Diffusion of Ti and some divalent cations in olivine as a function of temperature, oxygen fugacity, chemical potentials and crystal orientation

The diffusion of titanium along with Ni, Co and Mn in pure synthetic forsterite has been studied as a function of temperature, oxygen fugacity, crystallographic orientation and chemical potentials in the four-component system MgO-SiO₂-TiO₂-TiO1.5. In over 100 different experimental conditions, no Ti diffusion profiles with the classical shape of the error function were observed. Instead, the profiles vary between 'stepped' at high fO₂ (probably owing to diffusion on octahedral sites and trapping in tetrahedral sites), to hockey-stick shapes at low fO₂ owing to concentration-dependent diffusion. The profile shapes vary systematically between these end-members according to oxygen fugacity. These profile shapes are also reproduced in experiments using natural San Carlos olivine. The change in profile shape is interpreted to be a function of valence-state change from Ti⁴⁺ to Ti³⁺, shown by a sigmoidal log fO₂-diffusivity relationship. To distinguish between Ti⁴⁺ and Ti³⁺in forsterite, a diffusion profile was also investigated using 'hydroxylation spectroscopy' whereby a crystal from a diffusion experiment was hydroxylated by annealing in an H₂O-rich fluid at moderate pressure and temperature. The point defects associated with the added structural OH were then determined by infrared spectroscopy, providing evidence for the presence of Ti⁴⁺ and Ti³⁺ at intermediate oxygen fugacity. The transition fromTi⁴⁺ to Ti³⁺occurs at considerably higher fO₂ in these diffusion experiments than in equilibrium experiments in similar systems. Titanium diffusion, qualified using a mobility parameter (M), is fastest along the c-axis, at high activity of silica, low oxygen fugacity and high temperature. The rate of Ti diffusion is broadly similar to those of Mn, Ni and Co, and closer to published rates ofMg self-diffusion in olivine than Si self-diffusion. The observed Ti³⁺ and Ti⁴⁺ diffusion occurs on the M-sites. From statistical examination of the large Mn, Ni and Co diffusivity dataset, we determine that there is a 10^2/3 dependence of diffusion on aSiO₂; the cations diffuse more rapidly at high aSiO₂. In addition, diffusivity has a 10^-1/8 dependence on fO₂, suggested to be the result of increased concentration of loosely bound Ti³⁺ vacancy pairs, which enhance the mobility of other cations. This behaviour is likely to be present in natural systems where trivalent (e.g. Fe³⁺, Al³⁺, Cr³⁺) and divalent cations diffuse together. Consideration should be given to diffusive interference by highly charged, fast-moving cations when extracting time scales from frozen diffusion profiles.