HR: 14:55h
AN: V33D-06    [Abstracts]
TI: Water contents in mantle xenoliths from the Colorado Plateau and vicinity: Implications for the rheology and hydration-induced thinning of continental lithosphere
AU: * Li, Z A
EM: zxli@rice.edu
AF: Department of Earth Sciences, Rice University, 6100 Main St., Houston, TX 77005,
AU: Lee, C A
EM: ctlee@rice.edu
AF: Department of Earth Sciences, Rice University, 6100 Main St., Houston, TX 77005,
AU: Peslier, A H
EM: peslier@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058,
AU: Lenardic, A
EM: adrian@rice.edu
AF: Department of Earth Sciences, Rice University, 6100 Main St., Houston, TX 77005,
AU: Mackwell, S J
EM: mackwell@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058,
AB: Low angle subduction of the Farallon Plate beneath western North America during the early Cenozoic has been suggested to have introduced water into the overriding North America lithosphere. However, most of the evidence comes from trace element systematics and the presence of hydrous phases in lavas and some peridotite and eclogite xenoliths. Here, we present results from a systematic study on water contents in nominally anhydrous minerals (NAMs, including olivine, clinopyroxene and orthopyroxene) of peridotite xenoliths collected from the Colorado Plateau and vicinity. After water loss during magma ascent was corrected, olivines from 4 out of 5 sample localities are found to contain ~40 to 70 ppm H2O compared to ~10 to 40 ppm H2O in olivine from the fertile asthenosphere. Assuming this amount of water (~70 ppm H2O in olivine) was introduced into the base of the lithospheric mantle by hydration, application of an updated flow law for dislocation creep of wet olivine to lithospheric mantle conditions beneath the Colorado Plateau predicts that for given background shear stress, hydration alone could have resulted in a more than 1 order of magnitude viscosity drop at the base of the lithosphere. The reduction in viscosity would have eased any basal erosion of the lithosphere driven by the convective stresses associated with low angle subduction and Basin and Range extension. We show that hydration could have easily permitted more than 15 km of lithospheric thinning. Viscosity reduction and lithospheric thinning of even larger extents (up to ~100 km lithospheric thinning) are predicted to be possible when thicker lithosphere (such as Archean cratons) and larger water contents (up to water-saturated condition) are considered. If our interpretations are correct, the implications of our study go beyond western North America and hint at a plausible way of recycling continental mantle, including cratonic mantle, back into the convecting mantle.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3902 Creep and deformation
DE: 5120 Plasticity, diffusion, and creep
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting