Water diffusion in natural potassic melts

Water diffusion experiments were performed on a trachytic melt from the Agnano-Monte Spina explosive eruption (Phlegrean Fields, South Italy). Experiments were run in a piston cylinder apparatus at 1 GPa pressure, at temperatures from 1373 to 1673 K and for durations of 0 to 255 s, using the diffusion-couple technique. Water concentration profiles were measured by Fourier transform infrared spectrometry. Water diffusion coefficients at different temperatures and water concentrations were calculated from the total water profiles, using the Boltzntann-Matano technique. Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts (D(water)), m(2)/s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows: D(water) = exp(-11.924 - 1.003ln C(H2O)) exp(-(exp(11.836 - 0.139ln C(H2O)))/RT) where C(H2O) is the water concentration in wt%, R is 8.3145 (J K(-1) mol.(-1)) and T is the temperature in Kelvin. Water diffusivities in trachytic melts were compared with water diffusivities in rhyolitic and basaltic melts. The activation energies for water diffusivity in trachyte and basalt are comparable, and higher than in haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures.