Solubility of water in lunar basalt at low pH2O

Abstract

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractWe report the solubility of water in Apollo 15 basaltic 'Yellow Glass' and an iron-free basaltic analog composition at 1 atm and 1350 °C. We equilibrated melts in a 1-atm furnace with flowing H 2 /CO 2 gas mixtures that spanned ~8 orders of magnitude in fO 2 (from three orders of magnitude more reducing than the iron-wüstite buffer, IW−3.0, to IW+4.8) and ~4 orders of magnitude in pH 2 /pH 2 O (from 0.003 to 24). Based on Fourier transform infrared spectroscopy (FTIR), our quenched experimental glasses contain 69-425 ppm total water (by weight). Our results demonstrate that under the conditions of our experiments: (1) hydroxyl is the only H-bearing species detected by FTIR; (2) the solubility of water is proportional to the square root of pH 2 O in the furnace atmosphere and is independent of fO 2 and pH 2 /pH 2 O; (3) the solubility of water is very similar in both melt compositions; (4) the concentration of H 2 in our iron-free experiments is <~4 ppm, even at oxygen fugacities as low as IW−2.3 and pH 2 /pH 2 O as high as 11; (5) Secondary ion mass spectrometry (SIMS) analyses of water in iron-rich glasses equilibrated under variable fO 2 conditions may be strongly influenced by matrix effects, even when the concentration of water in the glasses is low; and (6) Our results can be used to constrain the entrapment pressure of lunar melt inclusions and the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads. We find that the most water-rich melt inclusion of Hauri et al.(2011) would be in equilibrium with a vapor with pH 2 O ~3 bar and pH 2 ~8 bar. We constrain the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads to be 0.0005 bar and 0.0011 bar respectively. We calculate that batch degassing of lunar magmas containing initial volatile contents of 1200 ppm H 2 O (dissolved primarily as hydroxyl) and 4-64 ppm C would produce enough vapor to reach the critical vapor volume fraction thought to be required for magma 2 fragmentation (~65-75 vol. %) at a total pressure of ~5 bar (corresponding to a depth beneath the lunar surface of ~120 m). At a fragmentation pressure of ~5 bar, the calculated vapor composition is dominated by H 2 , supporting the hypothesis that H 2 , rather than CO, was the primary propellant of the lunar fire fountain eruptions. The results of our batch degassing model suggest that initial melt compositions with >~200 ppm C would be required for the vapor composition to be dominated by CO rather than H 2 at 65-75 % vesicularity.

Model for the Origin, Ascent, and Eruption of Lunar Picritic Magmas

“…This gas volume is not sufficient to cause magma fragmentation (Papale 1999;Gonnermann and Manga 2013) at the Stage 2-3 transition (~500 m depth) of an OPGM with the initial C contents considered (4-50 ppm). Using the same MI data, and their O-H speciation and solubility data, Newcombe et al (2017) reach the same conclusion. They determine that the O-H gas phase alone would have caused fragmentation at a pressure of ~5 bars for batch (closed-system) degassing C-O-H species in the OPGM.…”

“…The velocity of the mixture at the vent could be more than twice this estimate given the addition of large volumes of H 2 O and S species transferred to the gas phase in the first 500 m of the Stage 3 degassing. Additionally, a large fraction (50-60%) of the H 2 O added to the gas in Stage 3 would have been present as H 2 (Newcombe et al 2017), which would decrease the density of the gas phase by a factor of 5 relative to a gas with just H 2 O. This would significantly increase the velocity of the ascending magma during Stage 3 both below the surface, and in the expanding hot gas cloud carrying the beads away from the vent.…”

“…1; Table 1). Given the H was dissolved as OH in the melt (Newcombe et al 2017) and is calculated to be present primarily as H 2 O and H 2 in the gas (Gaillard and Scaillet 2009), this gas formation would tend to produce melt oxidation (4OH = H 2 O + H 2 + 1.5O 2 ). Later in Stage 3, diffusive OH loss might be replaced by selective H 2 diffusion ) contributing further to oxidation of the melt.…”

Controlled Cooling‐Rate Experiments on Olivine‐Hosted Melt Inclusions: Chemical Diffusion and Quantification of Eruptive Cooling Rates on Hawaii and Mars

“…The single experimentally homogenized inclusion H7XC1 we measured has a water content of 383–388 ppm, suggesting that the inclusion lost most of its water during the isothermal homogenization step. This value is higher than but on the same order as the calculated solubility of ~250 ppm H 2 O in anorthite‐diopside liquid at the p H 2 O (≈0.12 bars) of the furnace atmosphere at the experimental conditions (Newcombe et al, ).…”

A Low O/Si Ratio on the Surface of Mercury: Evidence for Silicon Smelting?

“…At mantle pressures (>1 GPa), the solubility of H in silicate liquids is not markedly lower under highly reducing conditions compared to oxidizing conditions: however, the speciation of H is affected by oxygen fugacity (i.e., the H 2 /H 2 O abundance ratio is proportional to the f H 2 / f H 2 O ratio; Hirschmann et al, ). It also follows that H 2 O would be a very minor vapor species in favor of H 2 given the relationship between f H 2 / f H 2 O ratios and oxygen fugacity (Hirschmann et al, ; Newcombe et al, ; Sharp et al, ; Zolotov et al, ). In fact, the preferential loss of H 2 from H‐rich reduced magmas could lead to oxidation of the melt (Sharp et al, ), which would work against stabilizing metallic Fe or Si.…”