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 cry stals that accumulate at the site of cooling. The sharp decrease in th e solubility of water in silicate melts with decreasing pressure makes direct measurements of pre-eruptive concentrations in magmas difficul t 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 re sults 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 ig neous cumulates with a convergent-margin, back-arc or inter-arc tecton ic setting.