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 (

) 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, ) than in the presence of periclase (low ). Cr diffusion profiles were longer at lower with more extreme diffusive anisotropy at higher

. 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

corresponding to iron-wüstite at 200 µg/g Cr could be described using:

where 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

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 , Cr3+ uses M site vacancies ([vac]) for charge balance (i.e. ), whereas at low a new mechanism was observed with charge balance achieved by Mg2+ on the tetrahedral site producing a ‘spinel type’ substitution, . Cr2+ always substitutes directly for Mg2+, forming the

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. Show more