Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51c1438e&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P51C-1438
Mathematics
Logic
[5430] Planetary Sciences: Solid Surface Planets / Interiors, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The young Moon appears to have experienced a magma ocean of some depth, which resulted in the formation of an anorthosite flotation crust. Recent work has shown 4 to 46 ppm water in lunar volcanic glasses (Saal et al., 2008), and new work shows traces of water in magmatic accessory minerals (Boyce et al., 2010; McCubbin et al., 2010). Any water in the lunar magma ocean would have partitioned in minute quantities into the solidifying cumulate minerals, all of which are nominally anhydrous phases. Water, and all other incompatible elements, would therefore be progressively enriched in the evolving magma ocean liquids as solidification progresses. If the picritic glasses contained 500 ppm water before degassing and they were a 20% melt of their source region, then the source region contained about 100 ppm of water. This cumulate water content requires an initial magma ocean with 1 wt% water, probably an unrealistic value. The highest water contents would be in KREEP materials, which in such a magma ocean would contain nearly 5 wt% water; KREEP basalts, however, along with almost all other lunar samples, contain iron metal. To maintain an equilibrium between water and iron metal, a high partial pressure of hydrogen is required. We present the petrologic constraints on water in the lunar interior and the thermodynamic calculations that include these constraints, and conclude that unreasonably high hydrogen partial pressures are required to allow these water levels and iron metal to coexist. We suggest that the lunar basalts and glasses may have obtained water from reservoirs in the lunar conductive lid, not from their melting source regions. The highest water contents in the magma ocean, and therefore the time of highest volatile degassing, were present at the end of magma ocean solidification when residual liquids were immediately beneath the conductive anorthosite lid. This lid, therefore, may have been fluxed with volatiles and contain heterogeneous incompatible element and volatile reservoirs. Terrestrial magmas sometimes obtain volatiles while passing through the crust; the same process may explain trace water in igneous rocks on the Moon.
Elkins-Tanton Linda T.
Grove Timothy L.
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