Na-K interdiffusion in alkali feldspar melts

Abstract

Na-K chemical interdiffusion between albite and orthoclase melts has been measured at 1.0 GPa between 800 and 1600°C and at 2.0 GPa, 1400°C, in anhydrous melts and in hydrous, 5.5 wt% H 2O, melts at 1.0 GPa, 1200 and 1400°C. Anhydrous Na-K diffusivities at X or = 0.5 display Arrhenian behavior even through the inferred glass transition. After correction for diffusion occurring during heating Na-K interdiffusion can be described by D=2.39(+1.7,−0.98)×10 −5 exp(−145.8±6.8/ RT) where D is the diffusion coefficient in m 2s −1, 145.8 is the activation energy in kJ mol −1, R is the gas constant, and T is the temperature in kelvins. These diffusivities are four decades greater than estimated Si-Al diffusivities in the same melts due to the low alkali-oxygen bond valence, or cation field strength, and the abundance of appropriate charge-balanced locations for alkalis in alkali feldspar melts. Extrapolation of diffusion coefficients to 600–650°C for comparison with Na-K interdiffusion in alkali feldspar crystals demonstrates that melting increases diffusion by six orders of magnitude. The effect of pressure on Na-K diffusion at 1400°C is not measurable in this system and suggests that only minor dilation of the coordinating oxygen polyhedra which enclose alkalis in the melt is necessary for diffusion. A preliminary Arrhenius equation for alkali diffusion in silicic melts with 6 ± 0.5 wt% H 2O was calculated using results of this and previous studies D=4.97(+0.85,−0.73)×10 −6 exp(−115.8±1.8/ RT). The effect of H 2O only increases diffusivities by approximately a factor of four, distinctly different from the three orders of magnitude differences between Si-Al, Zr, and P diffusion in anhydrous and hydrous granitic melts. The differing behavior of alkalis and these other cations in hydrous silicic melts is associated with differences in the type and abundance of coordination polyhedra and the low bond valences and cation field strengths of alkalis.