Using Lunar Apatite to Assess the Volatile Inventory of the Lunar Interior

Potts, Nicola Jane (2017). Using Lunar Apatite to Assess the Volatile Inventory of the Lunar Interior. PhD thesis The Open University.



Recent in situ measurements of volatiles (H2O, Cl, F) within lunar melt inclusions, glass beads, and the mineral apatite have challenged the previously held notion that the Moon is depleted in volatiles. As the most widespread volatile-bearing phase, apatite has been the focus of many studies aiming to constrain the volatile inventory of the lunar interior. The thermodynamics of volatile partitioning into apatite, however, are poorly constrained. Here, Apollo mare basalts were investigated to provide an accurate composition for which apatite is found to crystallize under lunar conditions. This composition was used high temperature, high pressure experiments which constrained volatile partitioning between apatite and silicate melt. Combining experiments from this study, and those of the literature, an empirical model was developed to back-calculate melt volatile contents, at the time of apatite formation. Conservative estimates and preliminary findings suggest the amount volatiles in the at the time of apatite crystallization is highly variable but ranges up to 3.6 wt.% H2O, 5 wt.% F and 5.3 wt.% Cl. One of the reasons volatiles contents, in apatite and melt, vary so much is likely a result of degassing, as apatite is a late-stage mineral formed after ~95% melt solidification. Degassing, under lunar conditions, utilizing low-Ti and high-Ti compositions was constrained experimentally. The results from this investigation suggest that degassing is melt composition dependant and, as such, requires a thorough investigation of late-stage melt pockets to determine the amount of degassing that may occur prior to apatite growth. In addition, vapor-phase interaction was found to also alter volatile contents and δ37Cl isotope signatures in apatites from Apollo 14 samples. This vapor-phase is thought to be from volatile-release during a major impact event, similar to other old (sim4 Ga) samples found on the lunar nearside. To constrain whether this process was a global phenomenon samples from regions previously unexplored (including the lunar farside) are required. A mission scenario, optimising sample return from Schödinger basin is proposed here where volcanics and potentially mantle material could be collected. These samples would provide key insight into indigenous lunar mantle volatile contents and global differentiation processes.

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