Rates of grain boundary diffusion through enstatite and forsterite reaction rims

Abstract

 The growth rates of enstatite rims produced by reaction of Fo₉₂ and SiO₂ were determined at 250–1500 MPa and 900–1100°C for a wide range of water contents. Growth rates were also determined for forsterite rims between MgO and Mg₂Si₂O₆ and between MgO and SiO₂. Rim growth rates are parabolic indicating diffusion-controlled growth of the polycrystalline rims which are composed of ˜ 2 μm diameter grains. Rim growth rates were used to calculate the product of the grain boundary diffusion coefficient (D'_A) times the effective grain boundary thickness (δ) assuming in turn that MgO, SiO₂, and Mg₂Si₋₁ are the diffusing components (coupled diffusion of a cation and oxygen or interdiffusion of Mg and Si). The values for D'_MgOδ, D', and D' for enstatite at 1000°C and 700 MPa confining pressure with about 0.1 wt %  water are about five times larger than the corresponding D'_Aδ values for samples initially vacuum dried at 250°C. Most of the increase in D'_Aδ occurs with the first 0.1 wt %  water. The activation energy for diffusion through the enstatite rims (1100–950°C) is 162 ± 30 kJ/mole. The diffusion rate through enstatite rims is essentially unchanged for confining pressures from 210–1400 MPa, but the nucleation rate is greatly reduced at low confining pressure (for  ≤ 1.0 wt % water present) and limits the conditions at which rim growth can be measured. The corresponding values for D'_Aδ through forsterite rims are essentially identical for the two forsterite-producing reactions when 0.1 wt % water is added and similar to the D'_Aδ values for enstatite at the same conditions. The D'_Aδ values for forsterite are ˜ 28 times larger for samples starting with 0.1 wt %  water compared to samples that were first vacuum dried. Thus water enhances these grain boundary diffusion rates by a factor of 5–30 depending on the mineralogy, but the total range in D'_Aδ is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions.