Oceanic serpentinisation is the principal process of water incorporation into the oceanic lithosphere, thus playing a significant role in the elements cycle. The conditions of serpentinisation vary in temperature, water-rock ratio, and fluid composition, but the investigation of the interplay of these factors, rather than their individual variation, is rarely attempted. This study examines mid-ocean ridge (MOR) serpentinite samples from Mid-Atlantic Ridge 15°20′N Fracture Zone (Leg 209, Sites 1272 and 1274) and from Hess Deep (Leg 147, Site 895D and 895E) to gain insight into the combined effects of these variables. Various in situ geochemical tools are used to evaluate the mutual influence of the different factors in the successive stages of serpentinisation. The Cl/B content of serpentine is employed as a proxy for fluid salinity, enabling a more precise temperature calculation when the constant Cl/B correlates with varying δO. The combined in situ temperature-dependent oxygen and boron isotope compositions reveal instances of localised fluctuations in fluid pH that impact the boron isotope composition of serpentine. The δO compositions of serpentine vary between 0.8 and 7.8 ‰, implying temperature variations within the range of ∼115 °C to 290 °C. Where the δB compositions of serpentine exceed the expected temperature-related variations of circa 9–13 ‰, we propose that pH variations during progressive serpentinisation are the cause (δB up to 22 ‰ variability observed between serpentine textures from a single sample). Moreover, this study emphasises the divergence in serpentinisation conditions between MOR and passive margin settings. Serpentinisation can occur at higher temperatures (up to 290 °C) with a more saline fluid (Cl/B > 25) under variable pH conditions in MOR settings, while the serpentinisation in PaMa settings takes place at lower temperatures (< 200 °C) with a less saline fluid (Cl/B < 25) and probably more alkaline conditions.

中文翻译：

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大洋中脊蛇纹石化的历史：来自原位微量元素以及氧和硼同位素的见解

海洋蛇纹石化是水融入海洋岩石圈的主要过程，因此在元素循环中发挥着重要作用。蛇纹石化的条件因温度、水岩比和流体成分而异，但很少尝试研究这些因素的相互作用，而不是它们的个体变化。本研究检查了来自大西洋中脊 15°20′N 断裂带（段 209，站点 1272 和 1274）和赫斯深渊（段 147，站点 895D 和 895E）的大洋中脊 (MOR) 蛇纹岩样本，以深入了解这些变量的综合影响。各种原位地球化学工具用于评估蛇纹石化连续阶段中不同因素的相互影响。蛇纹石的 Cl/B 含量用作流体盐度的代表，当 Cl/B 常数与变化的 δ18O 相关时，可以进行更精确的温度计算。原位温度依赖性氧和硼同位素组成的组合揭示了流体 pH 值局部波动的情况，这种波动影响了蛇纹石的硼同位素组成。蛇纹石的 δ18O 组成在 0.8 到 7.8 ‰ 之间变化，这意味着温度变化在 ∼115 °C 到 290 °C 范围内。当蛇纹石的 δB 成分超过预期的大约 9-13 ‰ 的温度相关变化时，我们认为渐进蛇纹石化过程中的 pH 变化是原因（在单个样品的蛇纹石纹理之间观察到 δB 变化高达 22 ‰）。此外，这项研究强调了 MOR 和被动边缘设置之间蛇纹石化条件的差异。在 MOR 设置中，在可变 pH 条件下，使用含盐量更高的液体 (Cl/B > 25)，蛇纹石化可以在较高温度（高达 290 °C）下发生，而 PaMa 设置中的蛇纹石化则在较低温度（< 200 °C）下发生含盐量较低的液体 (Cl/B < 25) 和可能较多的碱性条件。