Noble Gas Reactivity in Planetary Interiors

Abstract

While the field of noble gas reactivity essentially belongs to chemistry, Earth and planetary sciences have brought a different perspective to the field. Indeed, planetary interiors are natural high pressure (P) and high temperature (T) laboratories, where conditions exist where bonding of the heaviest noble gases may be induced thermodynamically through volume reduction (Le Châtelier's principle). Earth and planetary sciences besides generate numerous and precise observations such as the depletion of the terrestrial and martian atmospheres in xenon, pointing to the potential for Xe to be sequestered at depth, potentially induced by its reactivity. More generally, this paper will review the advances on noble gas reactivity at the extreme P-T conditions found within planetary interiors from experiments and theoretical investigations. This review will cover the synthesis of cage compounds, stoichiometric oxides and metals, and non-stoichiometric compounds where noble gases are only minor or trace elements but could be essential in solving some Earth and planetary puzzles. An apparent trend in noble gas reactivity with P emerges. In the case of Xe which is the most documented, metals are synthesized above 150 GPa, i.e., at terrestrial core conditions, stoichiometric Xe-oxides between 50 and 100 GPa, i.e., in the P-T range of the Earth's lower mantle, but Xe-O high energy bonds may also form under the modest pressures of the Earth's crust (<1 GPa) in non-stoichiometric compounds. Most planetary relevant noble gas compounds found are with xenon, with only a few predicted helium compounds, the latter having no or very little charge transfer between helium and neighboring atoms.

Probing the partitioning behaviour of Xe using in situ X-ray synchrotron techniques at high P–T conditions

“…Xenon could have escaped from the atmosphere [8,9], and/or could be trapped at depth. The latter scenario is supported by the demonstration of Xe reactivity at much lower pressures as a trace/minor element with planetary materials such as olivine and quartz, through the formation of Xe-O bonds [10][11][12], also evidenced in deep crustal melts [13].…”

Hadean isotopic fractionation of xenon retained in deep silicates

“…Trapping-at-depth scenarios stem from the effect of high P to induce Xe chemistry at depth with planetary materials 16 . Xe may covalently bond to oxygen in pure oxides at the P-T conditions of the lower mantle 17 , it may also bond to nickel at the conditions of the Earth deep outer core 18 .…”

Editorial: “Changing the Perspective of the Noble Gas Reactivity”

“…The author beautifully shows the structure, stability and mode of formation of different such species that range from simple NgH + diatomic molecule to (H 3 + )(Ng) n . In a comprehensive review, Sanloup elaborates how high temperature and high pressure in planetary interiors induce interesting reactivity in Ng atoms ( Sanloup, 2020 ). This review shows different kind of cage compounds, stoichiometric oxides and metals, and non-stoichiometric compounds having Ng atoms (mostly Xe and in some cases He) which are formed in planetary interiors.…”