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 forste rite rims between MgO and Mg2Si2O6 and between MgO and SiO2. Rim growt h rates are parabolic indicating diffusion-controlled growth of the po lycrystalline rims which are composed of similar to 2 mu m diameter gr ains. Rim growth rates were used to calculate the product of the grain boundary diffusion coefficient (DIA) times the effective grain bounda ry thickness (6) assuming in turn that MEG, SiO2, and Mg2Si-1 are the diffusing components (coupled diffusion of a cation and oxygen or inte rdiffusion 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 confi ning pressure with about 0.1 wt% water are about five times larger tha n the corresponding D'(A) delta values for samples initially vacuum dr ied 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 t he enstatite rims (1100-950 degrees C) is 162 +/- 30 kJ/mole. The diff usion rate through enstatite rims is essentially unchanged for confini ng pressures from 210-1400 MPa, but the nucleation rate is greatly red uced at low confining pressure (for less than or equal to 1.0 wt% wate r present) and limits the conditions at which rim growth can be measur ed. The corresponding values for D'(A) delta through forsterite rims a re essentially identical for the two forsterite-producing reactions wh en 0.1 wt% water is added and similar to the D'(A) delta values for en statite at the same conditions. The DS values for forsterite are simil ar to 28 times larger for samples starting with 0.1 wt% water compared to samples that were first vacuum dried. Thus water enhances these gr ain boundary diffusion rates by a factor of 5-30 depending on the mine ralogy, 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 expecte d for most crustal conditions.