Chemistry of vent fluids and its implications for subsurface conditions at Sea Cliff hydrothermal field, Gorda Ridge

[ 1] We report here the first compositional data for fluids from the Sea Cliff hydrothermal field, northern Gorda Ridge, collected in 2000, 2002, and 2004. An unusual aspect of this site is its location similar to 2.6 km east of the axis of spreading, leading to speculation since its discovery in 1988 that this may be an older'' hydrothermal field, as it occurs on crust that the spreading rate would predict to be similar to 100,000 years old. Our results suggest this hydrothermal system is being driven by subsurface magma, as evidenced by ( 1) elevated He-3/heat ratios, ( 2) relatively high concentrations of He, and ( 3) chloride contents less than seawater in the hydrothermal fluids. The measured fluid temperatures were <= 308 degrees C, but we infer they were > 400 degrees C at depth. In spite of these elevated temperatures, the fluids exiting from these vents are clear, a consequence of extremely low transition metal concentrations. We attribute the low transition metal contents to loss of these metals below the seafloor, most likely as a result of the slightly elevated pH of the fluids. Neither the fluid compositions nor the setting provides evidence that buried sediments and/or organic matter are responsible for raising the fluid pH. Our favored explanation for the elevated pH is that calcite, deposited as a vein-filling mineral at this site, perhaps when it was closer to the axis and a hydrothermal downflow zone, is currently being dissolved by the hydrothermal fluids. This hypothesis is supported by our geochemical modeling results that suggest the fluids are close to saturation with calcite at in situ conditions. Elevated fluid pH is observed at a number of hydrothermal sites on the global mid-ocean ridge system, and the reason for this has not been well understood. Dissolution of previously deposited calcite may be a heretofore unrecognized mechanism that can explain these observations. Finally, our data suggest the compositions of these fluids are unchanged from 2002 to 2004 and are consistent with water column observations first made at this site in 1985. We therefore interpret the Sea Cliff site to be another hydrothermal area marked by long-term stability in fluid compositions and temperatures.