Kinetics study of dehydrogenation of pyrite-FeO2H above 1 Mbar using laser heating and time-resolved synchrotron x-ray diffraction

Abstract:

We have conducted time-resolved synchrotron x-ray diffraction of pyrite- FeO₂H at high pressures of 110 GPa and 120 GPa and high temperatures of 2100K and 2300K in a laser-heated diamond anvil cell (DAC). The accumulated laser-heating time extended up to 12 hours. The hydrogen concentration in FeO₂H_x is determined by comparing the cell volume of the sample after it was quenched to room temperature with those of the end members (FeOOH and FeO₂) at the same pressure and assuming a linear relationship between the molar volume and hydrogen concentration x in pyrite-FeO₂H_x. We found that pyrite-FeO₂H experiences a slow dehydrogenation process at these conditions. The derived x values as a function of the heating duration are fitted based on the first-order reaction equation dx/dt=-kx (where k is a time(t)-independent constant for a given environment) for the dehydrogenation: FeOOH=FeO₂H_x+½(1-x)H₂. The hydrogen concentration x approaches 0.76 at 110 GPa and 2300K when the heating time approaches infinity. The equilibrium x value decreases with temperature and increases with pressure, which explains the discrepancies in the hydrogen concentration among the previous experiments. These results indicate that FeOOH loses hydrogen through a dehydrogenation process at lower-mantle conditions and stabilizes at an intermediate composition, which remains capable of storing water as a reservoir at the bottom of the lower mantle.