Diffusion kinetics of Fe2+ and Mg in aluminous spinel:: Experimental determination and applications

The diffusion coefficients of Fe2+ and Mg in aluminous spinel at similar to20 kb, 950 to 1325degreesC, and at 30 kb, 1125degreesC have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO(2), was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe2+ and Mg at similar to20 kb, 950 to 1325degreesC, fit well the Arrhenian relation, D = D(O)exp(-Q/RT), where Q is the activation energy, with D-O(Fe) = 1.8 (+/-2.8) x 10(-5), D-O(Mg) = 1.9 (+/-1.4) x 10(-5) cm(2)/s, Q(Fe) = 198+/-19, and Q(Mg) = 202+/-8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of similar to5 cm(3)/mol. From analysis of compositional zoning in natural olivine-spinel assemblages in ultramafic rocks, previous reports concluded that D(Fe-Mg) in spinel with Cr/(Cr + Al) less than or equal to0.5 is similar to10 times that in olivine. The diffusion data in spinel and olivine have been applied to the problems of preservation of Mg isotopic inhomogeneity in spinel within the plagioclase-olivine inclusions in Allende meteorite and cooling rates of terrestrial ultramafic rocks. Copyright (C) 2002 Elsevier Science Ltd.