Self-diffusion and chemical diffusion in peridotite melt at high pressure and implications for magma ocean viscosities

Abstract

Self-diffusion coefficients of Si, O, Mg, and Ca and chemical diffusion coefficients of Ni and Co have been determined experimentally up to 24 GPa and 2623 K in anhydrous peridotite liquid using a multi-anvil press. Each diffusion couple consisted of a cylinder of isotopically-natural peridotite enriched in 1 wt% CoO that was mated to a cylinder of chemically similar peridotite enriched in 18O, 30Si, 25Mg, 44Ca, and 1 wt% NiO. Isotope abundance (18O, 30Si, 25Mg, 44Ca) profiles along each sample length and perpendicular to the diffusion interface were analyzed by secondary ion mass spectrometry, and Ni and Co concentration profiles were measured by electron microprobe analysis. The rates of self-diffusion of Si, O, Mg, Ca and chemical diffusion of Ni and Co decrease with increasing pressure from 4 to 8 GPa, where diffusivity reaches a minimum, and then increase with a further pressure increase to reach a maximum at ~12 GPa. The observed change in the pressure dependence at ~8–10 GPa is likely caused by pressure-induced structural changes and the formation of high-coordinated intermediate species. Above ~12 GPa, diffusivities again decrease weakly with increasing pressure most likely as a result of further melt compaction when the coordination changes are complete. Predicted viscosities from self-diffusion coefficients for oxygen, obtained using the Eyring equation, are similar to viscosities measured by falling sphere viscometry, including the observed changes of the pressure dependence. The effect of pressure on diffusion and viscosity in peridotite melt is small compared with the effect of temperature, indicating that the viscosity along the melting curve decreases with depth. Diffusion parameters determined in this study are used to estimate the viscosity of a magma ocean with a depth of ~700 km. The results indicate that the viscosity increases from ~10 mPa s near the surface to a reach a maximum of ~50 mPa s at a depth of ~250 km. At higher pressures the viscosity decreases to ~10 mPa s at a depth of ~300 km, below which it remains almost constant.