The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.

No abstract

[1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, f O2 = NNO + 2, H 2 O-saturated conditions for an equilibration period, decompressed to final pressures, P f , ranging from 175 to 5 MPa, and then held for 0.3-931 hours. According to the plagioclase liquidus curve in P H2O -T space for the relevant melt composition, these decompressions impose effective undercoolings, ÁT eff , of 34-266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 P f < 150 MPa (ÁT eff = 34-93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < P f < 50 MPa (ÁT eff = 125-241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low P f . Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ÁT eff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at $100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleationdominated regime.INDEX TERMS: 3630 Mineralogy and Petrology: Experimental mineralogy and petrology, 3640 Mineralogy and Petrology: Igneous petrology, 8434 Volcanology: Magma migration, 8439 Volcanology: Physics and chemistry of magma bodies;