microscopy, crystal size distributions, X-ray element mapping, and in situ techniques (e.g., EMPA, and LA-ICP-MS) we evaluate the petrographic history of lunar KREEP basalts 14155,11; 14160,214; 14310,25; 14431,2; 15386,3; 15434,181; and 72275,136. Using trace element analyses of pyroxenes, three distinct groups of equilibrium liquids were calculated for KREEP basalts. 1) highly enriched or “über”-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 possess Mg# < 80, they are typically OPX core compositions and likely to have formed first. 2) High-K KREEP-like pyroxene and plagioclase equilibrium liquids, possess chondritic to super-chondritic Zr/Hf and HFSE/LREE ratios. 3) LREE-depleted pyroxene-equilibrium liquids, possess sub-chondritic to super-chondritic Zr/Hf ratios and sub- chondritic to super-chondritic HFSE/LREE ratios and are thought to be later forming due to high-Fe and/or Ca content, and that they are found as rims/overgrowths on large pyroxene crystals and within the ground mass. Models were produced to replicate the ITE compositions found in the equilibrium liquids. One such model used supposed that a LREE-depleted orthopyroxenitic source region underwent two stages of partial melting first producing an über-KREEPy liquid and then a LREE depleted liquid as the degree of partial melting increased. These melts then mixed with each other, in varying amounts to reproduce the trace element signatures seen in the KREEP basalts.