Volatiles in the Lunar Volcanic Glasses, Evidence for the Presence of Indigenous Water in the Moon's Interior

Details Volatiles in the Lunar Volcanic Glasses, Evidence for the Presence of Indigenous Water in the Moon's Interior Volatiles in the Lunar Volcanic Glasses, Evidence for the Presence of Indigenous Water in the Moon's Interior

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.u13c..02s&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #U13C-02

Astronomy and Astrophysics

Astrophysics

1026 Composition Of The Moon

Scientific paper

The composition and origin of the lunar volatiles has been the subject of considerable interest and one of the most important unsolved questions regarding the formation of the Moon. Volatiles influence mantle melting, magma crystallization and volcanic eruption, and their abundances and spatial distribution provide important constraints on models for the thermal and chemical evolution of the Moon's interior. The general consensus today is that the Moon formed and evolved through a single or series of catastrophic heating events in which most of the highly volatile elements were either stripped or evaporated away. Hydrogen being the lightest element is thought to have been completely lost during this period. Most of the geochemical inferences about the deepest section of the moon have been based on studies of the most primitive melts erupted on the Moon's surface, the lunar volcanic glasses. We reported new volatile contents (CO2, H2O, F, S, Cl) for the lunar volcanic glasses, which by virtue of SIMS analysis provide improved detection limits by almost an order of magnitude. All volcanic glasses have CO2 below or within the 2 standard deviation of the detection limit for our runs (6 ppm CO2) and will not be considered further. The volatile contents after background correction have the following ranges: 4-46 ppm (±2 ppm 2σ) for H2O; 4-40 ppm (±0.2 ppm 2σ) for F; 115-576 ppm (±3 ppm 2σ) for S; and 0.06-2 ppm (±0.03 ppm 2σ) for Cl. Two outstanding features of the data is the significant correlation among H2O, Cl, F and S contents, and the clear relationship between the volatile and the major and trace element contents of the glasses. Most importantly, the correlations among the volatiles suggest that the measured H2O content in the volcanic glasses is indigenous. To test this hypothesis we determined concentration profiles for the volatiles within a single selected glass bead. Our data define concentration profiles with decreasing volatile contents from the core to the rim of the glass bead. This is especially significant for H2O, which decreases from ~30 ppm in the center to ~14 ppm in the margin of the bead, supporting the hypothesis of the indigenous origin of H2O subsequently affected by degassing processes during melt decompression and eruption. To our knowledge this is the first definitive confirmation of the presence of indigenous H2O in primitive lunar basalts. Our results suggest that, contrary to the prevailing ideas, the bulk Moon is not uniformly depleted in highly volatile elements, and the presence of water in particular must be included to constrain models for the thermal and chemical evolution of the Moon's interior.

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