Oceanic serpentinisation in the Samail ophiolites: evidence from trace elements and O and B isotopes

Oceanic serpentinisation represents a major link between the hydrosphere and the lithosphere, whereby aqueous fluids hydrothermally alter mantle rocks. The study of former oceanic lithosphere, now tectonically emplaced on land, can serve as a useful analogue to modern processes. In this study, we present the geochemical composition of serpentinised peridotites from the southern Samail ophiolites, a fast-spreading ridge axis active in the Neo-Tethys, in the Sumail and Wadi Tayin massifs. The combination of the fluid-mobile element (FME) composition, as well as O, B and Sr isotopes, of serpentine is utilised to decipher the source of the serpentinising fluid, which would constrain the loci of serpentinisation (i.e., oceanic domain, during obduction, after emplacement). The in situ analysis of the serpentine revealed FME contents ranging from 0.2 to 27.7 μg g⁻¹ B, 0.1–10.7 μg g⁻¹ Sr, and 600–1800 μg g⁻¹ Cl; high variability in δ¹⁸O values (+1.3 to +8.1 ‰); and δ¹¹B ranging from +33 to +41 ‰ in one sample, and between +2 and +47 ‰ for two others. A comparison of the data with the geochemical characteristics of potential sources of the serpentinising fluid (e.g., seawater, hydrothermally-altered fluid, subducted-slab-derived fluid, meteoric water) has failed to provide any evidence indicative of serpentinisation with subducted slab-derived fluid or meteoric water. Conversely, the geochemical characteristics of serpentine indicate that serpentinisation most likely occurred on the ocean floor in proximity to the spreading ridge axis, subsequent to reaction with seawater and hydrothermally-altered fluid following interaction with the overlying oceanic crust. Furthermore, the range of measured δ¹¹B in certain samples can be modelled on the basis of a two-step serpentinisation history in a closed system, at relatively low temperatures, calculated from the δ¹⁸O values (from 115 to 250 °C). This study demonstrates the efficacy of combining multiple in situ geochemical tools to trace the history of serpentinisation, particularly in obducted ophiolite where hydrothermal alteration may could have occurred on multiple occasions.