Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p41a..02b&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P41A-02
Physics
[1026] Geochemistry / Composition Of The Moon, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The dry Moon paradigm has been called into question by the recent discoveries of lunar materials with tens (glass spherules: Saal et al. 2008) to thousands (apatite: Greenwood et al. 2009; McCubbin et al. 2010; Boyce et al. 2010) of ppm H2O, including apatite with amounts of Cl and S that are also comparable to apatite from Earth. However, determining the original H content ([H]) from lunar melts based on analyses of glass spherules requires a large extrapolation to account for diffusive loss of H, and further uncertainties regarding magma generation. Similarly, the relationship between [H] in lunar apatite and the bulk lunar [H] is poorly constrained, and must include relevant uncertainties. A full treatment of this problem permits orders of magnitude variations in model bulk Moon [H]. Thus, it is possible that the presence of H-bearing apatite in lunar rocks may not in fact be in conflict with models that suggest that the Moon is very dry (e.g., Sharp et al. 2010). [H] in apatite constrains [H] in the bulk Moon only to the extent that we understand the origin and evolution of lunar magmas and the role of apatite in those processes. Even the least complicated model of lunar melt evolution is dependent on parameters such as degree of crystallization of the melt in equilibrium with the apatite (X), degree of melting of the source region in the lunar mantle (F), apatite-melt partitioning (D), and the fraction of the bulk Moon represented by the reservoirs that sourced the magma (B). This last parameter is particularly relevant for understanding lunar melts derived in part from a ‘KREEPy’ source. For D, the question is not only the confidence in the estimate of D, but also the degree to which the physical chemistry of experiments on terrestrial apatite are relevant to lunar apatite, which -- for example -- seems to incorporate more S than would be expected given the paucity of sulfate in lunar melts. The permitted ranges of X, F, and B are difficult to quantify, but are certainly too large to ignore. For D=0.25±0.15 (2σ), B=1, and non-correlated uniform distributions of X (0.94-0.99) and F (0.03-0.1), Monte Carlo modeling of measured [H] suggest that the 95% confidence interval of [H] in the lunar mantle ranges from ~300ppb to ~100ppm water (a factor of ~300). If B<1 (e.g., of the sources of apatite-bearing melts contain a KREEP component that is enriched in incompatible elements, including H), then the 95% confidence interval of [H] reported here could also be systematically too high by an order of magnitude or more. We conclude that even an order-of-magnitude estimate of bulk lunar [H] based on apatite [H] is impossible until the parameters relating these measurements to the bulk Moon are better constrained. The lower limits of model bulk lunar [H] may even be in agreement with both canonical views of a dry Moon and the model of Sharp et al. (2010). However, apatite is still by far the most concentrated reservoir of H in lunar samples, and there is much to learn from it. The utility of apatite in estimating bulk Moon [H] can be improved by partitioning experiments performed under lunar conditions, and by continued study of the lunar interior and lunar magma evolution.
Boyce Jeremy W.
Eiler John
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