HR: 0800h
AN: T21C-0538 INVITED     [Abstracts]
TI: Long-Term Observations of Active Hydrothermal Processes on the Gorda Ridge: The Sea Cliff Hydrothermal Field and Escanaba Trough
AU: * Von Damm, K L
EM: kvd@eos.sr.unh.edu
AF: Complex Systems Research Center, EOS, University of New Hampshire, Durham, NH 03824-3525 United States
AU: Parker, C M
EM: cheryl.parker@unh.edu
AF: Complex Systems Research Center, EOS, University of New Hampshire, Durham, NH 03824-3525 United States
AU: Lilley, M D
EM: lilley@u.washington.edu
AF: School of Oceanography, University of Washington, Seattle, WA 98195 United States
AU: Olson, E J
EM: olson@ocean.washington.edu
AF: School of Oceanography, University of Washington, Seattle, WA 98195 United States
AU: Zierenberg, R A
EM: zierenberg@geology.ucdavis.edul
AF: Dept. of Geology, University of California - Davis, Davis, CA 95616-8605 United States
AU: McClain, J S
EM: jsmcclain@ucdavis.edu
AF: Dept. of Geology, University of California - Davis, Davis, CA 95616-8605 United States
AU: Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AB: The two known sites of high temperature hydrothermal venting on the Gorda Ridge are the northerly Sea Cliff hydrothermal field and the southerly Escanaba Trough. Indications that the Sea Cliff field (GR-14) existed were first obtained from hydrographic work in 1985, and confirmed by the discovery of the site in 1988. Our cruise at the site in 2000 was the first time fluids were sampled, and we subsequently also collected fluids in 2002. One reason the Sea Cliff field is of interest is its location $\sim$3km east of the axis of spreading. It was suggested that the site might therefore be relatively 'old,' or at least located on 'older crust' at the 5.5cm/yr spreading rate. All of the hydrothermal fluids from Sea Cliff are low chlorinity, or vapor phase, contrary to its previous interpretation as an older hydrothermal system which are generally interpreted to vent fluids with chloride contents greater than seawater. Our other chemical data also support our interpretation that this system is not tapping highly altered crust. In spite of the $>$$300\deg$C measured temperatures of the vent fluids, they are strikingly clear, and have Fe contents $\sim30$ umoles/kg, about 2 orders of magnitude less than typical. The low Fe, and other transition metals, are likely a result of the slightly elevated pH (4.5 at $25\deg$C) of these fluids. Possible causes of the elevated pH include: incorporation of buried organic matter, dissolution of fracture-filling calcite, supercritical phase separation phenomena, and other reactions that may impact the proton balance in the fluids, as there is no sediment cover at this site, and no (chemical) evidence for buried sediments. The very low metal contents are in agreement with the water column signals observed in 1985, suggesting the fluids have not changed in $\sim20$ years, and that the site was not impacted by the seismic activity on the Gorda Ridge in 1996 and 2001. Hydrothermal fluids were first collected from Escanaba Trough (NESCA) in 1988, and this site was subsequently drilled by ODP in 1996. Our fluid samples collected in 2000 and 2002 have the same compositions as those collected in 1988. All of the fluids we collected venting from this site are high chlorinity (brines), while ODP also encountered low chlorinity vapors subsurface. We propose a model to explain why the brines are venting preferentially to the vapor phase at this site. Chemical stability in hydrothermal systems, as in these two cases from the Gorda Ridge, are in stark contrast to observations of variability elsewhere on the global mid-ocean ridge system. The time scales and explanations for the fundamental causes of these differences remain poorly understood.
DE: 9355 Pacific Ocean
DE: 4832 Hydrothermal systems
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 1050 Marine geochemistry (4835, 4850)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting