HR: 1340h
AN: MR43A-0868    [Abstracts]
TI: Diffusion in Zinc at High Pressure and Rheology of the Earth's Inner Core
AU: * Keshav, S
EM: s.keshav@gl.ciw.edu
AF: Geol Sc, CWRU, Cleveland, OH 44106
AU: Van Orman, J A
AF: Geol Sc, CWRU, Cleveland, OH 44106
AB: An attempt has been made here to estimate the viscosity of the Earth's inner core, and also to identify the primary mechanism by which the inner core deforms. Estimation of the viscosity and identification of the deformation mechanism(s) of the inner core require measurements of diffusion in the hcp (hexagonal close-packed, or epsilon) phase of iron, suggested to be stable at the Earth's inner-core pressure conditions. However, owing largely to experimental and analytical challenges, actual measurements on the diffusivity in this phase are non-existent. To overcome this problem, we have focused on the divalent transition metal, zinc (Zn), which has the hcp structure over a wide range of pressures. Hcp metals are known to have similar diffusivities at the same homologous temperature. A primary goal of this work was to explore the effect of increasing pressure on diffusion in zinc. Zinc has high compressibility, allowing diffusion measurements to be made on normalized pressures (P/K, pressure/bulk modulus) approaching those of Earth's core. We focused on diffusion of gold (Au), which has been extensively studied at atmospheric pressure. We find that with increasing pressure from 10 to 25 GPa, the diffusion coefficient of Au in Zn decreases, and the data at high pressures are in good agreement with that at 1 atm. However, in a plot of log D versus homologous temperature (Tm/T, where Tm is the melting point), the slope besides being slightly shallower, the high-pressure diffusivity values retrieved are higher than predicted from extrapolation of the 1 atm data. This trend is more prominent in plot of log D versus pressure, where instead of being linear, the diffusion coefficient shows a slight parabolic dependence, indicating that the activation volume decreases with pressure. High-pressure diffusivity values are higher than predicted either from homologous temperature scaling, or those retrieved assuming constant activation volume from one atmosphere data. To a good approximation, the inner core is in hydrostatic equilibrium with the surrounding fluid. The shear stress on the inner core is thought to be low, and grain sizes are larger than in the lower mantle. Temperature and pressure both influence the effective viscosity, and their role is often included in rheological models by referring the temperature to the melting temperature, Tm; the pressure dependence enters implicitly through its influence on Tm. Calculated values of viscosity of the Earth's inner core using the experimental philosophy outlined above are at least 8-9 orders of magnitude lower than estimated so far. These low viscosities have important consequences for the origin of seismic anisotropy and diffusion in the inner core. On the basis of these measurements, it appears that the inner core can quickly erase memory of deformation that occurred early in its history. Thus, search for alternative explanations of the anisotropy is required. Unraveling the clues left behind by the growth of the inner core offers the hope of new insights into the evolution of the Earth's deep interior.
DE: 8115 Core processes (1507)
DE: 7207 Core and mantle
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
DE: 1507 Core processes (8115)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting