Magnesium grain boundary diffusion rates in forsterite aggregates have been experimentally determined from 1000¿ to 1300 ¿C and 0.1 MPa to 10 GPa. The samples are fine-grained (mean linear intercept of 4.3 μm) hot-pressed aggregates. The technique employs a 26Mg-enriched surface layer and depth profiling using an ion microprobe. Values of the product of the grain boundary diffusion coefficient (D') and the effective grain boundary width (Δ) were calculated using appropriate analytical solutions to the grain boundary diffusion equation of Whipple (1954). The Arrhenius parameters for the 0.1 MPa data for samples annealed in H2+CO2 and CO+CO2 gas mixtures are D'0Δ=2.1¿10-10 and 7.7¿10-10 m3/s and Q=343¿27 and 376¿47 kJ/mol, respectively. The reproducibility of D'Δ measurements is a factor of 2. A determination of D' independent of Δ yields a calculated effective grain boundary width of ~3 nm, similar to the physical grain boundary width of 1--3 nm estimated from high-resolution transmission electron microscopy observations. The data indicate a very low pressure dependence for magnesium grain boundary diffusion in forsterite with an apparent activation volume for grain boundary diffusion of ≤~1 cm3/mol at 1100 ¿C. The results of this study provide information concerning diffusion creep in solid state deformation of forsterite aggregates over a broad range of pressures and temperatures. ¿ American Geophysical Union 1994