Chemical transfer during redox exchanges between H-2 and Fe-bearing silicate melts.

Abstract : Kinetics and reaction paths of Fe3+ reduction by H2 in high-Fe and low-H2O silicate melts have been investigated at 800 ∞C. Time-series experiments were performed in cold-seal pressure vessels at 50 bars of pure H2 using rapid-heating and rapid-quench strategies. Within the first minutes of the experiments, a fast partitioning of Na occurred between the gas and the melt due to the reducing conditions. Kinetically decoupled from the Na partitioning, the progression of a front of Fe3+ reduction within the quenched melt was observed and was identified as a diffusion-limited process. The growth of the reduced layer is accompanied by an increase in concentration of OH-groups suggesting that reduction operates through proton incorporation within the melt. As this growth rate is slightly faster than predicted from the diffusion of molecular H2O, a different and mobile waterderived species seems likely. One possible mechanism is the reduction of Fe3+ by the transport of molecular H2. As this process is limited by the flux of H2, it will depend on both diffusivity and solubility of H2 in the melt. Alternatively, migration of protons (H+) and electronic species within the melt could control the velocity of the reduction front. The increase in concentration of the reactionderived OH groups produces a water over saturation followed by partial dehydration of the melt. This dehydration leads to a change in the redox conditions within the gas that influences the Na partitioning between gas and melt.