INFLUENCE OF HYDROGEN ON Fe–Mg INTERDIFFUSION IN (Mg,Fe)O AND IMPLICATIONS FOR EARTH’S LOWER MANTLE

Abstract

Interdiffusion of Fe and Mg in (Mg,Fe)O has been investigated experimentally under hydrous conditions. Single crystals of MgO in contact with (Mg0.73Fe0.27)O were annealed hydrothermally at 300 MPa between 1,000 and 1,250°C and using a Ni–NiO buffer. After electron microprobe analyses, the dependence of the interdiffusivity on Fe concentration was determined using a Boltzmann–Matano analysis. For a water fugacity of ~300 MPa, the Fe–Mg interdiffusion coefficient in Fe x Mg1−x O with 0.01 ≤ x ≤ 0.25 can be described by ${\tilde{D} = \tilde{D}_{0} x^{{{B}}} \exp ^{{- (Q + {{C}}x)/{{R}}T}}}$ with ${ \tilde{D}_{0}= (5\pm 1)\times 10^{-4}\,\hbox{m}^{2}\,\hbox{s}^{-1}, Q = 270\pm 20\,\hbox{kJ\,mol}^{-1}, {B} = 0.8\pm 0.1},$ and C = −80 ± 10 kJ mol−1. For x = 0.1 and at 1,000°C, Fe–Mg interdiffusion is a factor of ~4 faster under hydrous than under anhydrous conditions. This enhanced rate of interdiffusion is attributed to an increased concentration of metal vacancies resulting from the incorporation of hydrogen. Such water-induced enhancement of kinetics may have important implications for the rheological properties of the lower mantle.