HR: 10:50h
AN: S11G-03    [PDF]
TI: Understanding the Mantle Through the Curious Topography on the 410 Discontinuity
AU: * Reif, C
EM: creif@ucsd.edu
AF: University of California San Diego SIO-IGPP, 9500 Gilman Drive MS-0225, La Jolla, CA 92093 United States
AU: Masters, G
EM: guy@igpp.ucsd.edu
AF: University of California San Diego SIO-IGPP, 9500 Gilman Drive MS-0225, La Jolla, CA 92093 United States
AU: Flanagan, M
EM: flanagan5@llnl.gov
AF: Lawrence Livermore National Laboratory Earth Sciences Division, EES, Bldg.1404, Room 105 PO Box 808, L-205, Livermore, CA 94551 United States
AU: Shearer, P
EM: pshearer@ucsd.edu
AF: University of California San Diego SIO-IGPP, 9500 Gilman Drive MS-0225, La Jolla, CA 92093 United States
AB: Our understanding of the 410 km and 660 km seismic discontinuities has grown tremendously over the past 15 years. The debate regarding whether the seismic discontinuities arise from chemical or phase changes seemingly has been put to rest by the multitude of seismic and mineralogical evidence supporting phase transformations in the olivine component at these depths. Assuming that the discontinuities are indeed due to mineral phase transformations, there is much that can be learned about the composition and dynamics of the mantle. The depth and sharpness of the discontinuities depends on composition, temperature, water content, and attenuation. The Clapeyron slopes (the gradient of the phase boundary in pressure/temperature space) of the olivine to wadsleyite transition at the 410 and the ringwoodite to perovskite and magnesiow\"{u}stite transition at the 660 are similar in magnitude but opposite in sign. Therefore, the topography of the two discontinuities should be anti-correlated in the presence of vertically coherent thermal anomalies. While this is observed in some subduction zones and hot spots, many global and regional studies do not observe the expected anti-correlation of topography. This lack of anti-correlation can be explained in some instances where the subducting slab broadens at the 660, causing a more extensive thermal anomaly than is present when the slab encounters the 410. However in areas where this is not the case, other explanations are necessary. We use an updated SS-S410S and SS-S660S (SS precursor) dataset that is nearly double the size of the Flanagan and Shearer (1998) dataset. Unlike previous studies that bin the SS precursor data from all azimuths to determine cap averages of topography, we are able to make SS precursor summary rays and correct for 3D velocity structure. However, we are best able to constrain the thickness of the region between the 410 and 660 (the transition zone thickness) since their differential times are not dependent on the upper mantle velocity model. While the transition zone thickness is dominated by the slab signature of the 660 topography, it is clear that the 410 topography is more often positively rather than negatively correlated with 660 topography. We find that this correlation is not an artifact of the modeling and therefore indicates either non-thermal effects or the presence of thermal anomalies that are not coherent between the two discontinuities.
DE: 3625 Descriptive mineralogy
DE: 7203 Body wave propagation
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting