Influence of pO₂ and pH₂O on the High Temperature Plasticity of Olivine

Abstract

Deformation processes in the Earth’s upper mantle control large-scale tectonic events. Their understanding requires deep knowledge of the deformation mechanism of olivine, which is the major component of the upper mantle. This paper deals with the dependence of the creep rate of olivine single crystals (San Carlos, Arizona) on pO₂ and pH₂O, the oxygen and water partial pressures in the experimental atmosphere. Up to now, the effect of the stress and temperature upon the strain rate and the dislocation microstructures of this material, have been largely documented. (e.g. Kohlstedt and Goetze, 1974, and Durham and Goetze, 1977). But the influence of the atmosphere surrounding the single crystal was less understood. This study brings new data after the recent investigation of Kohlstedt and Hornack (1981), who already showed an increasing creep rate when pO₂ is increased ($\dot \varepsilon \propto pO_2^m$ with 1/10< m <1/5) in a rather narrow range of oxygen partial pressure. Another attempt to determine m was performed by Jaoul et al. (1980) who found m = 1/5−1/7, but for only one temperature, and with an atmosphere containing 2% water. The present work which corresponds to part of the thesis of B. Poumellec (1981) is an attempt to investigate the ${\dot \varepsilon }$ vs pO₂ dependence over a large range of T and pO₂, where pO₂ is varied as an independent parameter. Moreover the influence of different gas mixtures (H₂/H₂O and CO/CO₂), giving the same values of pO₂, has been tested: one corresponds to “wet”conditions, the other to “dry” conditions.