High-pressure, high-temperature diffusion measurements in iron-nickel alloys to 65 GPa
Abstract
High-pressure, high-temperature diffusion measurements are crucial in constraining the dynamic behavior of deep-earth relevant materials. Of particular interest to geophysicists are creep rates and viscosities of these materials, which can be constrained by measuring diffusion rates at similar conditions. We performed single-sided laser heating of a Fe64Ni36 alloy substrate with a 300 nm layer of either Fe or Ni from 1 - 65 GPa and 1000 - 2800 K. The composition was chosen to maintain a single phase throughout the sample and the range of conditions. Subsequent to laser heating, the samples were recovered and a slice was removed through the center of the laser-heated spot using focused ion-beam milling (FIB). Once thinned to electron transparency, electron-dispersive x-ray spectroscopy (EDX) in a transmission electron microscope (TEM) produced compositional gradients into the sample as a function of distance across the laser-heated spot. The measured diffusion coefficients measured follow a homologous temperature relationship D=D_0 "exp" ((-19.3(2.7) T_m)/T) where D0 is 4.6 x 10-4 m2 s-1. This result is consistent with previous measurements at lower pressures and validates the homologous temperature assumption for extrapolating diffusivities to inner core conditions. For low deviatoric stress conditions of the inner core (~100 Pa) deforming via power-law creep, this predicts a viscosity of 1020 - 1022 Pa-s, requiring that observed seismic anisotropy develop through a combination of grain growth and rotation over the age of the Earth's inner core.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMMR54A..06R
- Keywords:
-
- 3902 MINERAL PHYSICS / Creep and deformation;
- 3924 MINERAL PHYSICS / High-pressure behavior;
- 3954 MINERAL PHYSICS / X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- 3994 MINERAL PHYSICS / Instruments and techniques