FY 1995

Geochemistry of hydrothermal fluids from Middle Valley, Juan de Fuca Ridge

Butterfield, D.A., R.E. McDuff, J.M. Franklin, and C.G. Wheat

Chapter 20 in Proceedings of the Ocean Drilling Program, Scientific Results, 139, M.J. Mottl, E.E. Davis, A.T. Fisher, and J.F. Slack (eds.), College Station, TX, 395–410 (1994)

Hydrothermal fluids were sampled with Alvin as part of the Ocean Drilling Program (ODP) site survey at Middle Valley in August, 1990. Hydrothermal fluids were recovered from ODP Sites 858 and 856. Endmember fluids from Site 858 (maximum measured temperature 276°C) have chlorinities near 580 millimoles/kilogram (mmol/kg), slightly higher than that of bottom seawater (540 mmol/kg). Endmember fluids from Site 856 (maximum measured temperature 264°C) have chlorinities of 412 mmol/kg. Except for the significant chlorinity depletion in the fluids from Site 856, the fluid chemistry is consistent with a seawater-derived source fluid reacting first with basalt at high temperature and subsequently with hemipelagic sediment. Both ammonia (~3 mmol/kg) and the chalcophillic metals Fe (10-20 µmol/kg), Mn (60-80 µmol/kg), Cu (0.3-1.3 µmol/kg), Zn (0.7-1.7 µmol/kg), and Pb (50-100 nmol/kg) are present at levels intermediate between Guaymas Basin and Escanaba Trough vent fluids, reflecting the influence of temperature on solubility. Trace metal concentrations in Middle Valley vent fluids are close to saturation with respect to magnetite and a suite of metal sulfides. Some reaction with organic matter is indicated by the elevated ammonia and bromide concentrations in the fluids. The alkali and boron systematics indicate a moderate contribution from sediments and a slightly higher water/sediment ratio for Middle Valley fluids compared to both Guaymas Basin and Escanaba Trough fluids. The site was resampled by Alvin in 1991 and by ROPOS (Remotely Operated Platform for Ocean Sciences), the Canadian remotely operated vehicle in 1992, but in both years, recovered samples contained a high percentage of entrained ambient seawater. The time series vent fluid data do not reveal significant changes in composition.