HR: 1340h
AN: B13A-0198 [Abstracts]
TI: Fluid-Rock Interaction in the Basement of the Lost City Vent Field: Insights from Stable and Radiogenic
Isotopes
AU: * Delacour, A
EM: adelie.delacour@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, 8092
Switzerland
AU: Frueh-Green, G L
EM: frueh@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, 8092
Switzerland
AU: Frank, M
EM: martin.frank@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, 8092
Switzerland
AU: Bernasconi, S M
EM: stefano@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, 8092
Switzerland
AU: Boschi, C
EM: chiara.boschi@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, 8092
Switzerland
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: Univ. of Washington, School of Oceanography, Seattle, WA 98195
United States
AB:
The Lost City Hydrothermal Vent Field (LCVF), with its characteristic carbonate-brucite chimneys, is located on a terrace, at
a water depth of 750 to 850m on the southern escarpment of the Atlantis Massif (Mid-Atlantic Ridge $30^{\circ}$N). The
Atlantis Massif, an oceanic core complex, consists of peridotites and gabbroic rocks that have undergone several phases of
serpentinization, talc-metasomatism and carbonate precipitation related to progressive deformation and interaction with
seawater during a long-lived exhumation history. We present stable and radiogenic isotope data from the serpentinized
peridotites and gabbros that provide constraints on the history of seawater-rock interaction and the role of serpentinization
in methane-production and sulfide mineral precipitation.
Early phases of serpentinization and metasomatism occurred at temperatures up to $\sim$250$^{\circ}$C, as indicated by
depleted bulk-rock O-isotope compositions of the serpentinites and gabbros. Sr- and Nd-isotope data allow modelling and
quantification of seawater-rock interaction. The isotopic compositions of the gabbros show heterogeneity, likely related to
variable interaction/exchange with hydrothermal fluids. The serpentinites have Sr- and Nd-isotope compositions close to
seawater values and correspond to high water/rock ratios (from 1.53 x 10$^{3}$ to 3.65 x 10$^{6}$), indicating large volumes
of seawater circulating through the massif during serpentinization. In contrast, the serpentine-talc schists, resulting from
high strain and focused fluid flow of Si-rich fluids during detachment faulting and exhumation, exhibit variable but low
fluid/rock ratios (from 10 to 150).
Analyses of bulk-rock carbon contents and carbon isotope compositions show total non-carbonate carbon contents of $<$600 ppm
with C-isotope compositions of -29 to -22$\permil$ (VPDB). The negative $\delta$$^{13}$C values are likely linked to
processes of fluid-rock interaction during serpentinization. C- and O-isotope compositions of carbonate-rich serpentinites
and carbonate veins reflect mixing of seawater and deep serpentinization-derived hydrothermal fluids, and a range of
precipitation temperatures from ambient conditions up to $\sim$180$^{\circ}$C at depth. Sulfur contents in bulk-rock
serpentinites indicate a significant addition of sulfur during serpentinization with S-isotope compositions close to seawater
values. $\delta$$^{34}$S ratios of extracted sulfides suggest that their formation is linked to sulfate reduction.
Collectively, the isotopic compositions of the vents and basement rocks, measured fluid temperatures, and $^{14}$C ages of
the vents, sediments and veins provide evidence that hydrothermal fluid flow and temperatures of
$\sim$100$^{\circ}$-150$^{\circ}$C have likely been sustained in the basement for many tens of thousands of years.
DE: 8135 Hydrothermal systems (8424)
DE: 7218 Lithosphere and upper mantle
DE: 3035 Midocean ridge processes
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting