The self-diffusivities of silicon and oxygen in diopside (CaMgSi2O6) liquid
have been measured at pressures and temperatures up to 15 GPa and 2300 deg
reesC, using a 1200-tonne multianvil apparatus. Diffusion couples were prep
ared using finely ground, diopside glass, half of which was enriched in tra
cts isotopes O-18 (5%) and Si-30 (12%). Results indicate that silicon and o
xygen self-diffusivities are coincident (within an accuracy range of 10% RS
D, 1 sigma) up to 13 GPa and show an initial decrease with pressure up to 1
1 GPa(oxygen self-diffusivities are 9.8 X 10(-10) m2/s at 3 GPa and 3.9 X 1
0(-10) m2/s at 11 cpa) after which there is an increase with pressure up to
15 GPa(9.1 X 10(-10) m2/s) at 2000 degreesC. The activation energy of self
-diffusion of both silicon and oxygen was calculated to be 267 kJ mol(-1),
with no observable pressure effect up to 3 GPa. Self-diffusivity is inverse
ly proportional to viscosity, the relationship between these properties bei
ng well approximated by the Eyring equation using the diameter of an oxygen
anion as the translation distance. The calculated viscosities are in good
agreement with previous direct viscosity measurements at lower pressure alt
hough the previously found high positive activation volumes were not reprod
uced. (C) 2001 Elsevier Science B.V. All rights reserved.