The Xe‐SiO₂ System at Moderate Pressure and High Temperature

Abstract

Xenon (Xe), the heaviest of the stable noble gases, is missing by a factor of 20 relatively to other noble gases, when comparing the Earth's and Mars's atmospheres with chondrites. In this work, the possibility of Xe retention in quartz, a major mineral of the continental crust, is tested. The Xe-SiO 2 system is investigated from 0.7 to 2.7 GPa and up to 1900 K, by in situ X-ray powder diffraction and infrared spectroscopy. Experimental data are complemented by ab initio calculations to retrieve Xe incorporation mechanisms. An excess of quartz unit cell volume up to 4.2% is observed, consistent with a Xe for Si substitution in the quartz structure. Xe has a linear oxygen environment composed of two O atoms located at 1.98-2.00 Å from the Xe atom. Moreover, a new phase is evidenced above Xe melting curve at 0.8-1.0 GPa. The new (Xe,Si)O 2 phase is orthorhombic and corresponds to an elongation of the quartz unit cell. In this phase, Xe substitutes for Si with a nearly planar oxygen environment composed of four O atoms located at 2.06-2.09 Å from the Xe atom. Xe incorporation in SiO 2 proves to be significant, as previously shown for olivine. Xe incorporation is therefore expected to occur in the whole range of lithospheric silicate minerals. These findings emphasize the need to consider Xe incorporation in crust and upper mantle minerals as a process to store and fractionate xenon, in the framework of the "missing xenon" issue.

Noble Gas Reactivity in Planetary Interiors

“…Figure 3) that appear under P upon heating above the melting curve of Xe, the reaction being thermodynamically favored by the volume reduction between reactants (i.e., liquid Xe and quartz or olivine) and products (i.e., Xe-doped quartz or Xe-doped olivine). Theoretical calculations have confirmed this mechanism for quartz [43,76,77] and olivine [42], and helped refine the crystal-chemistry of Xe in these minerals. Volume and cell-parameters vs. P relationships as well as Raman signature of Xe-doped silicates could be theoretically reproduced by substituting Xe to Si albeit in different geometries depending on the mineral: quasi-planar three-fold in olivine vs. linear twofold (2 nearest O atoms at 1.99 Å) with 2 next nearest O neighbors in orthogonal plan at 2.27 Å in quartz [43].…”

“…Xe-doped quartz transforms into a new (Xe,Si)O 2 phase upon increased heating above 1,700 K at 1 GPa [43]. (Xe,Si)O 2 structure bears similarities to the predicted XeO 2 structure [64] FIGURE 3 | Infrared spectra on Xe-doped silicates under high P-T conditions generated using resistive-heating diamond-anvil cells.…”

“…On the basis of ab initio calculations, FeO 2 He, is predicted to be stable at core-mantle boundary conditions [68]. Unlike Xe-oxides and KrO discussed above, there is almost no charge transfer [42], Xe-doped β-quartz [43], (Xe,Si)O 2 phase for which Xe site occupancy is unknown [43], cristobalite-He [41]. Color code for atoms: magnesium (orange), xenon (yellow), oxygen (red), silicon (dark blue), helium (white).…”

“…Volume and cell-parameters vs. P relationships as well as Raman signature of Xe-doped silicates could be theoretically reproduced by substituting Xe to Si albeit in different geometries depending on the mineral: quasi-planar three-fold in olivine vs. linear twofold (2 nearest O atoms at 1.99 Å) with 2 next nearest O neighbors in orthogonal plan at 2.27 Å in quartz [43]. Theoretically-derived solubility is up to 0.4 at% Xe in both phases [42,43].…”

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.…”