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.

本数据集采集自大洋钻探计划（Ocean Drilling Program, ODP）第169航次，于埃斯卡纳巴海槽中央山赋存块状硫化物的碎屑沉积序列内，获取了配套的孔隙流体与沉积物样品（ODP 1038站位）。
本研究报道了该批样品的碱金属元素、硼元素，以及锂与硼同位素数据。相较于位于高热流区外的对照站位1037，1038站位的孔隙流体氯含量波动范围为300~800 mM，且锂（最高6.2 mM）、硼（最高9.7 mM）、铯（最高5.0 mM）与铷（最高97 mM）的浓度显著更高。
研究表明，该孔隙流体源自延伸至基底洋壳的热液循环：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元素。但配套沉积物中并未出现这些元素的显著亏损，暗示流体从原始热液反应位点发生了平流输运。
孔隙流体的锂与硼同位素体系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，广泛形成了次生矿物。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对埃斯卡纳巴海槽海底下方块状硫化物的沉积位点提供重要约束。