Multiple magmatic processes and seawater circulation into the uppermost mantle of the Oman ophiolite: Evidence from Sr isotope and geochemical data for CM1A core samples from the Oman Drilling Project

Continuous sampling between the lower crust and uppermost mantle was achieved at the CM1A drill site of the Wadi Tayin massif, the southern Oman ophiolite, during the Oman Drilling Project. We analysed the ⁸⁷Sr/⁸⁶Sr and trace element compositions of relatively fresh gabbroic rocks and extensively serpentinised peridotites (dunite and harzburgite) from the drill core. Most trace element data combined with spinel Cr# (atomic Cr/[Cr + Al] ratios) indicate at least two magmatic processes. The initial ⁸⁷Sr/⁸⁶Sr ratios of the gabbroic rocks (⁸⁷Sr/⁸⁶Sr_96Ma = 0.70303–0.70320) and peridotites (⁸⁷Sr/⁸⁶Sr_96Ma = 0.70377–0.70418) are clearly higher than previously reported primary igneous rocks of the Oman ophiolite (⁸⁷Sr/⁸⁶Sr_100Ma = 0.70265, Kawahata et al., 2001), but significantly lower than seawater at 96 Ma/present and ophicarbonates formed during/after obduction in the Oman ophiolite. Estimated ⁸⁷Sr/⁸⁶Sr ratios of the involved fluids to the CM1A peridotites are consistent with those of endmember hydrothermal vent fluids from East Pacific Rise and Mid Atlantic Ridge. These ⁸⁷Sr/⁸⁶Sr signatures, together with mesh texture of serpentine, indicate seawater penetration into the uppermost mantle at temperatures of <500 °C in an off-axis setting. These observations suggest that the uppermost mantle was hydrated by hydrothermal fluids circulating probably through microcracks formed during serpentinisation, and that the ⁸⁷Sr/⁸⁶Sr ratios of fluids reaching the uppermost mantle were homogenised during their transport through the crust as a hydrothermal system at off-axis.