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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B9, 2429, doi:10.1029/2002JB001957, 2003

Chemistry of hydrothermal vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: Geochemical controls in the aftermath of June 1999 seismic events

W. E. Seyfried Jr.

Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA

J. S. Seewald

Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

M. E. Berndt

Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA

Kang Ding

Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA

D. I. Foustoukos

Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA

Abstract

In June 1999, an intense swarm of earthquakes occurred on the Endeavour segment of the Juan de Fuca Ridge influencing hydrothermal activity in and around the Main Endeavour Field (MEF). Here we report the dissolved concentrations of 31 species from five high-temperature vents sampled 3 months after the seismic event. The spatial variability of vent fluid chemistry is extreme. Vapor-dominated vent fluids at Cantilever and Sully sites have high measured temperatures (375°–379°C), high dissolved gas and boron concentrations, but low SiO₂. Modeling results indicate that these fluids can be accounted for by supercritical phase separation and brine condensation. Other vent fluids have moderate temperatures (340°–366°C) and chloride concentrations (208–426 mmol/kg), and may result from mixing of supercritical, vapor-rich fluids with evolved seawater. Phase equilibria calculations indicate that in addition to chloride, redox, temperature, and especially pressure play key roles in accounting for compositional variability of vent fluids at MEF. In comparison with earlier (1988) data, the 1999 data set reveals significantly lower chloride concentrations and higher boron, whereas alkali and alkaline earth cations are lower by 10–20% in keeping with chloride decrease. That dissolved chloride, boron, and other elements returned to preevent levels when again sampled in 2000 provide additional data documenting the inherently dynamic nature of hydrothermal systems at mid-ocean ridges.

Received 4 May 2002; accepted 12 May 2003; published 16 September 2003.

Index Terms: 3035 Marine Geology and Geophysics: Midocean ridge processes; 3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes; 4825 Oceanography: Biological and Chemical: Geochemistry.

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Citation: Seyfried, W. E., Jr., J. S. Seewald, M. E. Berndt, K. Ding, and D. I. Foustoukos (2003), Chemistry of hydrothermal vent fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: Geochemical controls in the aftermath of June 1999 seismic events, J. Geophys. Res., 108(B9), 2429, doi:10.1029/2002JB001957.

Copyright 2003 by the American Geophysical Union.