Barium geochemistry in sediment pore waters and formation waters of the oceanic crust on the eastern flank of the Juan de Fuca Ridge (ODP Leg 168)

Abstract

Sediment pore waters collected on the eastern flank of the Juan de Fuca Ridge at 10 sites drilled during Ocean Drilling Program (ODP) Leg 168 and basement fluids discharging at the seafloor on a basaltic outcrop called Baby Bare located at ∼100 km east of the Juan de Fuca Ridge were analyzed for barium. The data show an overall symmetrical behavior between Ba and sulfate, suggesting that the pore water Ba content is controlled by equilibrium with barium sulfate. Barite saturation indices calculated with a thermodynamic model of the Na‐K‐Ca‐Mg‐Ba‐Sr‐Cl‐SO4‐H2O system show that barite does not equilibrate with the pore waters at the shipboard conditions (20°C, 1 bar) but reflects an overall equilibrium between pore waters and barite at in situ temperatures and pressures, for depths below ∼20 m below the seafloor. Barite supersaturation is found for conditions in the upper 20 m of the sediment column. Ba concentrations in pore waters at the base of the sediment section exceed those in the underlying formation fluids, based on the direct sampling of these fluids in the open borehole at ODP Sites 1026 and the hydrothermal springs at Baby Bare. This suggests that Ba is supplied to basement by the sediment. Despite this, the formation fluids are undersaturated with respect to barite. These fluids, which contain ∼2–3 times more Ba than bottom seawater, vent at the seafloor resulting in a net flux of Ba to bottom seawater. The calculated flux from warm (63°C) ridge flank hydrothermal systems is insignificant to the oceanic budget for Ba, but the flux associated from cool (less than 25°C) ridge flank hydrothermal systems, which dispense most of the heat and fluid flux through the oceanic crust, is unconstrained.