Xenon and Argon: A contrasting behavior in olivine at depth

Xenon in the atmospheres of the Earth and Mars is characterized by a low abundance compared to other noble gases and by a depletion in light isotopes. By means of combined chemical analysis, in situ X-ray diffraction and Raman spectroscopy, we propose that Xe reacts with olivine at the high pressures and temperatures found in the upper mantle and in pre-terrestrial bodies. That provides a mechanism for the incorporation of Xe at depth and consequent isotopic fractionation. The substitution mechanism of Xe to Si depends on the type of silicate framework, forming XeO2XeO2 molecules in fully polymerized phases of silica, and XeO4XeO4 molecules in the isolated tetrahedra structure of olivine. Consequently, Xe retention in (Mg,Fe)2SiO4(Mg,Fe)2SiO4 olivine is less thermodynamically favored than in SiO2SiO2, implying lesser amounts of Xe trapped in olivine. This chemistry does not extend to the lighter noble gas Ar in the investigated pressure range. The incorporation of both Xe and Ar in olivine is correlated to its trace element content likely through the formation of vacancies, a pre-requisite for the retention of Xe on tetrahedral sites and Ar on octahedral sites.