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

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 CPa and from 1355 to 1662 degreesC. Glasses enriched in O-18 and S i-28 were synthesized and mated to their isotopically normal counterparts t o form diffusion couples used in the piston cylinder device (1 and 2 GPa) a nd multi-anvil apparatus (4 to 5.7 GPa). Profiles of isotope abundances wer e measured by secondary ion mass spectrometry. Self-diffusion coefficients for Si (D-Si(*)) are significantly lower than self-diffusion coefficients f or O (D-0*) at all run conditions; for example, D0* = 6.45 +/- 0.65 x 10(-1 4) m(2)/s and D-Si* = 1.45 +/- 0.45 x 10-14 m2/s at 1 GPa and 1355 OC. The temperature dependence is similar, but not identical, for Si and O self dif fusion 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 d ependence is similar for Si and O at all temperatures, giving activation vo lumes for Si and O that are -14.5 to -17.1 cm(3)/mol at 1460 degreesC, -9.8 to -8.7 cm(3)/mol at 1561 degreesC, and -8.8 to -9.3 cm(3)/mol at 1662 deg reesC. Self-diffusion coefficients for Si and O reach maximum values at rou ghly 5 GPa. The mode of Si and O self diffusion in dacitic liquids is const rained by the large activation volumes, D-0*= 2 D-Si*, and predictions usin g the Eyring equation, which suggest that Si and O diffuse as molecular spe cies at 1460 degreesC. At 1561 and 1662 degreesC, less negative activation volumes and predictions of the Eyring equation are consistent with diffusio n of Si and O by a combination of mechanisms, including the formation of a high-coordinated intermediate species.