HR: 1340h
AN: V23B-0630    [Abstracts]
TI: The Geochemical Consequences of Serpentinization and Weathering of Oceanic Peridotites
AU: * Humphris, S E
EM: shumphris@whoi.edu
AF: Geology and Geophysics Dept., Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Bach, W
EM: wbach@whoi.edu
AF: Marine Chemistry and Geochemistry Dept., Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: A suite of partially to completely serpentinized and weathered abyssal peridotites dredged from rift valley walls along the SW Indian Ridge (11-$13\deg$N), the equatorial Mid-Atlantic Ridge, and the Mid-Cayman Rise, and from the Bullard, Islas Orcadas and Bouvet fracture zones on the westernmost SW Indian Ridge have been analyzed to determine chemical changes during peridotite-seawater reactions, their impact on seawater chemistry, and their consequences for subduction zone fluxes. The serpentinites are dominated by lizardite that, together with minor magnetite, replaces olivine and orthopyroxene. Chrysotile occurs in microscopic veins with very fine-grained magnetite or in wispy, white monomineralic veins. At times, orthopyroxene is replaced by tremolite and minor talc. With increasing H$_{2}$O$^{+}$ content (an indicator of extent of serpentinization), Si, Fe, Mg, Zn, Cu and Cr concentrations remain relatively constant, while Ca and Ba contents decrease and V and Ni increase. Comparison of these compositions with those of derived fresh rock precursors suggest mostly minor elemental exchange during serpentinization. Two different types of low-temperature seafloor weathering can be distinguished. Type 1 is dominated by weathering of relict olivine to iddingsite (clay+goethite+aragonite) and results in increases in Ca, Sr, and C, and losses of Mg and Si. Type 2 is characterized by weathering of serpentine and magnetite to talc+hematite$\pm$dolomite, with the chemical consequences being increases in Si, Fe, C, Mn, La, V, Y, Zr, Cu, and Zn and relative losses in Mg and H2O. V/Ti, La/Ce, and Y/Ho correlate well with each other, while Cu and Zn correlate with Zr/Hf (which ranges between 9 and 172). We suggest that scavenging of La, V, Y, and probably Cu, Zn, and Zr occurs during Type 2 slow weathering of serpentinite. Preliminary estimates of the magnitudes and directions of elemental exchange during both serpentinization and low-temperature weathering of peridotites have been derived both by derivation of volume change resulting from mineral reaction equations, and by identification of immobile components for use in calculation of a mass change term. These provide some of the first estimates of peridotite-seawater reactions on global geochemical fluxes.
DE: 8135 Hydrothermal systems (8424)
DE: 3035 Midocean ridge processes
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting