THE INFLUENCE OF WATER ON THE PETROGENESIS OF SUBDUCTION-RELATED IGNEOUS ROCKS

THE presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1-3. This will alter the compositional path followed by residual liquids (the 'liquid line of descent'), and modify the types and compositions of crystals that accumulate at the site of cooling. The sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre-eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption. Here we report experimental results on the crystallization of basalts that contain substantial dissolved water (up to 6 wt%), and show how the results can be used to infer the amount of dissolved H2O involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent, or the accumulated minerals left behind after solidification. The presence of abundant magmatic water is a signature of subduction-zone volcanism4, and can be used to associate a suite of magmas or igneous cumulates with a convergent-margin, back-arc or inter-arc tectonic setting.