Strontium and boron geochemistry of ODP sites at Hydrate Ridge, eastern Pacific Ocean

ODP Leg 204, which drilled at Hydrate Ridge, provides unique insights into the fluid regime of an accretionary complex and delineates specific sub-seafloor pathways for fluid transport. Compaction and dewatering due to smectite-illite transition increase with distance from the toe of the accretionary prism and bring up fluids from deep within the accretionary complex to sampled depths (<= 600 mbsf). These fluids have a distinctly non-radiogenic strontium isotope signature indicating reaction with the oceanic basement. Boron isotopes are also consistent with a deep fluid source that has been modified by desorption of heavy boron as clay minerals change from smectite to illite. One of three major horizons serves as conduit for the transport of mainly fluid. Our results enable us to evaluate fluid migration pathways that play important roles on massive gas hydrate accumulations and seepage of methane-rich fluids on southern Hydrate Ridge.

在水合物脊（Hydrate Ridge）实施钻探的大洋钻探计划（Ocean Drilling Program, ODP）第204航次，为解析增生杂岩（accretionary complex）的流体活动体系提供了独特视角，并明确了流体运移的专属海底以下通道。伴随距增生楔（accretionary prism）前缘距离的增加，蒙脱石-伊利石（smectite-illite）转变引发的压实脱水作用逐渐增强，将增生杂岩深部的流体携带至取样深度（≤600米海底以下，meters below seafloor, mbsf）。这些流体呈现显著的非放射成因锶同位素指纹特征，表明其与洋壳基底（oceanic basement）发生了水岩反应。硼同位素数据同样印证了深部流体来源的结论——该流体因黏土矿物由蒙脱石向伊利石转变过程中重硼的解吸作用而发生组分改造。三个主要地层单元中的一个，是主要的流体运移通道。本研究结果可用于评估流体运移路径，此类路径对水合物脊南部的大规模天然气水合物藏（gas hydrate accumulations）形成以及富甲烷流体渗漏过程均具有关键调控作用。