HR: 17:15h
AN: V34A-06    [Abstracts]
TI: The Effects of Melt Depletion on the Density Structure of Cratonic Mantle, and its Implication for the Post-Laramide Convective Thinning of the Wyoming Craton
AU: * Schutt, D L
EM: schutt@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Dept. 3006 1000 University Ave., Laramie, WY 82071-3006, United States
AU: Lesher, C E
EM: lesher@geology.ucdavis.edu
AF: Geology Department, University of California at Davis, One Shields Avenue, Davis, CA 95616-8605, United States
AU: Dueker, K
AF: Department of Geology and Geophysics, University of Wyoming, Dept. 3006 1000 University Ave., Laramie, WY 82071-3006, United States
AB: To explain the stability of cratons, Jordan [1979] suggested that cratonic mantle is neutrally buoyant with respect to hotter asthenospheric mantle. This study hypothesized that extraction of partial melt-- which removes the incompatible elements and modifies the mineral proportions from that of fertile peridotite--reduces the density of the rock enough to counteract the effects of temperature. Many studies have elaborated on this phenomenon, and it is still generally considered that cratonic mantle is either neutrally or positively buoyant with respect to hotter advecting fertile mantle. We have evaluated the melting relations of peridotite to quantify the density effects of melt removal, and find that above about 110 km depth no amount of melt depletion can counteract the effects of temperature. This suggests that cratonic mantle from the Moho to ~110 km depth is negatively buoyant. To confirm this observation, we have examined density variations in more than 100 Kaapvaal craton xenoliths using principle component analysis to separate the effects of melt depletion from other geochemical variations such as silica enrichment. Similar to the isobaric melting results, we find a density cross-over at around 110 km. This implies that uppermost cratonic mantle is stabilized by its high viscosity with respect to asthenosphere, and not its density structure. A test of this hypothesis would be to see if reducing the viscosity of this uppermost mantle would cause it to convectively destabilize. This may have happened when Laramide aged subduction brought the Farallon slab to shallow depths beneath much of the western U.S. Hydration from the slab could have reduced the viscosity of the Wyoming craton, causing it to drip away. Noteably, teleseismic P-wave tomography finds a high velocity "drip" under the Wind River Range in western Wyoming. This feature is within a region of maximal Laramide foreshortening which would have promoted nucleation of the instability.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 7218 Lithosphere (1236)
DE: 7270 Tomography (6982, 8180)
DE: 8103 Continental cratons
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting