Fluid origins, paths, and fluid-rock reactions at convergent margins, using halogens, Cl stable isotopes, and alkali metals as geochemical tracers

Aqueous fluids play a major role in the physical and chemical evolution of subduction zone systems and profoundly impact the elemental and isotopic composition of the atmosphere, crust, mantle, and seawater. Chlorine stable isotope ratios [delta]³⁷Cl) and halogen concentrations (e.g. Br/Cl) in pore fluid samples and solid samples from three subduction zones, the Nankai Trough, Costa Rica, and Mariana Forearc, provide critical information on fluid sources, flow paths, and reaction conditions at subduction zones. At the Nankai and Costa Rica subduction zones, the [delta]³⁷Cl values of pore fluids are significantly more negative (up to -7.8 %₀, 2o⁻ ± 0.3%₀) than seawater value (0 %₀ ) and the Br/Cl ratios are higher than seawater value (1.5 x 10⁻³). Both [delta]³⁷Cl and Br/Cl show lateral seaward evolution along the transects. The data indicate the existence of a deep- sourced fluid, originated from high temperature hydrous mineral formations that preferentially incorporate the heavier Cl isotope (³⁷Cl), but exclude Br. In contrast, the [delta]³⁷Cl values in the pore fluids at the Mariana mud volcanoes range from +0.3 to +1.8 and the Br/Cl ratios are lower than the seawater ratio. The positive [delta]³⁷Cl values in the pore fluid and the high content of Cl with positive ]delta]³⁷Cl values (+1.2 to +6.0 %₀) in the serpentines, support that the origin of the upwelling pore fluids is the dehydration of the subducting serpentinized crust. These upwelling fluids, together with ridge crest hydrothermal fluids, having [dalta]³⁷Cl of + 2.4%₀ to +4.1%₀, constitute the positive ³⁷Cl fluxes into the ocean that compensate the negative [delta]³⁷Cl fluid flux at subduction zones. In an approach to estimate geochemical fluxes at mid-ocean ridges and subduction zones, hydrothermal experiments were conducted and the alkali metals (Li, K, Rb, and Cs) are used as tracers of fluid-rock reactions. The alkali concentration ratios together with Li isotope ratios may be used as approximate geothermometers. Hence, alkali elements could assist Cl isotopes and halogen concentrations to identify the temperature range and fluid-rock reactions at the fluid source in convergent margin