Uranium isotopes as tracers of serpentinite weathering

Abstract

The aqueous alteration of mantle rocks is considered to play a key role in Earth's global biogeochemical cycling. Uranium enrichment in fluid-altered mantle rocks is a common feature that may reflect redox state or fluid source, but it remains unclear when, why, and how uranium is mobilized within these systems. In this study we use uranium (U) isotopes (234U/238U and 238U/235U ratios) to deduce the timing, mobilization processes, and fluid sources responsible for weathering of subaerial and submarine serpentinites. We find that complex post-formation weathering processes continue to perturb serpentinite U isotope compositions and U concentrations long after initial serpentinization. Enriched 234U/238U ratios in dredged seafloor serpentinites require recent (<1.5 Myr) uptake of U, likely from derived from chemically evolved seawater that has circulated through igneous basement rocks. In contrast, depleted and variable 234U/238U ratios in a terrestrial serpentinite across <30 cm spatial scales from the Wadi Fins outcrop in the Samail Ophiolite, Oman, indicate alpha recoil effects, with differences in these ratios reflecting different grain sizes or shapes of U-hosting minerals in different weathering zones. Both seafloor and Wadi Fins serpentinites have 238U/235U ratios significantly different from seawater and mid-ocean ridge basalts, reflecting different initial protolith composition and/or overprinting of 238U/235U ratios during weathering. Using 238U/235U and U concentrations we calculate the isotopic composition of evolved fluids weathering the serpentinites at both localities. We also use 234U/238U and models of alpha recoil to determine the grain sizes of minerals hosting U from samples at Wadi Fins. Our results suggest that U isotopic compositions and concentrations are sensitive to post-serpentinization weathering processes in subaerial and submarine environments. The U isotopic composition of abyssal serpentinites is set by seawater that has chemically evolved during flow through basement rocks, questioning the utility of weathered serpentinites as an archive of paleo-seawater U isotope compositions, as well as other fluid-mobile elements modified during crustal fluid flow.