Using Apatite to Assess Volatile Contents of Primary Lunar Magmas: Potential Pitfalls

Details Using Apatite to Assess Volatile Contents of Primary Lunar Magmas: Potential Pitfalls Using Apatite to Assess Volatile Contents of Primary Lunar Magmas: Potential Pitfalls

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p41a..05n&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P41A-05

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[1060] Geochemistry / Planetary Geochemistry, [3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [5410] Planetary Sciences: Solid Surface Planets / Composition, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

Magmas reaching the low pressure lunar surface have potentially lost volatiles by exsolution of a fluid or vapor + liquid aqueous phases. With the recent SIMS determination of water, F, and Cl abundances in apatite from lunar samples [1-3], of primary interest is the use of these data to estimate volatile abundances in the lunar magmatic source regions for known lunar lithologies. However, textural evidence from the samples investigated suggests that the apatites analyzed are late-crystallizing phases. As such, the apatites may reflect the composition of a host melt that had already undergone fluid loss during decompression and/or crystallization. Although much remains unknown about the effect of mixed volatiles on the pressure of fluid exsolution or on fluid compositions for lunar basalts, the low water contents suggest that water will become significant in a fluid only at low pressure. At low pressures, the high water content of the vapor phase can result in significant dehydration of the melt if this phase is lost before the surface is reached. Furthermore, the differing effects of melt composition on F and Cl partitioning between melt and fluid suggest that apatite crystallizing on or near the surface may reflect dewatered, F-enriched late-stage residual melt compositions. First-order computations of crystallization pathways leading to fluid exsolution are being conducted in order to assess the effects of fluid exsolution and loss on melt volatile contents and on the volatile content of apatite crystallizing from the affected melts. Electron microprobe data on volatile heterogeneity in lunar apatites from a variety of lunar lithologies are being compared to these pathways in order to determine the possibility of fluid loss and therefore, the likelihood of underestimation of primary magma volatile contents by direct use of apatite volatile contents. References [1] McCubbin et al. (2010) Proc.Nat. Acad. Sci www.pnas.org/cgi/doi/10.1073/pnas.1006677107 . [2] Boyce et al. 2010 Nature 466 doi:10.1038/nature09274 [3] Greenwood et al (2010) Proc. Lunar Sci. Conf. 41th, 2439.

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