Geochemistry of pore fluids from ODP Leg 134 sites

Depending on the temperature and the extent of diagenetic alteration of fluid chemistry, fluid flow at convergent margins may transfer important quantities of heat and mass between the crust and seawater, thereby influencing global mass, isotopic and heat budgets. In the North Aoba Basin, an intra-arc basin located at the New Hebrides Island Arc, alteration of volcanic ash to clay minerals and zeolites forms a CaCl2 brine, perhaps in less than 1 to 3 m.y. The brine results from an exchange of Ca for Na, K, and Mg, and an increase in Cl concentrations to a maximum of 1241 mM. The Cl increase is partly due to the transfer of H2O from the pore fluid into authigenic minerals, but water mass balances, d18O-Cl correlations, and Br/Cl ratios suggest that there is a source of Cl in the sediments. Concentration profiles indicate that Li is transferred from the fluid to solid phase at depths <300 meters below seafloor (mbsf), but at greater depths it is transferred from the solid to fluid phase, at temperatures possibly as low as 25°C. […]

根据流体化学成岩蚀变的程度与温度条件，汇聚边缘的流体运移可在地壳与海水之间传递大量热量与物质，进而影响全球的物质、同位素及热收支平衡。北奥巴盆地是位于新赫布里底群岛弧（New Hebrides Island Arc）的弧内盆地（intra-arc basin），其内火山灰蚀变为黏土矿物与沸石的过程可形成氯化钙（CaCl₂）卤水，该过程耗时可能不足1至3百万年（m.y.）。该卤水源于钙与钠、钾、镁的离子交换，且氯浓度升高至1241 mM的最大值。氯浓度升高部分源于孔隙流体中的水向自生矿物的转移，但水质量平衡、δ¹⁸O-氯相关性以及溴氯比（Br/Cl）均表明，沉积物中存在氯的来源。浓度剖面显示，在海底以下300米（meters below seafloor，mbsf）以浅的深度范围内，锂（Li）从流体相向固相转移；而在更深的地层中，锂则从固相向流体相转移，该过程的温度可能低至25℃。[…]