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 FexMg1-xO with 0.01 ≤ x ≤ 0.25 can be described by D̃ = D̃0xB exp-(Q+Cx)/RT with D̃0 = (5 ± 1) × 10-4 m2 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°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.
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Acknowledgments SD thanks Christian Holzapfel for helpful advice at the early stage of the project, Mark Zimmerman for his generous assistance with running the Paterson apparatus, Nathalie Bolfan-Casanova for fervently and generously debating issues on point defects in (Mg,Fe)O, Catherine McCammon for Mössbauer analysis of the starting material, and Hans Keppler for access to his FTIR lab at the Bayerisches Geoinstitut. Electron microprobe analyses were carried out (with the help of Ellery Frahm) at the Electron Microprobe Laboratory, Department of Geology and Geophysics, University of Minnesota-Twin Cities. NSF supported the research through the grant NSF EAR-0337012 (to SJM) and NSF EAR-0439747 (to DLK). This paper is LPI publication #1326.
- Earth's mantle
- Fe-Mg interdiffusion