The diffusion coefficients of Fe2+ and Mg in aluminous spinel at ∼20 kb, 950 to 1325°C, and at 30 kb, 1125°C have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO2, was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe2+ and Mg at ∼20 kb, 950 to 1325°C, fit well the Arrhenian relation, D = Doexp(-Q/RT), where Q is the activation energy, with Do(Fe) = 1.8 (±2.8) × 10-5, Do(Mg) = 1.9 (±1.4) × 10-5 cm2/s, Q(Fe) = 198 ± 19, and Q(Mg) = 202 ± 8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of ∼5 cm3/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) ≤0.5 is ∼10 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.
ASJC Scopus subject areas
- Geochemistry and Petrology