HR: 1330h
AN: B12A-0776    [PDF]
TI: Chemistry of a serpentinization-controlled hydrothermal system at the Lost City hydrothermal vent field
AU: * Ludwig, K A
EM: kludwig@ocean.washington.edu
AF: School of Oceanography, University of Washington, Box 357940, Seattle, WA 98195 United States
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: School of Oceanography, University of Washington, Box 357940, Seattle, WA 98195 United States
AU: Butterfield, D A
EM: butterfield@pmel.noaa.gov
AF: University of Washington and National Oceanographic and Atmospheric Administration, Pacific Marine Environmental Laboratory, Seattle, WA 98195 United States
AU: Nelson, B K
EM: bnelson@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AU: Karson, J A
EM: jkarson@duke.edu
AF: Division of Earth and Ocean Sciences, Duke University, Durham, NC 27708-0230 United States
AB: The Lost City Hydrothermal Field (LCHF), at 30$\deg$N near the Mid-Atlantic Ridge, is an off-axis, low temperature, high-pH, ultramafic-hosted vent system. Within the field, carbonate chimneys tower up to 60 m above the seafloor, making them the tallest vent structures known. The chemistry of the vent structures and fluids at the LCHF is controlled by reactions between seawater and ultramafic rocks beneath the Atlantis massif. Mixing of warm alkaline vent fluids with seawater causes precipitation of calcium carbonate and growth of the edifaces, which range from tall, graceful pinnacles to fragile flanges and colloform deposits. Geochemical and petrological analyses of the carbonate rocks reveal distinct differences between the active and extinct structures. Actively venting chimneys and flanges are extremely porous, friable formations composed predominantly of aragonite and brucite. These structures provide important niches for well-developed microbial communities that thrive on and within the chimney walls. Some of the active chimneys may also contain the mineral ikaite, an unstable, hydrated form of calcium carbonate. TIMS and ICP-MS analyses of the carbonate chimneys show that the most active chimneys have low Sr isotope values and that they are low in trace metals (e.g., Mn, Ti, Pb). Active structures emit high-pH, low-Mg fluids at 40-90$\deg$ C. The fluids also have low Sr values, indicating circulation of hydrothermal solutions through the serpentinite bedrock beneath the field. In contrast to the active structures, extinct chimneys are less porous, are well lithified, and they are composed predominantly of calcite that yields Sr isotopes near seawater values. Prolonged lower temperature seawater-hydrothermal fluid interaction within the chimneys results in the conversion of aragonite to calcite and in the enrichment of some trace metals (e.g., Mn, Ti, Co, Zn). It also promotes the incorporation of foraminifera within the outer, cemented walls of the carbonate structures. The Lost City system represents a novel natural laboratory for observing hydrothermal and biological activity in a system controlled by moderate temperature serpentinization reactions. The LCHF is the only vent field of its kind known to date; however, it is likely not unique along the global mid-ocean ridge spreading network.
UR: http://www.lostcity.washington.edu
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 3035 Midocean ridge processes
DE: 4835 Inorganic marine chemistry
SC: Biogeosciences [B]
MN: 2003 Fall Meeting