LAUSR

Abstract The early Paleoproterozoic represents a period of rapid changes in Earth systems that could have affected the stable isotopic composition of seawater. The well-preserved pillow structures, hyaloclastites and komatiitic basalts of the 2.43–2.41 Ga Vetreny belt, Baltic Shield provide a record of high-temperature water-rock interaction induced by contemporaneous seawater. Here we present results of mineralogical, fluid inclusion, hydrogen, and triple oxygen isotopic analysis of hydrothermal alteration products. Emphasis is given to vein-filling quartz and epidote as they likely formed at high water-rock ratios. Ten minerals pairs, quartz-epidote and quartz-calcite, returned temperatures of isotopic equilibrium between 286 and 387 °C, which compares well to the homogenization temperatures measured for saline fluid inclusions hosted in vein quartz. The computed δD and δ18O values of equilibrium fluids range between −31 and +12‰, and −1.36 and +3.20‰, respectively, which overlap with the isotopic composition of ice-free world seawater and fluids generated at submarine hydrothermal systems. This is the earliest piece of evidence suggesting that early Paleoproterozoic seawater had a δD value close to 0‰. We also present triple oxygen isotopic composition of quartz and epidote that formed in similar facies of hydrothermal alteration from the relatively young (6–7 Ma) oceanic crust as sampled by the ODP Hole 504B in the eastern Pacific Ocean. These data show similarity to the triple oxygen isotope analyses of the Vetreny belt rocks indicating that the 2.43–2.41 Ga seawater had the Δ17O value close to that of modern-day seawater. Due to small fractionation at 300–390 °C (αepidote-water ≈ 1), epidotes present a strong evidence that equilibrium fluids had Δ′17O values close to 0‰. Based on the previously published quartz-water calibration, the computed Δ17O values of equilibrium fluids range between −0.11 and −0.03‰, significantly lower than that of seawater or inferred seawater-derived fluids at low water-rock ratios. This can be explained by multiple factors including phase separation of fluids or/and presence of low-temperature quartz overgrowths. Both are reflected in the fluid inclusion data and in situ δ18O measurements by ion microprobe (SIMS) presented here. Overall, our study suggests that the δ18O, Δ17O and δD values of the 2.43–2.41 Ga seawater were −1.7 ± 1.1, −0.001 ± 0.011 and 0 ± 20‰ respectively, similar to the modern values, which reflects the dominant role of submarine hydrothermal alteration in the stable isotopic budget of seawater throughout Earth’s history.