HR: 1330h
AN: B12A-0775    [PDF]
TI: Long-Lived Serpentinization and Carbonate Precipitation at the Lost City Hydrothermal Vent Field
AU: * Frueh-Green, G L
EM: gretli@erdw.ethz.ch
AF: ETH-Zurich, Department of Earth Sciences, Zurich, 8092 Switzerland
AU: Kelley, D S
AF: University of Washington, School of Oceanography, Seattle, WA 98195 United States
AU: Karson, J A
AF: Duke University, Division of Earth and Ocean Sciences, Durham, NC 27708-0230 United States
AU: Bernasconi, S M
AF: ETH-Zurich, Department of Earth Sciences, Zurich, 8092 Switzerland
AU: Proskurowski, G
AF: University of Washington, School of Oceanography, Seattle, WA 98195 United States
AU: Ludwig, K A
AF: University of Washington, School of Oceanography, Seattle, WA 98195 United States
AB: The discovery of spectacular, actively venting carbonate chimneys at the Lost City hydrothermal vent field (LCHF) on the Atlantis Massif (MAR $30^{\circ}$N) has stimulated great interest in the role of serpentinization in driving hydrothermal circulation in peridotite-hosted systems and in the biological communities that may be supported in these environments. The top of this fault-bounded, dome-like massif consists of variably deformed, talc-bearing serpentinites and gabbroic rocks ($\sim$1.5 Ma), unconformably overlain by polymictic sedimentary breccias and bedded pelagic limestones or chalks that form a flat-lying carbonate cap. The limestones and matrix of the breccias consist of highly indurated foraminiferal sand with a well-preserved sub-tropical fauna, which were at least locally deposited before the last glacial maximum. Calcite and/or aragonite veins are abundant; fractures in the basement are filled by carbonate sediments and lithic fragments. Veining generally pre-dates sedimentary fracture-infilling. The youngest hydrothermal phases include the LCHF chimneys and carbonate precipitates on outcrop surfaces, in cavities, and as growths protruding from fissures that are locally venting fluids. Sr-, C- and O-isotope analyses and radiocarbon age-dating indicate that this system is the integrated effect of tectonic activity, serpentinization, and hydrothermal flow that has lasted at least 30,000 years. C- and O-isotope compositions indicate a range of precipitation temperatures from ambient conditions up to $\sim 250^{\circ}$C at depth and reflect mixing of seawater and serpentinization-derived hydrothermal fluids. Analyses of separated fractions of sedimentary and hydrothermal components define a sedimentary end-member composition of $\delta^{13}C = 1.3 \pm 0.3$ and $\delta^{18}O = 1.5 \pm 0.5\permil$ (VPDB) and a hydrothermal end-member composition of $\delta^{13}C = 3.3$ and $\delta^{18}O = 5\permil$. Based on the present-day degree of serpentinization, the geophysical structure and age of the lithosphere at the Atlantis Massif, and the radiocarbon ages, we estimate a minimum rate of serpentinization of $1.2 \cdot 10^{-4}km^{3}/y$. Our field observations together with available gravity and seismic data indicate that a considerable proportion of the massif is relatively unaltered peridotite. The access of seawater to relatively cool, fresh peridotite, coupled with faulting, volumetric expansion and mass wasting processes are crucial to sustain such systems. In addition, diffusely percolating, high pH fluids emanating from the underlying serpentinites promote rapid sediment lithification, which offers an efficient mechanism for slowing heat loss and maintaining higher temperatures in the basement. Collectively these processes have the potential to prolong hydrothermal activity for tens of thousands of years.
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 7218 Lithosphere and upper mantle
DE: 8135 Hydrothermal systems (8424)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting