Mg diffusion in forsterite from 1250-1600 °c

²⁶Mg tracer diffusion coefficients were determined in single crystals of pure synthetic forsterite (Mg₂SiO₄). Isotopically enriched powder sources both acted as the ²⁶Mg source and buffered the activities of silica (aSiO₂) at forsterite + protoenstatite (Mg₂Si₂O₆) (high aSiO₂) and forsterite + periclase (MgO) (low aSiO2). Experiments were conducted at atmospheric pressure between 1250 and 1600 °C, and at oxygen fugacities (fO2s) between 10-12 bars (CO-CO2 mix) and 10-0.7 bars (air). The resulting diffusion profiles were measured along the three principal crystallographic axes (a, b, and c; ∥[100], ∥ [010], ∥ [001]) using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), with a quadrupole mass spectrometer. These measurements were corroborated by ion microprobe using the sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) instrument. Mg tracer diffusion is anisotropic, with D_[001] > D_[010] > D_[100], the difference in diffusion coefficients varying by about one order of magnitude at a given temperature with crystallographic orientation. Diffusion is faster in protoenstatite-buffered than periclase-buffered conditions, again with around one order of magnitude difference in diffusivity between buffering conditions. There is no apparent effect of fO₂ on diffusion. A global fit to all data, including data from Chakraborty et al. (1994) and Morioka (1981) yields the relationship: log₁₀D=log₁₀D₀(m²s^-1)+0.61(±0.03)log₁₀ αSiO₂+-359(±10)kJ/mol/2.303RT where log₁₀D₀ is -3.15 (±0.08), -3.61 (±0.02), and -4.01 (± 0.05) m² s^-1 for the [001], [010], and [100] directions, respectively (1 s.d.). The LA-ICP-MS technique reproduces diffusion coefficients determined by SHRIMP-RG, albeit with slightly different absolute values of isotope ratios. This shows that LA-ICP-MS, which is both accessible and rapid, is a robust analytical method for such tracer diffusion studies.