We report results from multi-anvil (MA) and laser-heated diamond anvil cell (LH-DAC) experiments that synthesize high-pressure phases, including bridgmanite, ferropericlase, stishovite, and ultramafic liquid, in the presence of an argon-rich fluid. The goal of the experiments is to constrain the equilibrium distribution of argon in magma ocean environments. Argon concentrations in LH-DAC experiments were quantified by electron microprobe analysis, while argon concentrations in MA experiments were quantified by laser-ablation mass spectrometry and electron microprobe analysis. Our LH-DAC experiments demonstrate that argon solubility in ultramafic liquid is near or above 1.5 wt.% at conditions between 13–101 GPa and 2300–6300 K. Argon concentrations in bridgmanite and ferropericlase synthesized in LH-DAC experiments range from below detection to 0.58 wt.%. Argon concentrations in bridgmanite and ferropericlase synthesized in MA experiments range from below detection to 2.16 wt.% for electron microprobe measurements and laser-ablation measurements. We interpret this wide range of argon concentrations in minerals to reflect the variable presence of argon-rich fluid inclusions in analytical volumes. Our analyses therefore provide upper limit constraints for argon solubility in high-pressure minerals (<0.015 wt.%) across all mantle pressures and temperatures. The combination of relatively high argon solubility in ultramafic liquid (∼1.5 wt.%) and low argon solubility in minerals implies argon incompatibility ( < 0.01, < 0.01) during magma ocean crystallization and that the initial distribution of argon, and likely other neutral species, may be controlled by liquids trapped in a crystallizing magma ocean. We thus predict a basal magma ocean would be enriched in noble gases relative to other regions of the mantle. Moreover, we predict that the noble gas parent-daughter ratio of magma ocean cumulates pile will increase with crystallization, assuming refractory and incompatible behavior for parent elements.

Colin R.M. Jackson, Curtis D. Williams, Zhixue Du, Neil R.Bennett, Sujoy Mukhopadhyay, Yingwei Fei, “Incompatibility of argon during magma ocean crystallization,” Earth Planet. Sci. Lett. 553, 116598 (2021). DOI: 10.1016/j.epsl.2020.116598 abstract

Representative images of MA experiments (MA_NG_EXP4). a) Secondary electron image showing texture of polished surfaces. Stishovite is high relief and Brg is low relief. b) Distribution of Si (EDS with focused 10 kV beam) highlights distribution and size of FP, stishovite, and Brg. c) Argon map shows distribution of micron-scale, Ar-rich “hotspots” in sample and the relative lack of Ar beyond hotspots. d) Horizontal transect across Ar map that intersects pair of Ar hotspots. Location of transect is given by pair of black markers on c).