Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin

Details Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin

Nov 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992e%26psl.113..495g&link_type=abstract

Earth and Planetary Science Letters, Volume 113, Issue 4, p. 495-510.

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57

Scientific paper

The trends delineated by D and 18O contents of water discharges, from a wide range of geothermal and volcanic systems along convergent plate boundaries around the Pacific, point to the existence of a common magmatic component with a narrow range of δD values of -20 +/- 10‰. The δD values for these waters, which are generally associated with andesitic magmatism, are well above those of -65 +/- 15‰ suggested for mantle water. The most likely source for this `andesitic' water is recycled seawater. It enters the subduction system in the form of porewater, or as the water of hydration in the clay minerals of accumulated marine sediments and is carried to the source region of arc magmas on the top of the subducting slab. Contributions from the continental and altered oceanic crusts appear to be minor, those from the mantle very minor (< 1%). Large proportions of other components (CO2, N2 and probably Cl) in volcanic and, by implication, geothermal fluid discharges along convergent plate boundaries are likely to be also of predominantly subducted marine origin. Rather than being due to water-rock isotope exchange within the crust, 18O shifts in high-temperature waters from thermal systems along convergent plate boundaries may largely reflect varying degrees of mixing with, in addition to D, 18O-enriched (+ 10 +/- 2‰), and thus isotopically `pre-shifted', andesitic waters. Because of similarities, in terms of their formation conditions and D contents, with `evolved seawaters' associated with sedimentary basins and an accretionary prism in New Zealand, the term `devolved seawater' is proposed for magmatic waters produced from subducted sediments.

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