Diffusion of Hydrogen in Iron-Bearing Olivine at 3 GPa

Abstract:

Physical and chemical properties of Earth’s mantle are affected by the interactions with volatiles, and especially by water and water-derived species. Thus, the characterization of solubility and kinetics of incorporation for hydrogen in nominally anhydrous minerals is important to understand the behavior of Earth’s deep material under hydrous conditions. Experimental studies on the olivine-water system indicate that significant amounts of hydrogen can be incoporated within olivine as point defects. Extending previous studies, we have focused on the kinetics of hydrogen diffusion in the iron-bearing olivine-water system, performing experiments of hydrogenation of crystallographically oriented olivine single crystals using multi-anvils apparatus at high temperature (900-1200°C) and high pressure (3 GPa). We use polarized Fourier transform infrared spectroscopy to characterize the speciation and the quantify the mobility water-derived defects in olivine, to determine diffusion coefficients under upper mantle conditions. Hydrogen diffusivities are obtained by fitting the hydrogen content measured as a function of position along [100] and [001] direction of the olivine sample, by a 1D and 3D numerical models of diffusion,

Our current results indicate that incorporation of hydroxyl species into iron-bearing olivine is a one-stage process with hydrogen chemical diffusion coefficients around 2.10^-12 m²/s at 900 °C parallel to [001] (with E // to [001]). The diffusivities are in the same order of magnitude than previous results from iron-bearing olivine at low pressure. The analysis of the different concentration profiles show an anisotropy of diffusion, with diffusion parallel to [001] faster than [100]. Consequences for electrical conductivity in the uppermost mantle will be discussed.

This study was financially supported by ANR JCJC “HyDeep” awarded to NBC.