Barium isotopes in mid-ocean ridge hydrothermal vent fluids: A source of isotopically heavy Ba to the ocean

Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO4) precip-itation during mixing between Ba-bearing vent fluids and SO4-bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36 degrees N and TAG 26 degrees N), the East Pacific Rise (EPR9-10 degrees N and 13 degrees N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 mu mol/kg and delta Ba-138/134 values from-0.26 to +0.91%, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid delta Ba-138/134 values, prior to barite precipitation, are between-0.17 and +0.09 parts per thousand, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Delta Ba-138/134(hyd-barite-fluid) =-0.35 +/- 0.10 parts per thousand (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: delta Ba-138/134(hyd) = +1.7 +/- 0.7 parts per thousand (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at approximate to 3-9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO4 concentrations in the past. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study reveals that hydrothermal vent fluids contain a variety of barium forms, but their initial composition changes due to barite precipitation. Water-rock interaction in the deep oceanic crust appears to occur without barium isotope fractionation. During mixing with seawater, barite precipitation selectively removes isotopically light barium from the fluids.