Titanium diffusion in olivine

Diffusion of Ti has been characterized in natural olivine and synthetic forsterite. Experiments on the natural olivines were run under buffered conditions (IW and NNO), and those on synthetic forsterite were run in air. Titanium diffusion appears relatively insensitive to crystallographic orientation and oxygen fugacity under the range of investigated conditions, and diffusivities are similar for Fe-bearing olivine and forsterite. For Ti diffusion in synthetic forsterite, we obtain the following Arrhenius relation for diffusion over the temperature range 900-1400 °C: DForst=5.97×10-14exp(-203±19kJ mol-1/RT)m2s-1. For Ti diffusion in San Carlos olivine, we obtain the following “global” fit to all diffusion data (incorporating diffusion parallel to b and c-axes, and experiments run under IW and NNO buffers), over the temperature range 1050-1254 °C: Dol-SC=2.11×10-14exp(-195±32kJ mol-1/RT)m2s-1. Titanium diffusivities in olivine are similar to those of the trivalent REEs, but considerably slower than those of Cr, Ca, and Fe-Mg in olivine. Titanium diffusivities in olivine, diopside and orthopyroxene are comparable over investigated temperature ranges, differing by about half a log unit at 900 °C, but have increasing variance at lower temperatures given the higher activation energies (by 40-70 kJ mol−1) for Ti diffusion in pyroxene compared with olivine. These large differences in cation mobility among Ti, Cr, Ca, Fe-Mg and the REE in pyroxene and olivine may allow us to distinguish dominant processes that give rise to the chemical disequilibria in olivine and pyroxene. With decreasing temperature, Ti is preferentially partitioned from olivine to pyroxene in ultramafic rocks, giving rise to characteristic reversed zoning in olivine and orthopyroxene and normal zoning in clinopyroxene. Diffusive exchange models with temperature-dependent diffusion and partition coefficients have been developed for the olivine-pyroxene bi-mineralic and tri-mineralic systems, allowing us to assess cooling rates and closure temperatures of the ultramafic and mafic rocks.