FeMg interdiffusion in single crystal olivine at very high pressure and controlled oxygen fugacity: technological advances and initial data at 7 GPa

Abstract

The interdiffusion of iron and magnesium in natural single crystals of San Carlos olivine (Fo 90) has been determined using a 2000 ton uniaxial split-sphere apparatus (USSA-2000) and Rutherford back-scattering spectrometry (RBS). Advances in the microanalysis technique made it possible to perform high-pressure experiments for short-duration runs at low temperatures in a controlled chemical and mechanical environment. Olivine crystals were coated with a thin film of fayalite, inserted in an Fe capsule and placed inside an 18 mm pyrophyllite octahedral cell assembly. After experiments at 7 GPa and 900°C for 6 h at pO 2 ≈ 10 −14 bar, the specimens were recovered with very few fractures and chemically unaltered. Analysis of RBS spectra from these crystals yields a value of D FeMg = 10 −13.7 cm 2 s −1. Comparison of this value with previous data at atmospheric pressure suggests that the activation volume for FeMg interdiffusion is V FeMg ∗ ≈ 2.2 ± 0.9 cm 3 mol −1 which, if proven correct, has important implications for electrical conductivity and creep in the upper mantle.