HR: 10:20h
AN: V32B-01    [Abstracts]
TI: Variations in Lower-Mantle Temperature and Chemistry From the Perspective of Seismic Tomography
AU: * Reif, C
EM: creif@ucsc.edu
AF: University of California San Diego Institute of Geophysics and Planetary Physics, 9500 Gilman Drive MS-0225, La Jolla, CA 92093 United States
AU: Masters, G
EM: gmasters@ucsd.edu
AF: University of California San Diego Institute of Geophysics and Planetary Physics, 9500 Gilman Drive MS-0225, La Jolla, CA 92093 United States
AU: Li, B
EM: bli@notes.cc.sunysb.edu
AF: SUNY Stony Brook Department of Geosciences, 255 Earth and Space Sciences (ESS), Stony Brook, NY 11794 United States
AB: For some time seismic tomography has revealed very slow velocity anomalies that extend several hundred kilometers above the core-mantle boundary in the central Pacific and southern Africa referred to as "superplumes". Since these slow anomalies are concentrated at a thermal and chemical boundary layer, it is necessary to distinguish between the possible combination of thermal and chemical contributions to the observed decrease in seismic velocity. Tomographic maps alone cannot distinguish between the two effects. However, distinctions can be made if shear and compressional velocity anomalies are compared to each other. Therefore, we have used cluster analysis to compile an extensive catalog of high-quality travel times of long-period P and S phases. Comparing the resulting compressional and shear velocity anomaly models shows that the ratio of the decrease in the shear versus the compressional velocity in the central Pacific superplume exceeds that which is possible for a purely thermal anomaly. In addition, using the data to invert for shear velocity and bulk sound speed anomalies we find the two are anti-correlated in the central Pacific, which also indicates a chemical anomaly. However, this still does not resolve the type of chemical anomaly present. Recently, Li ( in preparation), using an experimentally determined equation of state, solve for the sensitivities of variations in shear and compressional velocity and density to changes in temperature, mole fraction of perovskite, and mole fraction of iron in the lower mantle. Li ( in preparation) found that variations in shear velocity should be dominantly sensitive to temperature, in contrast with the findings of Trampert et al. (2004). Knowing these sensitivities, we directly invert our long-period seismic data and existing normal mode splitting data (Masters et al., 2000) for variations in temperature and chemistry of the lower mantle. We find that most of the high and low velocity and density anomalies in the lower mantle can be explained by variations in temperature. However, in the central Pacific, compositional variations are also present in the form of an increase in the mole fraction of perovskite or an increase in the mole fraction of iron in the perovskite. Our thermo-chemical models demonstrate that long-period body wave data can be used to not only indicate, but also quantify variations in lower mantle temperatures and major element chemistry.
DE: 1025 Composition of the mantle
DE: 7270 Tomography (6982, 8180)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005