HR: 1340h
AN: V13A-0521    [Abstracts]
TI: Cr-Al Diffusion in Chromite Spinel at High Pressure
AU: * Suzuki, A
EM: aysuzuki@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Yasuda, A
EM: yasuda@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ozawa, K
EM: ozawa@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-0033 Japan
AB: Compositional zoning in chromite spinel gives us important information to constrain thermal and deformation history of ultramafic-mafic rocks. For the quantitative estimation, diffusivity of elements in spinel is a critical parameter. Although the Mg-Fe²⁺ interdiffusion coefficient in MgAl₂O₄ spinel has experimentally studied by Freer & O'Reilly (1980) and Liemann & Ganguly (2002), Cr-Al interdiffusion coefficient has not been determined yet. In this study, we have experimentally determined Cr-Al interdiffusion coefficient in chromite spinel at temperatures ranging 1400-1700 °C and pressures ranging 3-7 GPa, by using diffusion couple of natural single crystals of spinel and chromite. Experiments were carried out with a multi-anvil type (MA-8 type) high-pressure apparatus at the Earthquake Research Institute, University of Tokyo. After experiments, the samples were cut perpendicular to the contact plane and analyzed with EPMA and EBSD. The elemental mapping showed that Cr, Al, Fe³⁺, Fe²⁺, and Mg diffused perpendicular to the contact plane. The Cr-Al diffusion profiles are complementary with each other and asymmetric with steeper profile in the spinel side suggesting a compositional dependence of Cr-Al diffusion in spinel. The Cr-Al interdiffusion coefficient was estimated by the Boltzmann-Matano method. The coefficient decreases with Cr# (=Cr/(Cr+Al)) of spinel, which varies more than one order of magnitude as the Cr# changes from 0.1 to 0.85 at 3 GPa and 1600 °C. It is concluded that the self-diffusion coefficient of Al is more than one order of magnitude larger than that of Cr. The Cr-Al interdiffusion coefficient is expressed by D=D₀exp(-Q/RT), where D₀=2.8×10^-2 m²/s and Q=498 kJ/mol at Cr#=0.2. This relation is applicable up to Cr#=0.5. Extrapolation of the self-diffusion coefficient of Cr to the lower temperature shows that Cr is the slowest diffusion species in chromite spinel including oxygen. This extremely slow Cr self-diffusion is consistent with the Cr-Al zoning observed in chromite spinel in peridotites, which is inferred to have formed by diffusion creep (Ozawa, 1989).
DE: 3630 Experimental mineralogy and petrology
DE: 3902 Creep and deformation
DE: 5120 Plasticity, diffusion, and creep
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005