Structure and chemistry of noble gases at Earth’s core conditions

Noble gases are important geochemical tracers allowing to reconstruct key planetary formation processes such as the origin of Earth’s atmosphere. It is presently assumed that noble gases have almost completely outgassed from the condensing magma ocean because they dissolve preferentially into silicate melts and exhibit high incompatibilities in solid phases at moderate pressures. Any storage in the solidified Earth’s mantle and liquid core has been therefore considered insignificant. In this presentation, I will show that the second most abundant lower mantle mineral, (Mg_1-x,Fe_x)O and liquid iron-alloys at conditions of up to 115(1) GPa and 3700(150) K exhibit extremely high krypton storage capacities. From μ-XAS and μ-XRF measurements coupled to electron microscopy analysis, we demonstrate that up to 3 wt% of krypton can be stored in (Mg_1-x,Fe_x)O and 3000 ppm in the liquid metal. For both phases storage capacities increase with pressure which is remarkably different to the solubility drops reported for silicate melts. At diluted lower mantle conditions krypton can preferentially substitute into the anion site of (Mg_1-x,Fe_x)O. Because (Mg_1-x,Fe_x)O exhibits higher solubilities compared to bridgmanite and the liquid outer core, we suggest that (Mg_1-x,Fe_x)O plays an important role as deep krypton reservoir. In order to better constrain the proposed model, we have also studied the compression behavior of pure solid krypton. Similarly to xenon, solid krypton undergoes a pressure induced martensitic phase transition from a face centred cubic (fcc) to an hexagonal close packed (hcp) structure. These two phases coexist in a very wide pressure domain inducing important modifications of the bulk properties of the resulting mixed phase. I will present new results from detailed in situ XAS and XRD revealing the influence of the fcc-hcp phase transition on the compression behaviour of solid krypton in an extended pressure domain up to 140 GPa.