HR: 08:15h
AN: T21A-02    [PDF]
TI: Upper Mantle and Transition Zone Geochemical Heterogeneity: Seismic Constraints on the ~80% Peridotite Solution
AU: * Williams, Q
EM: quentw@emerald.ucsc.edu
AF: Dept. of Earth Sciences, U. of California, Santa Cruz, CA 95064 United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: Dept. of Geology and Geophysics, U. of Minnesota, Minneapolis, MN 55455 United States
AU: Rost, S
EM: srost@es.ucsc.edu
AF: Dept. of Geology, Arizona State University, Tempe, AZ 85287 United States
AB: The size of mantle geochemical heterogeneities spans from the length-scale of grains to the thousands of km lateral length-scales associated with subducted slabs. Grain-scale heterogeneities are difficult to probe remotely, while the large-scale heterogeneities associated with recent subduction have been extensively investigated. At intermediate length-scales, observational evidence is accumulating that abundant mantle heterogeneities exist in the 10 to ~100 km size range: such sizes of heterogeneity may be associated with either moderately stirred ancient subduction, or with regional magmatic events. The ability to seismically detect such intermediate-sized features hinges on their type of mineralogic heterogeneity, and thus the velocity- or impedance-contrasts of such regions with the normal background of upper mantle peridotite. Two distinct types of mineralogic heterogeneity can be seismically detected in the upper mantle: (1) assemblages bearing free silica, such as occurs in many eclogites at pressures above about 2 GPa; and (2) at depths less than 300 km, highly garnet-enriched assemblages. Abundant amounts of eclogite can be detected through the observation of seismic discontinuities generated by phase transitions within their silica component: such eclogite could be generated through remixing of subducted oceanic crust or delamination of basaltic lower crust. At depths less than 300 km, garnet-rich zones have substantially higher seismic velocities than those of coexisting peridotite: such garnetite zones may be generated by magmatic processes, and have been documented to occur beneath the Pitcairn hot-spot chain. In the case of both silica and garnet-enrichment, seismic observations indicate that, within different geodynamic environments, geochemical heterogeneities can be detected at the 10 to 100 km length scale. Such detections provide critical constraints on regional deviations from a peridotite-only mantle, and thus on the nature and length-scale of mantle heterogeneity.
DE: 7207 Core and mantle
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting