THE EFFECT OF BULK COMPOSITION ON THE SPECIATION OF WATER IN SUBMARINE VOLCANIC GLASSES

Submarine volcanic glasses ranging in composition from basalts to dacites from a variety of tectonic settings have been studied for their total (H2O + OH-) and molecular water contents using combined high-temperature mass spectrometric and Fourier-transform infrared spectroscopic techniques. A molar absorptivity of 61 +/- 11/mol-cm was determined for the fundamental OH-stretch at 3550 cm-1 for these glasses. It is found that the relative abundances of molecular water and hydroxyl groups depend not only upon total dissolved water but also bulk silicate chemistry as well. For glasses with total H2O contents between 0.5 and 1.0 wt% the ratio of molecular to total water varies approximately tenfold from 0.03 to 0.30. Increasing silica content and/or decreasing the concentration of non-tetrahedral cations enhances the relative abundance of molecular water.