HR: 0800h
AN: U21A-0003    [Abstracts]
TI: Constraints on Upper Mantle Temperature from Seismic Attenuation
AU: * Cammarano, F
EM: fabio@seismo.berkeley.edu
AF: UC Berkeley, 215 Mc Cone Hall, Berkeley, CA 94720, United States
AU: Romanowicz, B
EM: barbara.romanowicz@gmail.com
AF: UC Berkeley, 215 Mc Cone Hall, Berkeley, CA 94720, United States
AB: Imperfections in the crystalline structure of any mineral cause non-elastic effects, which in turn lead to dissipation and dispersion of wave propagation at seismic frequencies. The mechanisms are analogous to the creep (or deformation) mechanisms occurring at much lower frequency and are thermally activated. Owing to the advances in experimental techniques, it is now possible to have reliable laboratory measurements of seismic attenuation (Q) at temperatures (T) and pressures (P) corresponding to the uppermost mantle. The available data can be represented with appropriate P, T and grain-size (GS) dependent models. Extrapolation to high pressure is uncertain and relies on mostly unconstrained values of activation volume or on an empirical homologous temperature approach. We test the experimental Q models against available compilations of attenuation measurements. These include surface waves and free oscillations fundamental modes, free oscillation overtones and longer period fundamental measurements. Despite the uncertainties in seismic observations (e.g., the deviation between surface waves and normal mode measurements) and in the mineral physics parameters (e.g., the pressure dependence), available knowledge of anelastic behavior already puts a tight constraint on absolute average temperature (or grain size) and its gradient with depth in the first 400 km of the mantle. The same experimental P, T and GS dependent Q model, coupled with elastic data of mantle minerals, are used for the interpretation of long-period seismic waveforms. In a previous study, we found that long period seismic waveforms require, globally, an increase in shear velocity between 250km and 350km that is compatible with either a gradual enrichment in garnet with depth, or a negative thermal gradient, or an increase in grain size. Although a negative thermal gradient or an increase in grain size (or a combination of both) can explain the isotropic features without invoking a compositional change, both factors affect the Q model significantly. The resulting Q profiles obtained with all available combinations of elastic and anelastic models do not fit the attenuation data well, while observations are generally consistent with an adiabatic thermal gradient and a constant grain size within this depth range. This supports the possibility of a compositional change with depth throughout the upper mantle, which can explain the isotropic profile without affecting the Q profile.
DE: 7200 SEISMOLOGY
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
SC: Union [U]
MN: 2007 Fall Meeting