HR: 11:05h
AN: MR12A-04 INVITED     [Abstracts]
TI: Deep Mantle Melts: Seismic Constraints
AU: * Garnero, E J
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287 United States
AU: Williams, Q
EM: quentw@pmc.ucsc.edu
AF: University of California, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064 United States
AU: Rost, S
EM: srost@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287 United States
AU: Thorne, M
EM: mthorne@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287 United States
AB: For nearly two decades, seismologists have illuminated significant lowermost mantle heterogeneity in elastic properties beyond radial averages. In the last decade, this heterogeneity has been imaged at a variety of lateral scale lengths: 1000's of km from tomographic inversion for global mantle structure, 100's of km from broadband waveform studies, down to 1-10 km from analyses of short period seismic array data. The nature of lower mantle seismic heterogeneity has remained elusive, however, owing to uncertainties in the seismic modeling, as well as a limited knowledge of the detailed nature of both P and S wave heterogeneity in any given location. Nonetheless, there are several noteworthy seismic findings: at long scale lengths, several deep mantle regions show uncorrelated P and S wave velocity heterogeneity, suggesting chemical heterogeneity. Deep mantle low velocities, on the other hand, commonly underlie surface hotspots, suggestive of a strong thermal component to the origin of velocity depressions. Recently, ultra low velocity zones (ULVZ) have been mapped in thin zones right at the core-mantle boundary (CMB), which in some cases have Vs reductions of near 30%, and shear reductions that are 3 times larger than that of Vp, supporting a partial melt origin. In a detailed array analysis, we can constrain the partial melt-bearing aggregate to be ~10% denser than the surrounding mantle, and probably be connected with the root of a mantle plume. The magnitude of density contrast implies that the ULVZ's are iron-enriched, but do not approach core compositions: Fe/Fe+Mg ratios of 0.3-0.5 could generate the observed density contrasts. The most parsimonious viewpoint is that iron- and possibly volatile-enriched partial melts descend into D'', where they pond at the base of the mantle. Here we predict that the largest magma chambers on Earth may in fact be ULVZs at the CMB, with geographical correlation to zones of plume genesis.
UR: http://garnero.asu.edu
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3924 High-pressure behavior
DE: 7208 Mantle (1212, 1213, 8124)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005