HR: 1340h
AN: MR43A-0866    [Abstracts]
TI: Interdiffusion of Iron and Nickel at High Pressure
AU: * Yunker, M L
EM: mxy38@case.edu
AF: Case Western Reserve University Department of Geological Sciences, 10900 Euclid Avenue, Cleveland, OH 44106 United States
AU: Van Orman, J A
EM: jav12@case.edu
AF: Case Western Reserve University Department of Geological Sciences, 10900 Euclid Avenue, Cleveland, OH 44106 United States
AB: To better understand diffusion-controlled properties of Earth's inner core, such as viscosity, we performed a series of multi-anvil experiments to measure Fe-Ni interdiffusion coefficients at pressures up to 23 GPa and temperatures up to 1800 $\deg$C. Diffusion couples consisting of a pure Fe and a pure Ni rod were compressed, heated and held at constant temperature for 0.5 to 6.5 hours, then quenched. An electron microprobe was used to analyze Fe and Ni concentrations perpendicular to the diffusion interface, and diffusion coefficients were determined as a function of alloy composition using the Boltzmann-Matano method. In all experiments, Fe-Ni interdiffusion coefficients were found to increase smoothly across the diffusion couple from pure Fe to pure Ni, with diffusion in almost pure nickel about two orders of magnitude faster than in iron. The diffusion coefficient at a given alloy composition decreases with increasing pressure, but the data are not consistent with a constant activation volume; instead the activation volume decreases with pressure. The entire data set for Fe-Ni interdiffusion in iron-rich alloys, including 1 atm and 4 GPa data of Goldstein et al. (1965; Trans AIME 233:812-820), is reproduced with reasonable accuracy by a simple exponential function of the reduced melting temperature, D=D$_{0}$exp(-bT$_{m}$/T), where b is a constant. This homologous temperature relation appears to be the most accurate method for extrapolating diffusion coefficients to high pressure. At inner core conditions (T/T$_{m}$ $\sim$ 0.9), the Fe-Ni interdiffusion coefficient for an alloy with the fcc structure is predicted to be $\sim$4$\times$10$^{-14}$ m$^{2}$/s. Diffusion coefficients in the inner core, which is thought to be composed of an iron-nickel alloy with the hcp or bcc structure, are likely to be higher since diffusion in hcp and bcc metals is faster than in fcc metals at the same homologous temperature.
DE: 5724 Interiors (8147)
DE: 3630 Experimental mineralogy and petrology
DE: 3902 Creep and deformation
DE: 1015 Composition of the core
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting