Diffusion and mobility of electrically conducting defects in olivine |
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Authors: | S. Constable A. Duba |
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Affiliation: | (1) Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla California 92093-0225, USA e-mail: sconstable@ucsd.edu, US;(2) Earth and Planetary Sciences Department, American Museum of Natural History, Central Park West at 79th St, NYC New York 10024-5192, USA, US |
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Abstract: | Electrical conductivity of lherzolite (65% olivine), measured as a function of time after changes in the oxygen fugacity (f o2) of the surrounding CO2/CO atmosphere, is used to infer the diffusivity of the point defects responsible for conduction in olivine. A total of 63 equilibration runs at temperatures of 900, 1000, 1100, and 1200 ∘C were fit using nonlinear parameter estimation to recover time constants (directly related to diffusivity) and conductivity steps. An observed f o2 dependence in the time constants associated with re-equilibration implies two defect species of fixed diffusivity but with f o2-dependent concentrations. Although the rate-limiting step may not necessarily be associated with a conducting defect, when time constants are converted to diffusivities, the magnitudes and activation energies agree extremely well with the model for magnesium vacancies (the slower species) and small polarons (holes localized on Fe3+) derived by Constable and Roberts (1997). This earlier study used an independent method of simultaneous modeling of thermopower and electrical conductivity as a function of f o2 and temperature, on data from a different type of sample (a dunite). We observe that at high f o2 where polarons dominate over magnesium vacancies in the defect population, re-equilibration is dominated by magnesium vacancy diffusion, and vice versa (at low f o2 magnesium vacancies dominate and re-equilibration proceeds at the faster rate associated with polaron mobility). We interpret this to suggest association between the cation vacancies and polarons, as has been suggested by Tsai and Dieckmann (1997), making the concentration of the minority defect the rate-limiting step in the oxidation/reduction reactions. Received: 18 October 2000 / Accepted: 7 May 2002 |
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Keywords: | Defect chemistry Electrical conductivity Olivine |
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