Sulphur mineralogy and geochemistry of serpentinites and gabbros from the Atlantis Massif

In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150-200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high d34Ssulfide values and low d34Ssulfate values, which reflect a multistage history primarily controlled by seawater-gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity.

在大西洋中脊（Mid-Atlantic Ridge，30°N）亚特兰蒂斯海山（Atlantis Massif）中央穹丘处的原位抬升洋壳地块，由综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）第304、305次航次完成钻探，并采获了一段长1.4 km、以辉长岩类岩石为主且夹少量超镁铁质岩的岩心序列。本研究对采自1309D钻井的蛇纹石化橄榄岩、富橄榄石橄长岩及各类辉长岩类样品的硫矿物学与地球化学特征变化进行了系统表征。上述数据被用于限定蚀变过程与氧化还原条件，并与南侧5 km处赋存失落城热液场（Lost City Hydrothermal Field）的亚特兰蒂斯海山南壁基底岩石开展对比。中央穹丘处的洋壳以富镍硫化物为特征，反映了还原环境与有限的海水循环。在抬升与折返过程中，以辉长岩为主的区域内海水-岩石相互作用有限——辉长岩与富橄榄石橄长岩的类地幔硫含量与同位素组成均一，可佐证这一点。类地幔组成的局部偏差与岩浆演化差异，或与沿断裂带运移的海水源流体的相互作用有关。富橄榄石橄长岩中伴生的马基诺矿（mackinawite）与辉长岩中的硬石膏脉，共同限定了700 m深度以下沿局部脆性断层循环的晚期流体温度范围为150~200 ℃。与之相反，中央穹丘处的超镁铁质岩岩性代表了海水通量更高、蚀变程度更强的区域，其硫地球化学特征存在显著差异。钻井上部的蛇纹岩以高总硫化物含量、高δ³⁴S硫化物值与低δ³⁴S硫酸盐值为特征，反映了以海水-辉长岩相互作用及后续蛇纹石化为主控因素的多阶段演化历史。与失落城热液系统的蛇纹岩与辉长岩相比，中央穹丘处的基底岩石记录了更低的氧逸度与更有限的流体通量。本研究结果与前人研究一致，表明蚀变洋壳地幔序列的硫物种与硫同位素组成通常随时间演化。硫地球化学的非均质性反映了蛇纹岩对流体通量、温度、氧与硫逸度以及微生物活动的局部变化具有极高敏感性。