Laboratory experiments were conducted in order to determine simultaneously the self diffusivities of Si and O in synthetic dacite melt (NBO/T = 0.1) from 1 to 5.7GPa and 1355 to 1662C. Glasses enriched in 18O and 28Si were synthesized and mated to their isotopically normal counterparts to form diffusion couples used in the piston cylinder device (1 and 2GPa) and multi-anvil apparatus (4 to 5.7GPa). Profiles of isotope abundances were measured by secondary ion mass spectrometry. The self-diffusion coefficients for Si were significantly lower than the self-diffusion coefficients for O under all run conditions. Thus, DO* = 6.45 x 10-14m2/s and DSi* = 1.45 x 10-14m2/s at 1GPa and 1355C. The temperature dependence was similar, but not identical, for Si and O self-diffusion at all pressures; yielding activation energies of 293-380kJ/mol at 1GPa, 264-305kJ/mol at 2GPa and 155-163kJ/mol at 4GPa. The pressure dependence was similar for Si and O at all temperatures; giving activation volumes for Si and O that were -14.5 to -17.1cm3/mol at 1460C, -9.8 to -8.7cm3/mol at 1561C and -8.8 to -9.3cm3/mol at 1662C. The self-diffusion coefficients for Si and O reached maximum values at roughly 5GPa. The mode of Si and O self-diffusion in dacitic liquids was constrained by the large activation volumes, DO*≈2DSi* and predictions using the Eyring equation, which suggested that Si and O diffused as molecular species at 1460C. At 1561 and 1662C, less negative activation volumes and predictions of the Eyring equation were consistent with the diffusion of Si and O via a combination of mechanisms, including the formation of a high-coordinated intermediate species.

Self Diffusion of Si and O in Dacitic Liquid at High Pressures. D.Tinker, C.E.Lesher: American Mineralogist, 2001, 86[1-2], 1-13