Geochemistry of Atlantis Massif serpentinites

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34-91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in d11B (11-16 per mil) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of d11B values (9-10 per mil), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.亚特兰蒂斯地块南部（大西洋中脊，30°N）的失落城热液系统（The Lost City hydrothermal system）为研究蛇纹石化作用（serpentinization）、超镁铁质岩宿主热液系统的时间演化，以及洋核杂岩形成与就位过程中的蚀变条件提供了天然实验室。本研究发布了硼、氧及锶同位素数据，旨在探究亚特兰蒂斯地块内拆离断层作用及岩石圈序列折返过程中的流体-岩石相互作用与质量转移过程。研究数据表明，广泛的蛇纹石化作用以海水为主导，主要发生于150~250℃的温度区间及高累计水岩比（W/R）条件下，该过程造成了显著的硼富集（34~91 ppm）。蛇纹石化过程中从海水中移除的硼与δ¹¹B的变化（11~16‰）呈正相关，但与氧同位素组成无明显关联。模拟结果显示，蛇纹岩的硼浓度及同位素值受瞬态温度-pH条件调控。与前人研究不同，本研究认为低温海洋风化作用对亚特兰蒂斯地块蛇纹岩的硼地球化学特征无显著影响。富滑石及角闪石的断层岩形成于拆离断层带内，形成温度约为270~350℃，且水岩比较低。该地块超镁铁质区域内的滑石形成于蛇纹石化早期阶段之后，其受控于海水-辉长岩相互作用产生的富硅流体的补给。滑石置换蛇纹石的过程造成了硼的流失及δ¹¹B值的显著降低（9~10‰），我们将该过程建模为硼逐步萃取的产物。本研究为大洋扩张脊的硼地球化学循环提供了新的约束条件，并指出与超镁铁质岩宿主热液系统相关的蛇纹石化作用，可能对俯冲带环境中的硼行为具有重要启示意义。