HR: 09:15h
AN: T31B-06 INVITED     [PDF]
TI: Inferring Chemical, Thermal and Mechanical Heterogeneities in the Upper Mantle From Seismological Observations
AU: * Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AU: Shito, A
EM: azusa.shito@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AB: Inferring heterogeneity in the mantle is critical for our understanding of evolution and dynamics of this planet. Most previous efforts in this direction have been concerned with mapping anomalies in temperature, partial melting and/or major element chemistry. We show that in addition to these anomalies, anomalies in trace elements such as hydrogen (water) and the stress level can now be mapped using seismological observations when combined with the latest results of mineral physics. The effects of hydrogen on seismic wave propagation are mostly through its effects on attenuation (Q) and anisotropy. A theoretical analysis shows that the effects of water on attenuation and seismic wave velocities can be parameterized using the rheologically effective temperature (Karato, 2003). This formulation predicts that the velocity heterogeneity caused by anomalies in temperature or water content must have correlation with anomalies in Q. Furthermore the slopes of correlation between Q and velocity anomalies are different between thermal and water origin. Consequently a comparison of anomalies in average seismic wave velocities and Q provides a useful tool to identify the cause of these anomalies. Such an analysis on wedge mantle in the western Pacific suggests that significant heterogeneity in major element chemistry is present in the shallow ($<$200km) upper mantle whereas anomalies in the deep upper mantle are most likely attributed to the heterogeneity in water content (Shito and Shibutani, 2003). Recent laboratory studies also show that the nature of seismic anisotropy is sensitive to various parameters including water content, temperature and stress magnitude (Jung and Karato, 2001; Katayama et al., 2003). A commonly observed trend of fast shear wave polarization (trench parallel near trench to trench normal anisotropy away from trench) can be attributed to the regional variation in stress level (and water content) in the subduction zone: high stress (plus high water content) near slabs (leading type-B lattice preferred orientation of olivine) and low stress (low water content) away from slabs (type-A or C). Deviations from this trend may imply different stress distributions caused by different temperature and/or water content.
DE: 3655 Major element composition
DE: 3909 Elasticity and anelasticity
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2003 Fall Meeting