Silicon diffusion rates were determined in fine-grained (~4.5 &mgr;m) hot-pressed forsterite aggregates at 900¿ to 1200 ¿C and 0.1 MPa using a 30Si-tracer and standard SIMS analysis. There is no measurable difference in diffusion rates between samples buffered by excess MgO or SiO2, nor samples run in dry N2 or H2/CO2. The results yield the Arrhenius parameters D0,bulk=5.4¿10-9 m2/sec and Q=203¿36 kJ/mol. At 1200 ¿C, the grain boundary diffusion rate of 30Si is similar to 26Mg (Si ~10 times greater), ~102 less than Fe, and ~105 less than O. In addition, the 30Si grain boundary diffusion rate is ~109 greater than the Si volume diffusion rate and indicates that diffusional transport of Si in forsterite aggregates is dominated by grain boundaries. ¿ 2000 American Geophysical Union