ANDESITIC WATER - A PHANTOM OF THE ISOTOPIC EVOLUTION OF WATER-SILICATE SYSTEMS - COMMENT

On account of the low porosity of the lithosphere, intracrustal fluids behave very differently from surface fluids, in that they are changin g their geochemical and isotopic labels according to the geological en vironment. Given a heat source, meteoric waters can be supplied plenti fully and their rates of throughput in geothermal systems are sufficie ntly high to exhaust the compositional signals of a given rock buffer. In contrast, fluids exsolved from magma, and subducted fluids, would be supplied at less than about one tenth of the meteoric rate over the life time of a system. Based on up-to-date flow models, the isotopic evolution of meteoric water interacting with crustal rock follows a cu rved to L-shaped track in the delta D versus delta(18)O plot. Instanta neous (present-day) tie-lines between recharge and discharge are secan ts of such tracks and can have a range of slopes. At the start of an i nteraction, waters have delta D and delta 18O values approaching equil ibrium with the original rock composition (water ''W1''). Using known hydrogen isotope fractionation factors, W1 values generally plot in th e region of ''andesite'' or ''andesitic'' waters of various authors. S ince the W1 waters have delta D values that are on average more positi ve, and also less variable than those of the meteoric recharges, most tracks and tie-lines have positive slopes, and the plotting of a large number of tie-lines will produce a focus on the field of W1 waters, r egardless of the original water source.