THE CHEMISTRY OF HYDROTHERMAL FLUIDS FROM THE BROKEN SPUR SITE, 29-DEGREES-N MID-ATLANTIC RIDGE

Hydrothermal fluids have been collected from three high temperature (3 60-364 degrees C) vents from Broken Spur (29 degrees 10.08'N, 43 degre es 10.46'W; water depth - similar to 3100 m). This is only the fourth site on a slow spreading ridge from which such fluids have been collec ted. Compared to other vent sites, the hydrothermal fluids are enriche d in Li (1035 mu M) and have lower dissolved Mn (similar to 250 mu M) and Sr (43 mu M) concentrations. The boron isotope systematics indicat e that substantial removal (>50%) of seawater B has occurred in the lo w-temperature portion of the hydrothermal convection cell. In addition , low temperature removal of seawater Sr is similar to 10% greater at Broken Spur compared to similar vent sites in the Pacific where spread ing rates are faster. A low Eu anomaly (11 +/- 3) and a Cs/Rb ratio (1 0.8) intermediate between pristine and weathered basalt suggest that t he vent fluids have interacted with a component of basalt that has pre viously undergone low-temperature weathering. The fluids are 14% deple ted in Cl relative to seawater. Supercritical phase separation appears to be the only reasonable process that may account for the fluid depl etion.