HR: 1330h
AN: T32A-0910    [PDF]
TI: Mapping the Mariana Seismogenic Zone Through the Measurement of Geochemical Tracers in Serpentinite Seamounts
AU: * Hulme, S M
EM: shulme@mlml.calstate.edu
AF: Moss Landing Marine Laboratories, 8272 Moss Landing Rd., Moss Landing, CA 95039 United States
AU: Wheat, C G
EM: wheat@mbari.org
AF: GURU, University of Alaska, 7700 Sandholdt Rd. Bldg. D, Moss Landing, CA 95039 United States
AU: Mottl, M J
EM: mmottl@soest.hawaii.edu
AF: SOEST, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822 United States
AU: Fryer, P
EM: pfryer@soest.hawaii.edu
AF: SOEST, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822 United States
AB: The Mariana forearc contains tens of seamounts up to 2 km high and 20-50 km in diameter. These seamounts were formed by serpentinite mud volcanism, sometimes in combination with uplift of serpentinized forearc mantle blocks, in which fluids driven off of the subducting slab infiltrated the overlying mantle and serpentinized the harzburgite and dunite rocks creating a density imbalance within the mantle. The resulting fluid-rock matrix flows along faults and exposes mantle-sourced serpentinite muds, blueschist facies metamorphosed mafic clasts, and slab-sourced fluids at the seafloor. The protrusion of these materials allows direct observation of active subduction zone components that are elsewhere buried beneath kilometers of rock and sediment. A multi-disciplinary survey of the Mariana Forearc was conducted in the spring of 2003 to study the biogeochemical properties of this mud volcanism. Seven different seamounts were sampled using shipboard and subsea coring techniques employing RV Thomas G. Thompson and ROV Jason II, respectively. Pore waters were extracted from these sediment cores and analyzed for several chemical constituents at sea. The measured values were consistent with preliminary work from 1997. Systematic trends in chemical composition of these high pH fluids (up to 12.3) are observed with distance from the trench (proxy for the depth to slab). These trends include low alkalinity and high Ca near the trench (e.g., Blue Moon Seamount; 0.26 mmol alkalinity/kg and 55 mmol Ca/kg), and high alkalinity and low Ca further from the trench (e.g., Big Blue Seamount; 69 mmol alkalinity/kg and 0.14 mmol Ca/kg) consistent with carbonate dissolution at the top of the plate between depths of 17 km and 22 km. Here we report results from trace element analyses that similarly show trends across the forearc region. For example, fluids upwelling at Baby Blue Seamount have; 58 $\mu$mol Sr/kg, 31 $\mu$mol Li/kg, 1.4 $\mu$mol Rb/kg, 10 nmol Cs/kg, 0.2 $\mu$mol Ba/kg, 0.1 $\mu$mol Mo/kg, 0.6 nmol U/kg, and 5 nmol Y/kg. In contrast, fluids from Big Blue Seamount, only 23 km away, have; 10 $\mu$mol Sr/kg, 0.7 $\mu$mol Li/kg, 6 $\mu$mol Rb/kg, 0.2 $\mu$mol Cs/kg, 60 nmol Ba/kg, 0.1 $\mu$mol Mo/kg, 0.2 $\mu$mol U/kg, and 0.1 nmol Y/kg.Trace element analyses for a host of chemical species and for each of the sampled seamounts offer insight into the conditions and interactions currently occurring within the Mariana seismogenic zone.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3099 General or miscellaneous
SC: Tectonophysics [T]
MN: 2003 Fall Meeting