Experimental constraints on the solidification of a hydrous lunar magma ocean

Abstract

The identification of hydrogen in a range of lunar samples and the similarity of its abundance and isotopic composition with terrestrial values suggest that water could have been present in the Moon since its formation. To quantify the effect of water on early lunar differentiation, we present new analyses of a high-pressure, high-temperature experimental study designed to model the mineralogical and geochemical evolution of the solidification material equivalent to 700 km deep lunar magma oceans first reported in Lin et al. (2017a). We also performed additional experiments to better quantify water contents in the run products. Water contents in the melt phases in hydrous run products spanning a range of crystallization steps were quantified directly using a secondary ion mass spectrometry (SIMS). Results suggest that a significant but constant proportion (68 AE 5%) of the hydrogen originally added to the experiments was lost from the starting material independent of run conditions and run duration. The volume of plagioclase formed during our crystallization experiments can be combined with the measured water contents and the observed crustal thickness on the Moon to provide an updated lunar interior hygrometer. Our data suggest that at least 45-354 ppm H 2 O equivalent was present in the Moon at the time of crust formation. These estimates confirm the inference of Lin et al. (2017a) that the Moon was wet during its magma ocean stage, with corrected absolute water contents now comparable to estimates derived from the water content in a range of lunar samples.

Employing Magma Ocean Crystallization Models to Constrain the FeO Content of the Moon

“…They found that olivine is the liquidus phase for all relevant LMO compositions and pressure ranges in the early phase of magma ocean crystallization. Lin et al (2017Lin et al ( , 2020 performed experiments simulating fractional crystallization of a 700 km deep magma ocean (corresponding to crystallization pressures of 0 -3 GPa) with LPUM composition and various water contents, demonstrating that the water content influences the plagioclase/clinopyroxene ratio that crystallizes from the melt, as it has been shown in earlier studies (e.g. Sisson and Grove, 1993).…”

Oxygen controls on magmatism in rocky exoplanets

“…Studies of the effect of f O 2 on magma oceans have, to date, been limited to quantifying the link between f O 2 variations and changes in the redox states of the major cation iron ( 13 , 26 , 46 ), or linked directly to models of gas speciation in atmospheres influenced by magma ocean outgassing ( 35 , 36 , 47 ). Variations in solidus and liquidus temperatures of cooling magma oceans have largely been assessed in terms of variations in volatile abundances ( 48 , 49 ). In this study we provide experimental evidence for a direct effect of oxygen fugacity on rock melting properties, in the absence of valence state variations in iron and in the absence of volatiles.…”

Interiors of Earth-Like Planets and Satellites of the Solar System

“…Together with the geophysical techniques discussed in the following section, they form a powerful method to elucidate planetary interiors, even in the case of planets and moons for which we will never be able to obtain direct deep subsurface samples. Lin et al 2020). This experiment illustrates conditions at which the lunar mantle would be partially molten and partially solidified.…”

“…Fig.2a Back-scatter electron (BSE) microscope image of a polished section through the run product of a high-pressure experiment mimicking the interior of the Moon (afterLin et al 2020). This experiment illustrates conditions at which the lunar mantle would be partially molten and partially solidified.…”