Carbon and oxygen geochemistry of the mantle peridotites of the Wadi Tayin Massif, Samail Ophiolite, ICDP Expedition 5057 Oman Drilling Project

A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective sink for CO2. Understanding carbonation mechanisms within ophiolites is almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400 m deep) and BA3A (300 m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (•31 to > 50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26 to 43 °C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1000 yr. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro-fractures and grain boundaries, causing large-scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids with •18O lower than modern ground- and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.

水合大洋岩石圈的很大一部分由蛇绿岩中出露的蛇纹岩构成。蛇纹岩是具有反应活性的化学与热力系统，可能是二氧化碳的有效汇。目前对蛇绿岩内碳化机制的理解几乎完全基于露头研究，这可能限制对古热液系统的解释。我们提供稳定碳与放射性碳数据，为国际大陆科学钻探计划（ICDP）阿曼钻探项目BA1B孔（深400米）和BA3A孔（深300米）中橄榄岩碳化的同位素趋势及流体演化提供见解。对蛇纹岩中碳酸盐的地球化学研究表明，其形成于过去50千年内，这意味着萨迈勒蛇绿岩经历了与初始大洋水合及蛇纹岩化显著不同的蚀变阶段。最古老的碳酸盐（约31至>50千年）为局部产出的方解石、白云石和文石脉，形成于26至43°C之间，与集中流体流动相关。随后在过去1000年内沉淀了广泛分布的少量分散碳酸盐。橄榄岩的宏观角砾化与脉体发育表明，碳化受构造特征影响，这些构造允许流体在不同构造层次（如断裂面、微裂隙和晶界）长期渗透，导致蛇绿岩的大规模水合。分散碳酸盐的形成与δ¹⁸O低于现代地下水和大气降水的渗透流体有关。本研究表明，放射性碳研究是解读碳化历史的关键工具，仅依靠稳定氧和碳同位素可能导致模糊的解释。