HR: 1330h
AN: T32B-0925    [PDF]
TI: Evidence for Chemical Heterogeneities Derived From Poisson's Ratio in the Lower Mantle Beneath Alaska.
AU: * Stutzmann, E
EM: stutz@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75005 France
AU: van der Hislt, R
EM: hilst@mit.edu
AF: MIT, 77 Massachusetts Ave, Cambridge, MA 01239 United States
AU: Saltzer, R
EM: rsaltzer@mit.alum.edu
AF: MIT, 77 Massachusetts Ave, Cambridge, MA 01239 United States
AB: Deep mantle composition can be investigated from seismic velocities only by considering both P-wave and S-wave velocity anomalies. Using a waveform cross-correlation technique we have measured $\sim$17,000 P and S wave differential travel times from earthquakes and stations near a great circle path from Japan, across Alaska, to North America. We jointly invert the data for variations in shear and bulk sound speed and derive Poisson's ratio variations in the mantle. We explore the model space using different levels of regularization and we keep a range of models that give acceptable data fits. These models are discussed only in the lower mantle well resolved areas. The correlation between Vs and Vp variations is good to $\sim$1500 km depth but gradually degrades in the bottom 1000 km, whereas the ratio R=dlnVs/dlnVp is between 1.5 and 2.0 at most depths. Our data suggest that there is no significant correlation between bulk and shear wave speed, but they appear anti-correlated in several regions. We estimate the effect on the Poisson's ratio of changes in temperature, iron content, and the magnesiowustite (mw) to perovskite (pv) ratio. At all depths the effect of temperature is largest, and that of pv and mw ratio smallest. Our data cannot resolve the trade-off between thermal and compositional effects, but explaining the Poisson's ratio variability by temperature alone would require unrealistic perturbations and it would not explain the inferred anti-correlation between shear and bulk sound speed. Our results suggest that the elastic parameter variations in the deep mantle beneath Alaska reflect a combination of thermal and compositional effects, with dT of $\sim$300-800K, variation in Fe contnet of $\sim$4%, and Pv enrichment (or depletion) of up to $\sim$10% just sufficient for explaining the data.
DE: 6982 Tomography and imaging
DE: 7207 Core and mantle
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting