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 are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope 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°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. 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 δ¹⁸O 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.

中文翻译：

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瓦迪泰因地块活跃蛇纹石化地点的碳地球化学：来自 ICDP 阿曼钻探项目第二阶段的见解


水合海洋岩石圈的很大一部分由暴露在蛇绿岩中的蛇纹岩组成。蛇纹岩构成反应性化学和热系统，并可能代表CO ₂的有效汇。了解蛇绿岩内的碳化机制几乎完全基于对露头的研究，这可能限制对化石热液系统的解释。我们提供稳定的放射性碳同位素数据，为 ICDP 阿曼钻探项目 BA1B（400 米深）和 BA3A（300 米深）钻孔中橄榄岩碳化的同位素趋势和流体演化提供见解。对蛇纹岩中碳酸盐的地球化学研究表明其形成于最近 50 kyr，这意味着与萨迈伊蛇绿岩最初的海洋水化和蛇纹石化阶段明显不同的蚀变阶段。最古老的碳酸盐岩（∼31 至 >50 kyr）是局部方解石、白云石和文石脉，形成于 26°C 至 43°C 之间，与集中流体流动有关。随后在过去的 1000 年里，普遍存在少量分散的碳酸盐沉淀。橄榄岩的宏观角砾化和脉纹表明，碳化作用受到构造特征的影响，允许流体在较长时期和不同结构水平（例如沿着断裂面、微裂缝和晶界）渗透，导致蛇绿岩大规模水化。分散碳酸盐的形成与δ ¹⁸ O低于现代地下水和大气水的渗滤流体有关。我们的研究表明，放射性碳研究是解释碳化历史的重要工具，而稳定的氧和碳同位素本身可能会导致模糊的解释。