Cloride and bromide concentrations and Br-/Cl- molar ratios in pore fluids at ODP Site 134-833@en

Other than halite diagenesis and organic matter degradation, Cl- and Br- are considered to be conservative in marine pore fluids. Consequently, Br-/Cl- ratios should remain constant during most diagenetic reactions. Nonetheless, Br-/Cl- molar ratios decrease to 0.00127 (~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 little organic matter. Sediment at this site is largely volcanic ash, which becomes hydrated with depth as it converts to clay and zeolite minerals. These hydration reactions remove sufficient water to increase the concentrations of most solutes including Cl- and Br-. The resulting concentration gradients drive diffusion, but calculations indicate that diffusion does not decrease the Br-/Cl- ratio. Some Cl- may be leached from the ash, but insufficient 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.除石盐（halite）成岩作用与有机质降解外，氯（Cl⁻）与溴（Br⁻）在海洋孔隙流体（marine pore fluids）中通常被视为保守性组分。据此，多数成岩反应过程中溴氯摩尔比（Br-/Cl- ratios）应维持恒定。然而，新赫布里底汇聚边缘青叶盆地833站位的孔隙流体中，溴氯摩尔比降至0.00127（较海水值低约18%），尽管该区域未发生石盐成岩作用，且有机质含量极低。该站位沉积物以火山灰为主，随埋藏深度增加，火山灰会发生水合作用并转化为黏土与沸石矿物。此类水合反应可脱除足量水分，使包括氯、溴在内的多数溶质浓度升高，由此产生的浓度梯度会驱动扩散作用，但计算结果显示，扩散并不会导致溴氯摩尔比降低。部分氯可能从火山灰中淋滤而出，但淋滤量不足以引发观测到的溴氯摩尔比下降。有限的氯来源表明，相较于氯，有比例更高的溴从流体中流失至成岩固相之中。在洋脊轴部与侧翼的流体循环过程中，氯与溴可能也会出现类似的非保守性行为，进而影响地壳内的卤化物分布。