Diffusion of dissolved SiO2 in H2O at 1?GPa, with implications for mass transport in the crust and upper mantle

Cation diffusion rates at 690 ± 30 °C have been calculated by inverse modelling of observed manganese (Mn) zonation profiles in 40 garnets from two kyanite-bearing metapelite samples from the High Himalayan Crystalline Series, Zanskar, northwest India. Knowledge of the initial growth profile of Mn in garnet is a pre-requisite for this technique. Following previous workers we model Mn partitioning into growing garnet in terms of a Rayleigh fractionation process, and demonstrate that the Mn_{garnet:whole rock} partition coefficient is 60–100. Three-dimensional zonation profiles were obtained by successively grinding and polishing ∼1 cm slabs of each sample at 0.1–0.2 mm intervals and analysing the garnets at each stage, thus ensuring that core sections were measured. The diffusion model assumes that garnet has a spherical geometry and behaves as a closed system, and simulates diffusive modification of the hypothetical Mn Rayleigh growth profile for each garnet. The derived measure of the time-integrated diffusion history for each garnet is then combined with radiometric and field-relation constraints for the duration of the Himalayan metamorphic event to calculate cation diffusion rates. The average cation interdiffusion rate calculated for garnets in the two samples examined is (6 ± 3.2) × 10⁻²³ m²s⁻¹. This interdiffusion rate pertains to a temperature of 690 ± 30 °C, which is 0.97 × T _PEAK, the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques. Garnet compositions are P_y2–17Alm_65–77Gro_6–16Sp_1–17. These new diffusion data are consistent with, and more precise than, existing high-temperature (>1000 °C) experimentally determined diffusion data, although some uncertainties remain difficult to constrain. Qualitative comparison between diffusively modified Mn growth profiles in garnets from the Scottish Dalradian and the Himalayan garnets suggests that the duration of metamorphism affecting the Dalradian garnets was 10–20 times longer than that endured by the Himalayan garnets. Received: 5 June 1996 / Accepted: 29 January 1997