HR: 16:30h
AN: T44C-03    [Abstracts]
TI: Petrology, Pressures, and Temperatures Along the Mariana Sub-Forearc Slab-Mantle Interface
AU: * Gharib, J J
EM: jim_gharib@aoageophysics.com
AF: AOA Geophysics, 2500 Tanglewilde, Suite 120, Houston, TX 77063, United States
AU: Fryer, P
EM: pfryer@hawaii.edu
AF: University of Hawaii at Manoa, Department of Geology and Geophysics, 1680 East-West Rd., Honolulu, HI 96822, United States
AU: Mottl, M J
EM: mmottl@soest.hawaii.edu
AF: University of Hawaii at Manoa, Department of Oceanography, 1000 Pope Rd., Honolulu, HI 96822, United States
AU: Ross, K
EM: kentross@berkeley.edu
AF: University of California at Berkeley, Department of Earth and Planetary Science, 307 McCone Hall, Berkeley, CA 94720, United States
AU: McConnell, D
EM: dan_mcconnell@aoageophysics.com
AF: AOA Geophysics, 2500 Tanglewilde, Suite 120, Houston, TX 77063, United States
AB: The protrusion of sedimentary serpentinite deposits from the Mariana forearc on the seafloor has constructed large km-scale seamounts, probably upwelling through fault-bounded conduits in the suprasubduction-zone mantle before reaching the seafloor. Clasts of basic metamorphic rocks and minerals, interpreted to come from the subducting Pacific plate, have been sampled in these serpentinites and are typically of the moderate- to high- pressure and low-temperature blueschist facies conditions expected for sub-forearc subduction zone environments. The majority of minerals have no indication of retrograde metamorphism or metasomatism, indicating that most of the clasts ascended relatively rapidly through the suprasubduction-zone mantle wedge and forearc crust without any significant chemical interaction with the surrounding environment. This material is from the top of the slab and therefore reflects the conditions at the slab-mantle interface. The material is likely to have interacted mechanically and perhaps chemically with the serpentinites and peridotites from the base of the forearc mantle wedge. Furthermore, the temperatures encountered will be hotter than those of the cool interior of the slab. Many terrestrial blueschist deposits originally from slab interiors would therefore not be representative of these boundary conditions. The Mariana blueschists allow these conditions to be estimated in an active modern subduction zone. Bulk rock compositions have been calculated for these clasts using a combination of element mapping and modal analysis. Bulk compositions are generally basaltic, but with unusually high Mg and low Al, resembling altered and possibly alkalic basalt (possibly ocean island basalt), potentially remnants of underplated seamounts that have been smeared off the slab. Inferred conditions of metamorphism are approximately 0.45 to 0.6 GPa (~20 km below seafloor) and approximately 250 to 300 C. These temperatures are among the most robust sub-forearc slab temperature estimates to date and imply 12-20% shear-heating along the slab-mantle interface using the Peacock (1992) thermal model.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting