Self diffusion of Si and O in dacitic liquid at high pressures

Abstract

Laboratory experiments have been conducted to determine simultaneously the self diffusivities of Si and O in synthetic dacite melt (NBO/T = 0.1) from 1 to 5.7 GPa and from 1355 to 1662 °C. 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 2 GPa) and multi-anvil apparatus (4 to 5.7 GPa). Profiles of isotope abundances were measured by secondary ion mass spectrometry. Self-diffusion coefficients for Si (D*Si) are significantly lower than self-diffusion coefficients for O (D*0) at all run conditions; for example, D*0 = 6.45 ± 0.65 × 10-14 m2/s and D*Si = 1.45 ± 0.45 × 10-14 m2/s at 1 GPa and 1355 °C. The temperature dependence is similar, but not identical, for Si and O self diffusion at all pressures, yielding activation energies of 293-380 kJ/mol at 1 GPa, 264-305 kJ/mol at 2 GPa, and 155-163 kJ/mol at 4 GPa. The pressure dependence is similar for Si and O at all temperatures, giving activation volumes for Si and O that are -14.5 to -17.1 cm3/mol at 1460 °C, -9.8 to -8.7 cm3/mol at 1561 °C, and -8.8 to -9.3 cm3/mol at 1662 °C. Self-diffusion coefficients for Si and O reach maximum values at roughly 5 GPa. The mode of Si and O self diffusion in dacitic liquids is constrained by the large activation volumes, D*0 ≈ 2 D*Si, and predictions using the Eyring equation, which suggest that Si and O diffuse as molecular species at 1460 °C. At 1561 and 1662 °C, less negative activation volumes and predictions of the Eyring equation are consistent with diffusion of Si and O by a combination of mechanisms, including the formation of a high-coordinated intermediate species.