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 H 2 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 H 2 O and H 2 could be close to equilibrium during the reduction of water to hydrogen by the FeO component of silicates. Key words: δD, oxido-reduction, high-temperature, isotopic fractionation, iron-bearing silicate, Archean ocean Introduction D/H ratios have been widely used to study the water evolution of terrestrial planet atmospheres. The D/H ratio of surficial water intimately depends on the evolution mode of the atmosphere and its interactions with inner envelopes of terrestrial planets. Water dissociation and hydrogen escape are responsible for high deuterium enrichment of Martian's and Venusian's atmospheres relative to 1788-2281/$ 20.00 © 2011 Akadémiai Kiadó, Budapest Central European Geology, Vol. 54/1–2, pp. 81–93 (2011) DOI: 10.1556/CEuGeol.54.2011.1–2.8 ESIR 2011 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) Laurent Simon, Christophe Lécuyer François Robert François Martineau Laboratoire de Minéralogie, Laboratoire de Géologie de Lyon, Lyon Muséum National d'Histoire Naturelle, Paris Address of corresponding author: C. Lécuyer: UMR CNRS 5276, Université Lyon 1 et Ecole Normale Supérieure de Lyon, 69622 Villeurbanne, France, e-mail: clecuyer@univ-lyon1.fr Received: May 12, 2011; accepted: May 26, 2011