H2-H2O immiscibility in Earth’s upper mantle

Immiscibility between water and hydrogen-rich fluids may be responsible for the formation of super-reduced mineral assemblages and for the early oxidation of Earth´s upper mantle. In the current study, we present new data on the critical curve in the H2-H2O system to 1400 ℃ and 4 GPa. We utilized a synthetic fluid inclusion method to trap fluids at high P–T conditions within quartz and olivine crystals. Experiments were performed in a piston-cylinder type apparatus, employing a double-capsule technique. The inner capsule contained the crystal and fluids of interest, while the outer served as oxygen fugacity buffer, maintaining f(O2) at the iron-wüstite (Fe-FeO) equilibrium. Our results suggest that below ~ 2.5 GPa, the critical curve has a mostly linear slope of 200 ℃/GPa, while at more elevated pressure it becomes significantly steeper (400 ℃/GPa). This implies that in most of the modern, reduced upper mantle, water and hydrogen are immiscible, while localized heating events, such as rising plumes, may close the miscibility gap. The steep increase of the critical curve at high pressure observed in this study implies that even for very hot geotherms in the early Archean or the late Hadean, H2-H2O immiscibility likely occurred in the deeper parts of the upper mantle, thus making a plausible case for rapid H2 loss as a mechanism of early mantle oxidation. To constrain the geochemical fingerprint of this process, we performed a series of element partitioning experiments to unravel how the H2-H2O unmixing may affect element transfer. Noble gases such as Xe as well as methane are preferentially incorporated in the hydrogen-rich phase, with a XeH2O/XeH2 ratio of ~ 8. This partitioning pattern may, for example, explain the underabundance of Xe isotopes produced by fission of Pu in the mantle. These Xe isotopes may have been removed by a primordial H2-H2O unmixing event during the early stages of planetary evolution.