We have investigated grain boundary diffusion rates in enstatite by he
ating single crystals of quartz packed in powdered San Carlos olivine
(Mg0.90Fe0.10)(2)SiO4 at controlled oxygen fugacities in the range 10(
-5.7) to 10(-8.7) atm and temperatures from 1350 degrees to 1450 degre
es C for times from 5 to 100 h at 1 arm total pressure. Following the
experiments, the thickness of the coherent polycrystalline reaction ri
m of pyroxene that had formed between the quartz and olivine was measu
red using backscatter scanning imaging in the electron microprobe. Qua
ntitative microprobe analysis indicated that the composition of this r
eaction phase is (Mg0.92Fe0.08)(2)Si2O6. The rate of growth of the pyr
oxene increases with increasing temperature, is independent of the oxy
gen fugacity, and is consistent with a parabolic rate law, indicating
that the growth rate is controlled by ionic diffusion through the pyro
xene rim. Microstructural observations and platinum marker experiments
suggest that the reaction phase is formed at the olivine-pyroxene int
erface, and is therefore controlled by the diffusion of silicon and ox
ygen. The parabolic rate constants determined from the experiments wer
e analyzed in terms of the oxide activity gradient across the rim to y
ield mean effective diffusivities for the rate-limiting ionic species,
assuming bulk transport through the pyroxene layer. These effective d
iffusivities are faster than the lattice diffusivities for the slowest
species (silicon) calculated from creep experiments, but slower than
measured lattice diffusivities for oxygen in enstatite Thus silicon gr
ain boundary diffusion is most likely to be the rate-limiting process
in the growth of the pyroxene rims. Also, as oxygen transport through
the pyroxene rims must be faster than silicon transport, diffusion of
oxygen along the grain boundaries must be faster than through the latt
ice. The grain boundary diffusivity for silicon in orthopyroxenite is
then given by (D) over bar(Si)(gb) delta = (3.3+/-3.0) x 10(-9) f(O2)(
0.0) e(-400+/-65/RT) m(3)s(-1) where the activation energy for diffusi
on is in kJ/mol, and delta is the grain boundary width in m. Calculate
d growth rates for enstatite under these conditions are significantly
slower than predicted by an extrapolation from similar experiments per
formed at 1000 degrees C under high pressure (hydrous) conditions by Y
und and Tullis (1992), perhaps due to water-enhancement of diffusion i
n their experiments.