GRADIENTS IN THE COMPOSITION OF HYDROTHERMAL FLUIDS FROM THE ENDEAVORSEGMENT VENT FIELD - PHASE-SEPARATION AND BRINE LOSS

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a la rge vent field near 47-degrees-57'N on the Endeavour segment of the Ju an de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on a pproximately 10 large sulfide structures, which are localized along fa ults and fault intersections across the vent field. Each sulfide struc ture has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given st ructure. However, there are large gradients in fluid composition acros s the vent field, with endmember chlorinity increasing from approximat ely 255 mmol/kg in the SW to 505 mmol/kg in the NE. End-member concent rations of major elements are well correlated with chlorinity, and end member volatile concentrations in the lowest chlorinity fluids are app roximately twice as high as in the highest chlorinity fluids. The grad ients in composition across the vent field and measured vent fluid tem peratures >400-degrees-C are consistent with supercritical phase separ ation and loss of brine phase below the seafloor. The factor-of-2 vari ation in CO2 (and H2S) is larger than expected for loss of a very high -chlorinity brine. Concentrations of iron and manganese are not positi vely correlated with chlorinity, suggesting that temperature and pH ar e more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor inte raction between the hydrothermal fluids and organic matter, and high b oron concentrations point to a sedimentary source.