A solar wind-derived water reservoir on the Moon hosted by impact glass beads

The past two decades of lunar exploration have seen the detection of substantial quantities of water on the Moon’s surface. It has been proposed that a hydrated layer exists at depth in lunar soils, buffering a water cycle on the Moon globally. However, a reservoir has yet to be identified for this hydrated layer. Here we report the abundance, hydrogen isotope composition and core-to-rim variations of water measured in impact glass beads extracted from lunar soils returned by the Chang’e-5 mission. The impact glass beads preserve hydration signatures and display water abundance profiles consistent with the inward diffusion of solar wind-derived water. Diffusion modelling estimates diffusion timescales of less than 15 years at a temperature of 360 K. Such short diffusion timescales suggest an efficient water recharge mechanism that could sustain the lunar surface water cycle. We estimate that the amount of water hosted by impact glass beads in lunar soils may reach up to 2.7 × 1014 kg. Our direct measurements of this surface reservoir of lunar water show that impact glass beads can store substantial quantities of solar wind-derived water on the Moon and suggest that impact glass may be water reservoirs on other airless bodies

Water Diffusion in Calc-Alkaline Silicate Melts.

Water diffusion in a series of calc-alkaline melts, including rhyolite, dacite, and haploandesite, was investigated at various temperatures, pressures, and water contents. FTIR microspectroscopy was used to analyze profiles of H2O concentration on quenched glasses. Molecular H2O (H2Om), rather than OH, is the dominating diffusion species. For rhyolite, diffusion couple experiments were carried out at 680-1902 K, 0.95-1.9 GPa, and 0.2-5.2 wt.% H2O in a piston-cylinder apparatus. Negative pressure effect on H2O diffusion was observed. With literature data incorporated, H2Om and total H2O (H2Ot) diffusivity models in rhyolite were constructed for 676-1902 K, 0-1.9 GPa, and 0.1-7.7 wt.% H2O. For dacite, diffusion couple experiments were performed at 786-893 K, 0.48-0.95 GPa, and 0-8 wt.% H2O. H2Om and H2Ot diffusivity models in dacite were presented for 786-1798 K, 0-1 GPa, and 0-8 wt.% H2O. For haploandesite, hydrous melts with ≤2.5 wt.% H2O were dehydrated at 743-873 K and 0.1 GPa Ar atmosphere in cold-seal pressure vessels. At a given water concentration and temperature, there is more OH in haploandesite than in rhyolite or dacite. At ≤873 K, H2Ot diffusivity increases from andesite to dacite to rhyolite, which is in contrary to the trend at superliquidus temperatures. These water diffusivity models can be applied to various magmatic and volcanic processes involving the transport of water, such as bubble growth in explosive volcanic eruptions. In addition, the exchange of oxygen isotopes between coexisting minerals during cooling was examined. The mineral pair with the largest isotopic fractionation (PLIF) can bracket their apparent equilibrium temperature (Tae) within their Dodson closure temperatures. This special feature of PLIF may be used to constrain the thermal history of slowly cooled plutonic and metamorphic rocks.Ph.D.GeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/62368/1/hni_1.pd

Diffusion Data in Silicate Melts

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In situ observation of crystal growth in a basalt melt and the development of crystal size distribution in igneous rocks

International audienceTo understand the solidification processes of natural magma and the texture evolution of igneous rocks, we have carried out in situ observation of the crystallization of a high-K basaltic melt cooling from ~1,240 °C in a moissanite cell. In a series of experiments with different thermal history, olivine or clinopyroxene (cpx) appeared as the liquidus phase before the formation of plagioclase. During cooling at 100 °C/h, the morphology of olivine and cpx transited from tabular to hopper habit. To first order approximation, crystal grow rate (2 × 10−9 to 7 × 10−9 m/s for olivine and 6 × 10−9 to 17 × 10−9 m/s for cpx), probably limited by chemical diffusion, is proportional to crystal size. In one experiment dominated by olivine crystallization, the good image quality allows the analysis of texture evolution over an extended period. Nucleation of olivine occurred only in a narrow temperature and time interval below the liquidus. Two-dimensional length- and area-based crystal size distributions (CSDs) show counterclockwise rotation around axes of 8 μm and 100 μm2, which is consistent with the proportionate crystal growth. Both CSDs and direct observation show the dissolution of small crystals and Ostwald ripening. These data suggest that conventional analyses of crystal size distributions of igneous rocks may be in error—the slope of the CSD cannot be interpreted in terms of a uniform growth rate, and the intercept with the vertical axis does not correspond to a nucleation density