Effect of H+ on Fe-Mg interdiffusion in olivine, (Fe,Mg)2SiO4

To quantify the effect of hydrogen on the kinetics of Fe-Mg interdiffusion in olivine, diffusion couples composed of crystals with Mg/(Mg+Fe) ratios of 0.91 and 0.83 were annealed under water-saturated conditions at T = 1373 K and P = 300 MPa., With f(o2) buffered at the Ni-NiO phase boundary, f(H2O) approximate to 300 MPa. The resulting interdiffusivity, (D) over tilde (Fe-Mg) approximate to 1 x 10(-16) m(2)/s, is approximately one order of magnitude larger than that measured under anhydrous conditions. The enhancement in diffusivity in the presence of water results from an increase in the concentration of cation vacancies associated with the introduction of high concentrations of protons as point defects into the olivine structure. (C) 2004 American Institute of Physics.