Diffusion-controlled melting in granitic systems at 800-900 ℃ and 100-200 MPa: Temperature and pressure dependence of the minimum diffusivity in granitic melts

摘要

This paper presents the temperature and pressure dependence of the minimum binary diffusivity in granitic melts. The minimum diffusivities are determined by monitoring the temporal development of the diffusion-controlled melt layer(DCM) in granitic systems (albite (Ab)-quartz (Qtz)-H_2O and orthoclase (Or)-Qtz-H_2O) gathered during 31 melting experiments under conditions of 800-900 ℃ and 100-200 MPa for durations of 19-72 h. The DCM is formed between single crystals (Ab or Or crystals) and powdered quartz in all runs and is characterized by a distinct concentration gradient. The maximum thickness of the DCM increases systematically with temperature, pressure, and run duration. Temporal development of the DCM obeys the parabolic growth rate law, using which the diffusivity can be estimated. Plots of concentrations along the diffusion paths in ternary diagrams (Na_2O-Al_2O_3-SiO_2 diagram for the Ab-Qtz-H_2O system and K_2O-Al_2O_3-SiO_2 diagram for the Or-Qtz-H_2O system) show linear trends rather than S-shaped trends, indicating that binary nature of diffusion occurs in these systems. Therefore, the diffusive component can be interpreted as an albite component or orthoclase and quartz components (SiO_2) rather than an oxide or a cation.