MAGNESIUM GRAIN-BOUNDARY DIFFUSION IN FORSTERITE AGGREGATES AT 1000-DEGREES-1300-DEGREES-C AND 0.1-MPA TO 10-GPA

Magnesium grain boundary diffusion rates in forsterite aggregates have been experimentally determined from 1000 degrees to 1300 degrees C an d 0.1 MPa to 10 GPa. The samples are fine-grained (mean linear interce pt of 4.3 mu m) hot-pressed aggregates. The technique employs a Mg-26- enriched surface layer and depth profiling using an ion microprobe. Va lues of the product of the grain boundary diffusion coefficient (D') a nd the effective grain boundary width (delta) were calculated using ap propriate analytical solutions to the grain boundary diffusion equatio n of Whipple (1954). The Arrhenius parameters for the 0.1 MPa data for samples annealed in H-2+CO2 and CO+CO2 gas mixtures are D-0'delta = 2 .1 x 10(-10) and 7.7 x 10(-10) m(3)/s and Q = 343+/-27 and 376+/-47 kJ /mol, respectively. The reproducibility of D'delta measurements is a f actor of 2. A determination of D' independent of delta yields a calcul ated effective grain boundary which of similar to 3 nm, similar to the physical grain boundary width of 1-3 nm estimated from high-resolutio n transmission electron microscopy observations. The data indicate a v ery low pressure dependence for magnesium grain boundary diffusion in forsterite with an apparent activation volume for grain boundary diffu sion of less than or equal to similar to 1 cm3/mol at 1100 degrees C. The results of this study provide information concerning diffusion cre ep in solid state deformation of forsterite aggregates over a broad ra nge of pressures and temperatures.