HR: 17:30h
AN: V12J-07    [PDF]
TI: Variable Seawater-Peridotite Interactions - First Insights From ODP Leg 209, MAR $15\deg$N
AU: * Bach, W
EM: wbach@whoi.edu
AF: WHOI, 360 Woods Hole Rd., Woods Hole, 02543 United States
AU: Garrido, C J
EM: carlosg@ugr.es
AF: University of Granada, Dept of Mineralogy and Petrology, Granada, 18002 Spain
AU: Harvey, J
EM: j.harvey@open.ac.uk
AF: The Open University, Dept. of Earth Sciences, Milton Keynes, MK7 6AA United Kingdom
AU: Paulick, H
EM: Holger.Paulick@uni-bonn.de
AF: University of Bonn, Mineralogisch-petrologisches Institut, Bonn, 53115 Germany
AU: Rosner, M
EM: rosner@gfz-potsdam.de
AF: GFZ-Potsdam, Telegrafenberg B122, Potsdam, 14473 Germany
AU: ODP Leg 193 Shipboard Science Party,
AF: Ocean Drilling Program, TAMU 1000 Discovery Drive, College Station, 77845 United States
AB: Serpentinization of peridotites at slow-spreading mid-ocean ridges has important consequences for the rheology of the oceanic lithosphere, geochemical budgets of the oceans, and microbial processes within, at, and above the seafloor. ODP Leg 209 recovered peridotites that show a remarkable variability of hydrothermal alteration reactions and intensities, including talc-tremolite alteration of pyroxenes associated with incipient serpentinization of olivine, complete alteration of peridotites to serpentine and magnetite followed by the destruction of magnetite and replacement of serpentine by talc, variable degrees of serpentine-brucite alteration, and replacement of brucite by iowaite. These rocks provide a unique opportunity to calibrate our observations against recent results from experimental/theoretical geochemical studies and further our understanding of serpentinization and its role in tectonic accretion and microbial colonization of oceanic lithosphere at slow and ultraslow spreading ridges. We propose that at temperatures above $250\deg$C, pyroxenes react to form serpentine, talc, and tremolite, releasing Ca, Si, H$_{2}$, and acidity to the reacting fluids that may cause rodingitization in adjacent gabbro bodies. Overall however, rodingites are rare, which may reflect the depleted nature of the mantle protoliths. In the absence of pyroxenes (in dunites) - or at temperatures below $250\deg$C, where pyroxenes react very slowly - the fluids do not become enriched in Ca and Si and serpentine, magnetite, and brucite will form. Many serpentinites lack brucite, tremolite, and talc, because changes in fluid pH and silica activity of the interacting fluids, following the exhaustion of either olivine or pyroxene, caused reaction of these phases to serpentine. Extensive talc alteration of serpentinites and gabbros is usually observed at the intrusive contacts, but large-scale silica metasomatism (or Mg-loss) must be invoked to explain the overall abundance of talc at Site 1268. Significant pyrite/marcasite/hematite veining at Site 1268 indicates fairly oxidizing conditions consistent with the presence of sulfate in the interacting fluids. The first discovery of iowaite in mid-ocean ridge serpentinites (at Site 1272) and the abundant carbonate/Fe-oxyhydroxide alteration, locally extending down to 90 meters below seafloor, indicate that water-rock reactions continue at low temperatures and under strongly oxidizing conditions. Pore fluids from nontronite-bearing serpentine muds in fault gouges may provide information about the nature of these late-stage circulating fluids and potential microbial activity.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 4832 Hydrothermal systems
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
DE: 8045 Role of fluids
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting