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Geological Society, London, Special Publications; 2003; v. 213; p. 53-62;
DOI: 10.1144/GSL.SP.2003.213.01.04
© 2003 Geological Society of London

Part I Magma Degassing: Models and Experiments

Water diffusion in natural potassic melts

C. Freda1, D. R. Baker2, C. Romano3 & P. Scarlato1

1 Instituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605-00143, Rome, Italy freda{at}ingv.it
2 Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal H3A 2A7, Canada
3 Dipartimento di Scienze Geologiche, Università degli Studi di Roma Tre., Largo San Leonardo Murialdo, 1-00146, Rome, Italy

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 Boltzmann-Matano technique.

Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts (Dwater), m2/s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows:

Formula
where CH2O 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 the haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures.