Rates of grain boundary diffusion through enstatite and forsterite reaction rims

The growth rates of enstatite rims produced by reaction of Fo(92) and SiO2 were determined at 250-1500 MPa and 900-1100 degrees 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 similar to 2 mu m diameter grains. Rim growth rates were used to calculate the product of the grain boundary diffusion coefficient (DIA) times the effective grain boundary thickness (6) assuming in turn that MEG, SiO2, and Mg2Si-1 are the diffusing components (coupled diffusion of a cation and oxygen or interdiffusion of ME and Si). The values for D'(MgO)delta, D'(sio2)delta, and D'(Mg2Si-1)delta for enstatite at 1000 degrees C and 700 MPa confining pressure with about 0.1 wt% water are about five times larger than the corresponding D'(A) delta values for samples initially vacuum dried at 250 degrees C. most of the increase in D'(A) delta occurs with the first 0.1 wt% water. The activation energy for diffusion through the enstatite rims (1100-950 degrees 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 less than or equal to 1.0 wt% water present) and limits the conditions at which rim growth can be measured. The corresponding values for D'(A) delta through forsterite rims are essentially identical for the two forsterite-producing reactions when 0.1 wt% water is added and similar to the D'(A) delta values for enstatite at the same conditions. The DS values for forsterite are similar to 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'(A) delta is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions.