Experimental study of D/H fractionation between water and hydrogen gas during the oxidation of Fe-bearing silicates at high temperatures (600 °C–1200 °C)

Abstract

Hydrogen gas is produced during the oxidation of the FeO component of silicates by water. This redox reaction occurs during the high-temperature (400 °C–800 °C) hydrothermal alteration of oceanic crustal rocks, and is responsible for H2 production at mid-ocean ridges. Samples of international reference biotite NBS30 (δD = −65.7‰) were reacted at high temperatures (600–1200 °C) in a high-vacuum line, releasing both structural water and hydrogen gas. An apparent fractionation factor α, derived from D/H measurements of water and hydrogen gas, is linearly dependent on T−2 following the equation α = 1.024 + 2477296.T−2 with a residual standard deviation σ = 0.023. The apparent D/H fractionation factors between water vapor and hydrogen gas during biotite oxidation show a dependency on T−2 that resembles those measured either by equilibration experiments or calculated from partition functions. Moreover, the apparent fractionation factors we measured are close to those determined at equilibrium in the same temperature range by Cerrai et al. (1954). This observation suggests that the D/H fractionation between H2O and H2 could be close to equilibrium during the reduction of water to hydrogen by the FeO component of silicates.