Nonconservative behavior of Br-/Cl- ratios during alteration of volcaniclastic sediments

Other than halite diagenesis and organic matter degradation, Cl- and Br- ar e considered to be conservative in marine pore fluids. Consequently, Br-/Cl - ratios should remain constant during most diagenetic reactions. Nonethele ss, Br-/Cl- molar ratios decrease to 1.27 x 10(-3) (approximate to 18% less than seawater value) in pore fluids from Site 833 in the Aoba Basin of the New Hebrides convergent margin despite the lack of halite diagenesis and l ittle organic matter. Sediment at this site is largely volcanic ash, which becomes hydrated with depth as it converts to clay and zeolite minerals. Th ese hydration reactions remove sufficient water to increase the concentrati ons of most solutes including Cl- and Br-. The resulting concentration grad ients drive diffusion, but calculations indicate that diffusion does not de crease the Br-/Cl- ratio. Some Cl- may be leached from the ash, but insuffi cient amounts are available to cause the observed decrease in Br-/Cl- ratio . The limited source of Cl- suggests that proportionately more Br- than Cl- is lost from the fluids to the diagenetic solids. Similar nonconservative behavior of Cl- and Br- may occur during fluid circulation at ridge crests and flanks, thereby influencing the halide distribution in the crust. Copyr ight (C) 1999 Elsevier Science Ltd.