Influence of hydrogen on Fe-Mg interdiffusion in (Mg,Fe)O and implications for Earth's lower mantle

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 degrees 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 similar to 300 MPa, the Fe-Mg interdiffusion coefficient in FexMg1-xO with 0.01 <= x <= 0.25 can be described by D-similar to D(0)x(B) exp(-(Q-Cx)/RT) with D-0(similar to) = (5 +/- 1) x 10(-4) m(2)s(-1); Q = 270 +/- 20 kJ mol(-1); B = 0: 8 +/- 0.1, and C = -80 +/- 10 kJ mol(-1). For x = 0.1 and at 1,000 degrees C, Fe-Mg interdiffusion is a factor of similar to 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.