New constraints on Xe incorporation mechanisms in olivine from first-principles calculations

Abstract

International audienceStorage of Xe at depth in silicate minerals has recently been proposed to explain the low Xe abundance in the Earth’s and Mars’ atmospheres compared to other noble gases (the so-called ‘Missing Xenon’ issue). Evidences for incorporation, and thus reactivity of Xe in olivine at high pressure and high temperature are based on variations in cell parameters and the appearance of a new Raman band. To constrain the, so far only hypothetical, Xe incorporation mechanism in olivine, we theoretically investigated models of Xe-bearing olivine using density functional theory. Three types of incorporation mechanisms are tested: Xe for Si and Xe for Mg substitutions, and interstitial Xe. Xe for Si substitution, implying an oxidation of Xe, is found to be the only mechanism consistent with experimental observations, leading to an increase of cell parameter a and the appearance of a new Raman band around 720–750 cm−1 associated with XeO stretching vibrations. Raman spectroscopy makes it possible to identify Xe incorporation site, even at low Xe content, due to high Xe polarizability. An estimation of Xe content in olivine, based on present work and previous in situ experimental results, shows that up to 0.4 at.% Xe could be stored in olivine at depth

The Xe‐SiO2 System at Moderate Pressure and High Temperature

“…The significantly smallest excess volume is obtained in the case of Xe for Si substitution, with even a decrease at 0 GPa and for 0.41 at% Xe. In line with our previous study on Xe‐bearing olivine (Crépisson et al, ), this small excess volume and the formation energies suggest that Xe for Si substitution is the likeliest Xe incorporation mechanism in quartz.…”

“…As reported for olivine (Crépisson et al, ; Sanloup et al, ), the unit cell volume of quartz is systematically larger by up to 4.2%, in the presence of Xe, than the volume predicted by the equations of state of plain quartz based on in situ X‐ray diffraction data from both XRD1 and XRD2 data sets (Table ). The presence of Ar is unlikely to affect the unit cell volume of quartz as Ar is classically used as a P ‐transmitting medium.…”

“…All calculations were done within the DFT and using the generalized gradient approximation (GGA) with Perdew‐Burke‐Ernzerhof (PBE) pseudopotentials of Perdew et al (), with the PWscf code of the Quantum espresso package http://www.quantum-espresso.org/; Giannozzi et al, ). The same norm‐conserving pseudopotentials (Troullier & Martins, ) as in Crépisson et al () are used. Electronic wave functions (charge density) are expanded up to an energy cutoff of 80 (480) Ry.…”

“…High Xe reactivity has been proved, with the synthesis of several Xe compounds (Grochala, ), among which Xe oxides at ambient conditions (Brock & Schrobilgen, ), at high P ‐ T conditions (Dewaele et al, ), and Xe‐bearing perovskite (Britvin et al, ). Furthermore, Xe can be incorporated as a minor or trace element in SiO 2 phases (Sanloup et al, , ) and in olivine (Crépisson et al, ; Sanloup et al, ) at upper crustal and mantle conditions with up to 0.4 at% Xe possibly stored in olivine. However, there are still few data regarding Xe incorporation in minerals, due to Xe high volatility, which requires in situ investigation at high pressure ( P ) and temperature ( T ).…”

Perspectives on Atmospheric Evolution from Noble Gas and Nitrogen Isotopes on Earth, Mars & Venus

“…Recent experimental studies propose that missing Xe might be stored inside silicate structures in the deep Earth (Crépisson et al 2018;Crépisson et al 2019;Sanloup 2005 (Pepin 1991). This Xe component has been nicknamed U-Xe and has resisted decades of investigation (Pepin 1994 Fig.…”