Establishing consistent equations of state for solid noble gases: Implication for partitioning behaviors of noble gases in the lower mantle

Abstract

Noble gases (He, Ne, Ar, Kr and Xe) are important geochemical tracers for exploring the volatile cycles in the whole Earth system through geological time. Noble gases are dissolved more in bridgmanite and ferropericlase, the dominant minerals of the lower mantle, than in the minerals of the upper mantle. In order to understand the incorporation mechanism of noble gases into the crystal structures, it is necessary to derive atomic radii of the noble gases at pressure-temperature conditions of the lower mantle, which depend on accurate P-V-T equations of state (EOSs). On the other hand, extensive P-V EOSs have been reported on noble gases, on the basis of isothermal compressional measurements in the diamond anvil cell (DAC). However, severe discrepancies exist among the reported EOSs for these phases with pressure differences up to 30%. We converted all the pressures to be consistent with the inter-calibrated MgO-Au-Pt-ruby pressure scales from Ye et al. (2018)High Press. Res., and found that for each noble gas, different measurements agree well with each other with pressure discrepancies generally within ±5 GPa. Hence, the discrepancies of EOSs mainly come from inconsistencies among various pressure scales. After pressure conversion, consistent P-V-T EOSs were constructed among noble gases. In addition, we also established modified P-V EOSs for these phases, which are independent of selection of the volumes (V0) or bulk moduli (KT0 and K0’) at ambient pressure. For each phase, good agreement was observed between the normal and modified EOSs, with discrepancy within the measurement uncertainties throughout the experimental pressure range. The oxygen site Young's moduli are also given for bridgmanite and ferropericlase, and then established lattice strain models at the P-T conditions in the lower mantle. Consequently, solubilities of Ne, Ar, Kr, and Xe can be constrained at various depths in the lower mantle, which are crucial to the noble gas fractionation during formation of the lower mantle from a magma ocean in the Archean. Variation of the primordial stable isotope ratios (130Xe/36Ar and 20Ne/36Ar) between magma ocean and present OIBs are generally consistent with the solubility ratios of Ne: Ar: Xe from this model. On the other hand, our model also supports much more Ar and Kr being stored in the lower mantle than Xe, which could be associated with the depletion of Xe in the present atmosphere, as compared with the abundances in chondrites. The lower mantle is therefore an important reservoir for noble gases, and has not yet been completely degassed.