HR: 0800h
AN: V31A-1424    [Abstracts]
TI: Cycling of Halogens Through the Cascadia Subduction Zone and the Cascade Volcanic arc: Insights From Thermal Springs in Central Oregon, USA
AU: * Hurwitz, S
EM: shaulh@usgs.gov
AF: U.S. geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Mariner, R H
EM: rmariner@usgs.gov
AF: U.S. geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Fehn, U
EM: fehn@earth.rochester.edu
AF: University of Rochester, Department of Earth and Environmental Sciences Hutchison Hall 227, Rochester, NY 14627 United States
AU: Snyder, G T
EM: gsnyder@rice.edu
AF: Rice University, Earth-Science-MS 126 P.O. Box 1892, Houston, TX 77251-1892 United States
AB: We use concentrations of halogen elements and isotopic ratios of 36Cl/Cl and 129I/I to examine the cycling of halogen elements through the Cascadia subduction zone and volcanic arc. The samples come from thermal springs in the most volcanically-active segment of the Cascades Range (north-central Oregon) and mineral springs in the forearc (Willamette Valley), and are compared to halogen concentrations in pore fluids from the accretionary prism. Mass-balance, element-ratio, and isotopic considerations suggest that a significant part of the Cl, as well as other halogens discharged from the thermal springs are derived from magma degassing. Given published estimates of the subducted halogen flux, mass balance suggests that only about 3% of the Cl, 1.5% of the Br and 0.5% of the I are discharged via thermal springs along the arc. These fluxes are low compared to those in other volcanic arcs and may indicate that most devolatilization from the slab occurs at shallow depths, perhaps related to the elevated temperatures of the subducted Juan de Fuca plate. Total Cl and I discharge along the 260-km segment of the arc is one order of magnitude lower than fluxes from fumaroles in individual volcanoes along the arcs of Central America, Japan, and New Zealand. If our halogen mass-balance calculations are indicative of water loss from the subducting Juan de Fuca Plate, then this would imply a relatively dry sub-arc mantle wedge, and limited melting. Our estimate is in good agreement with the observation that the degree of volcanic vigor in the Cascade Range is low compared to other volcanic arcs surrounding the Pacific Rim, expressed in the paucity of Holocene volcanoes, low-frequency of known eruptions and large volumes of basaltic andesite compared to the more silicic compositions in other arcs.
DE: 8424 Hydrothermal systems (8135)
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting