The history of serpentinisation at mid-ocean ridges: Insights from in situ trace elements coupled with oxygen and boron isotopes

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 δ18O. 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 δ18O compositions of serpentine vary between 0.8 and 7.8 ‰, implying temperature variations within the range of ~115 °C to 290 °C. Where the δ11B compositions of serpentine exceed the expected temperature-related variations of circa 9–13 ‰, we propose that pH variations during progressive serpentinisation are the cause (δ11B 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.