DETERMINATION OF SIDEROPHILE ELEMENT CHARACTERISTICS THROUGHOUT LUNAR HISTORY: IMPLICATIONS FOR THE LUNAR MAGMA OCEAN AND LATE HEAVY BOMBARDMENT
Walker, Richard J
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Examining the chemical behavior of highly siderophile elements (HSE) in impact events and during planetary differentiation can illuminate geologic processes that have affected the Moon. This dissertation addresses impactor compositions during the putative late heavy bombardment and the chemical composition of the evolving lunar magma ocean at both the times of core segregation and crust formation. Concentrations of the HSE Re, Os, Ir, Ru, Pt, and Pd and 187Os/188Os isotopic compositions are reported for seven Apollo 17 and four Apollo 16 impact melt rocks. Most Apollo 17 samples examined here as in prior studies are characterized by very similar HSE signatures, consistent with a common impactor that had suprachondritic Ru/Ir, Pd/Ir, and Re/Os. In contrast to the Apollo 17 signature, the Apollo 16 impact melts have a wider range of Ru/Ir, Pd/Ir, and Re/Os. This compositional range might be the result of sampling at least three impactor signatures at this site. Experimentally determined plagioclase-melt partition coefficients are also presented. These partition coefficients are used to estimate the concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Plagioclase-melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may have concentrated in the residual liquid during fractional crystallization, due to their incompatibility. Experimentally determined metal-silicate partition coefficients are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The resulting lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Together, these new results are used to present a compositional timeline for the Moon between the crystallization of the lunar magma ocean and the late heavy bombardment.