The role of water in the high-temperature deformation of clinopyroxene

The effect of water on the creep strength of a natural coarse-grained clinopyroxenite and a fine-grained diopside fabricated from natural diopside single crystals was investigated. Samples were deformed under hydrous conditions at temperatures between 1373 and 1473 K. Experiments were carried out at confining pressures of 150 and 300 MPa and with Ni/NiO and Fe/FeO buffers in order to vary in water fugacity and oxygen fugacity. Deformation of coarse-grained clinopyroxene occurred in the dislocation creep regime with the creep rate proportional to the water fugacity to the 3.0 +/- 0.6 power and an activation volume of 0 m3/mol. One possible water weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions. Deformation of fine-grained diopside took place in the transition between diffusion creep to dislocation creep at the higher oxygen fugacity (Ni/NiO) but in the diffusion creep regime at the lower oxygen fugacity (Fe/FeO). Samples deformed at the lower oxygen fugacity and water fugacity creep over 5 times faster at a given temperature, confining pressure and differential stress. Another possible mechanism for the observed water-weakening effect is due to increases in the diffusivities of both silicon and oxygen ions. Compared to other major minerals in Earth's lower crust, specifically olivine and plagioclase, the water-weakening effect is the most significant for clinopyroxene.; A hydrothermal annealing experiment was performed on diopside single crystals at a temperature of 1473 K and a confining pressure of 300 MPa under Fe/FeO and Ni/NiO buffers to investigate the effect of water fugacity and oxygen fugacity on the water solubility in diopside. Diopside hydrothermally annealed with a Fe/FeO buffer has a water solubility 1.5 times higher than diopside hydrothermally annealed with a Ni/NiO buffer. One possible hydrogen incorporation mechanism in diopside is that hydrogen exists mostly in forms of OH˙ o and H/Me with the charge neutrality condition given by Fe/Si +H/Me =OH˙O . For clinopyroxene, water solubility increases with increasing concentration of trivalent cations.