HR: 14:10h
AN: V43C-03    [Abstracts]
TI: Insights Into Peridotite-Hosted Hydrothermal Systems From Petrological and Geochemical Studies of the Lost City Hydrothermal System
AU: * Fruh-Green, G L
EM: frueh@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Boschi, C
V43C-03 AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Kelley, D S
V43C-03 AF: Univ. of Washington, School of Oceanography, Seattle, WA 98195 United States
AU: Delacour, A
V43C-03 AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Bernasconi, S M
V43C-03 AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Karson, J A
V43C-03 AF: Duke University, Division of Earth and Ocean Sciences, Durham, NC 27708 United States
AB: The Lost City hydrothermal field is a rare example of an active peridotite-hosted hydrothermal system, in which reactions between seawater and ultramafic rocks produce high pH (9 to 11) fluids that result in the formation of up to 60m tall carbonate-brucite structures. The 25 to 90° C fluids are enriched in hydrogen, methane and other hydrocarbons, and support dense microbial communities dominated by archaea [Kelley et al., 2005]. We present an overview of field, petrological, and geochemical studies of the basement rocks and hydrothermal precipitates, which provide important insights into the complex interplay between deformation, fluid flow, mass transfer and hydrothermal activity during a long-lived history of detachment faulting, uplift and seawater-peridotite interaction in the Atlantis Massif. Sampling by submersible of this fault-bounded, dome-like oceanic core complex recovered a heterogeneous suite of variably altered and deformed peridotites and gabbroic rocks, overlain by less abundant basalts, breccias and pelagic carbonates. The harzburgitic peridotites are 70-90% serpentinized and have variable mineral and bulk rock chemistries that reflect heterogeneous mantle compositions, varying degrees of melt impregnation, and multiple phases of serpentinization, metasomatism and veining. Talc-amphibole-chlorite metasomatism is a key element of the detachment fault zone at the top of the massif and reflects a deformation and metamorphic history that is distinct from the underlying serpentinites. Major and trace elements compositions of the metasomatic rocks indicate complex interaction between mafic and ultramafic rocks during detachment faulting and suggest localized circulation of oxidizing, Si-Al-Ca-rich fluids in high strain deformation zones. Extensive seawater-peridotite interaction under varying temperatures and high fluid-rock mass ratios in the basement at Lost City are evident by distinct enrichments in B and U contents in the serpentinites, by systematic changes in Sr- and Nd-isotope ratios towards seawater compositions, and by highly depleted bulk rock O-, H-, and B- isotopic compositions. These data together with C-, H-, O- and B-isotope compositions of end-member vent fluids and hydrothermal carbonates document mixing of seawater and deep, serpentinization-derived, CO₂-poor, H₂-rich hydrothermal fluids under a range of temperatures from ambient conditions to >200° C at depth. Our field observations indicate that outflow at the vent sites is controlled by fractures and faults in the basement, and that much of the subsurface flow is along subhorizontal surfaces producing carbonate deposits parallel to the basement foliation and subparallel to gently west-dipping faults. References: Kelley, D.S. et al., (2005) Science, 307(5714), 1428-1434.
UR: http://www.oceanexplorer.noaa.gov/explorations/05lostcity/welcome.html
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3617 Alteration and weathering processes (1039)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005