Experimental study of water diffusion in haplobasaltic and haploandesitic melts

The diffusion of water in polymerized silicate melts has been studied extensively, but only limited data are available for water diffusion in depolymerized melts such as andesite and basalt. We report here results of eight experiments to determine the diffusivity of water (DH2O, defined to be the diffusivity of total H2O; i.e., regardless of speciation) in haplobasalt and haploandesite melts at 1300 °C and PH2O = 0.3–100 MPa using hydration and diffusion-couple methods. Water contents of the melts ranged from < 0.1 to ∼ 4 wt.%. Our results contribute to understanding the dependence of water diffusivity on melt composition, especially the concentration of water itself. Diffusion experiments were conducted for 420–1500 s using an internally heated pressure vessel. Water concentration vs. distance profiles in quenched glasses was determined using Fourier transform infrared (FTIR) spectroscopy. DH2O values were determined by fitting these profiles using either Boltzmann–Matano methods, simple functional forms for the relationship between water content and DH2O, or the DH2O vs. water content function implied by a simple water speciation model. For both of the compositions studied, DH2O increases with increasing water content. The increase in DH2O with water content can be modeled as exponential over the range investigated, and it is generally consistent with simple models of speciation in which dissolved molecular water is mobile and hydroxyl groups are immobile. The following equations can be used to calculate water diffusivity at various water contents in haplobasaltic and haploandesitic melts at 1300 °C:DH2O=1.27⋅exp(0.538·CH2O)⋅10−10haplobasaltDH2O=4.90⋅exp(0.493·CH2O)⋅10−11haploandesitewhere DH2O is in m2/s and CH2O is in weight percent.Comparison of our results with those in the literature for other compositions suggest an overall correlation between water diffusivity and melt viscosity for CH2O ≤ 3 wt.%. Given the availability of algorithms relating viscosity to melt composition, this correlation can be used to estimate DH2O in melts for which it has not been experimentally determined and to model igneous processes for which DH2O plays a role over a range of magma composition spanning rhyolite to basalt. The following equations can be used to calculate water diffusivity from the viscosity of depolymerized and polymerized melt compositions:logDH2O=−8.87−0.578⋅logηdepolymerized meltlogDH2O=−9.65−0.269⋅logηpolymerized meltwhere η is in Pa s and DH2O in m2/s.

Research highlights
► Experimentally produced H2O diffusion profiles in basic melts were measured by FTIR.
► Diffusivities determined by various numerical methods and models are compared.
► General exponential DH2O – CH2O relationships fit the profiles well.
► Diffusivity–viscosity relationships are given for polymerized and depolymerized melts.