Substitution and diffusion of Cr2+ and Cr3+ in synthetic forsterite and natural olivine at 1200–1500 °C and 1 bar

Abstract

The diffusion and substitution mechanisms of Cr in forsterite were studied as a function of crystallographic orientation and the chemical potentials of all four components in the system MgO-SiO2-Cr-O. Oxygen fugacity (fO2) was varied over 15.4 log units at 1400 °C and was fixed at the iron-wüstite equilibrium for a temperature series (1200–1500 °C). The valence state changes of Cr along some diffusion profiles was also investigated using X-ray absorption near edge structure spectroscopy.Cr diffusion was found to be anisotropic (fastest along the c axis), and considerably faster in the presence of protoenstatite (high silica activity, aSiO2) than in the presence of periclase (low aSiO2). Cr diffusion profiles were longer at lower fO2 with more extreme diffusive anisotropy at higher fO2. Determined Cr diffusion coefficients were similar to those of Fe-Mg inter-diffusion in olivine at equivalent conditions.The diffusivity of Cr was found to be a function of its own concentration, but, as an approximation, Cr diffusion (in m2/s) along the c axis of pure forsterite, at an fO2 corresponding to iron-wüstite at 200 µg/g Cr could be described using:logDCr=-4.37±0.26m2/s+-316±8kJ/mol2.303RTwhere R is the gas constant in kJ/Kmol and T is the temperature in Kelvin.In forsterite, the valence state ratio of chromium (Cr2+/ΣCr, where ΣCr= Cr2++Cr3+) changed systematically along diffusion profiles. The equilibrium Cr2+/ΣCr ratio imposed by the fO2 of the experiment was observed at the diffusion interface, and this changed to a constant value in the crystal interior.Cr3+ always substitutes onto an octahedral site in olivine. At high aSiO2, Cr3+ uses M site vacancies ([vac]) for charge balance (i.e. Cr4/33+[vac]2/3SiO4), whereas at low aSiO2 a new mechanism was observed with charge balance achieved by Mg2+ on the tetrahedral site producing a ‘spinel type’ substitution, Cr23+MgO4. Cr2+ always substitutes directly for Mg2+, forming the Cr22+SiO4 substitution.Comparative experiments using San Carlos olivine showed slightly faster Cr diffusion, a lesser concentration dependence of diffusion and no Cr valence state change along diffusion profiles.