The alkali element and boron geochemistry of ODP Site 169-1038 in the Escanaba Trough, East Pacific

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the clastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We report the alkali element and boron, and Li and B isotope data for these samples. Relative to a reference site (Site 1037) located outside the zone of high heat flow, pore fluids from Site 1038 show a wide variation in Cl (300-800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Cs (up to 5.0 mM), and Rb (up to 97 mM). We show that the pore fluids are derived from hydrothermal circulation that has extended into the basement oceanic crust, with input of the alkali elements and B as the rising hydrothermal fluids interact geochemically with the overlying clastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during cooling of the fluids from ~350 to ~20ºC. Precipitation of metal-rich sulphides would have occurred prior to the formation of these minerals, thus, the pore fluid Li and B isotope data can place important constraints on the locus of sulphide deposition beneath the seafloor at Escanaba.

本数据集涵盖大洋钻探计划（Ocean Drilling Program, ODP）第169航次期间，从埃斯卡纳巴海槽（Escanaba Trough）中央山丘赋存块状硫化物（massive sulphides）的碎屑沉积序列（clastic sedimentary sequences）内采集的共轭孔隙流体（pore fluid）与沉积物样品，采样站位为ODP 1038；本研究报道了该批样品的碱金属元素（alkali element）、硼、锂及硼同位素数据（isotope data）。相较于位于高热流带（high heat flow zone）外侧的参考站位（reference site）1037，1038站位的孔隙流体氯含量（300~800 mM）波动范围更广，且锂（最高可达6.2 mM）、硼（最高可达9.7 mM）、铯（最高可达5.0 mM）与铷（最高可达97 mM）的浓度显著高于参考站位1037。研究表明，该孔隙流体源自延伸至基底洋壳（oceanic crust）的热液循环（hydrothermal circulation）：上升热液流体与上覆碎屑沉积物发生地球化学相互作用，带入了碱金属元素与硼；然而，共轭沉积物中未出现这些元素的显著贫化，指示流体从初始热液反应位点发生了平流输送（advective transport）。孔隙流体的锂与硼同位素体系（isotope systematics）模拟结果佐证了这一结论：模拟显示流体从约350℃冷却至约20℃的过程中，广泛发育次生矿物（secondary minerals）形成过程；富金属硫化物的沉淀应发生于这些次生矿物形成之前，因此孔隙流体的锂与硼同位素数据可为约束埃斯卡纳巴海槽海底下方硫化物沉积的赋存位点（locus）提供重要限定依据。