The chemistry of hydrothermal fluids from the Broken Spur site, 29°N Mid-Atlantic ridge

Hydrothermal fluids have been collected from three high temperature (360–364°C) vents from Broken Spur (29°10.08′N, 43°10.46′W; water depth ∼ 3100 m). This is only the fourth site on a slow spreading ridge from which such fluids have been collected. Compared to other vent sites, the hydrothermal fluids are enriched in Li (1035 μM) and have lower dissolved Mn (∼250 μM) and Sr (43 μM) concentrations. The boron isotope systematics indicate that substantial removal (>50%) of seawater B has occurred in the low-temperature portion of the hydrothermal convection cell. In addition, low temperature removal of seawater Sr is ∼10% greater at Broken Spur compared to similar vent sites in the Pacific where spreading rates are faster. A low Eu anomaly (11 ± 3) and a View the MathML source ratio (10.8) intermediate between pristine and weathered basalt suggest that the vent fluids have interacted with a component of basalt that has previously undergone low-temperature weathering. the fluids are 14% depleted in Cl relative to seawater. Supercritical phase separation appears to be the only reasonable process that may account for the fluid depletion.