Global Tomography -150 km depth Continental cratons - cold, rigid material Spreading centers & mountains - warm mantle

P,S, and Q Tomography - Tonga Arc Velocity tomography shows anomalies relative to average model [Conder and Wiens, 2005]; Q tomography shows log(Q) from new tomographic Inversion of data from Roth et al [1999]

How are geophysical observations related to material properties? Geophysical observables: P velocity S velocity Attenuation (1/Q) Velocity Anisotropy Electrical Conductivity Material Properties: Temperature Melt content Composition Water (+ other volatiles?) ? Complex and Difficult Inverse Problem !

What about composition? Density Shear Velocity Fe-Mg ratio in mantle xenoliths More iron gives higher density & lower velocity But is there a competing trend in Al ?

More complex models (Schutt & Lesher 2006)

Experiments - Q and Vs at high pressure and temperature

Jackson et al., 2004

Extrapolation in grain size

How do material properties affect mantle seismic observables? Temperature effect on seismic velocity -- no melt present Experimental results: P and S velocities are controlled by anharmonic temperature derivatives at temperatures below about 900°C --- relatively linear dVP/dT ~ 0.6 m/s/K ( 0.8 % per 100°C); dVS/dT ~ 0.45 m/s/K (1 % per 100°C) Above 900°C the relationship is non-linear due to attenuation effect Attenuation is also a function of frequency, grain size, and depth (Faul andJackson, 2005) Shear Velocity Velocity derivative

Temperature and depth dependence of dV/dT Studies linking seismic velocities and temperature often use a single value of dV/dT However, dV/dT has strong temperature and depth dependence due to anelastic contribution Temperature derivative drops by a factor of two between 50 and 350 km depth ν = dlnVs/dlnVp = (ΔVs/Vs)/ (ΔVp/Vp) values greater than 1.6 are often said to indicate melt However, temperature variations allow large ν values without melt Depth variation of dVs/dT dlnVs/dlnVp vs Temperature

Melt - Possible attenuation mechanisms grain boundary sliding can be - elastically accommodated: unique equilibrium state -> attenuation peak - diffusionally accommodated: continuous -> no peak

Melt Geometry q < 60 q > 60 Node q q Tubule The effect of melt on seismic velocity is a function of the melt geometry There is still a controversy about melt geometry and how it varies with percent melt Melt geometry is also related to porosity and permeability and how fast melt escapes q < 60 q > 60 Node q q Tubule Wark et al., 2003

Melt Geometry from Experiments Faul et al., [1994]

Three-grain edges: melt-free vs melt-bearing

Shear Velocity Reduction and Attenuation for Olivine containing Melt Modulus Reduction and Attenuation Mechanism Melt and seismic attenuation Line thickness gives melt content; line color gives grain size For a given grainsize, 1% melt gives nearly an order of magnitude increase at 1 Hz Seismic velocity reduction occurs through both “melt squirt” and grain boundary sliding Faul et al., 2004

Effect of Water? 810 ppm H/Si = .005 wt % water - normal MORB Karato, 2003 810 ppm H/Si = .005 wt % water - normal MORB Mariana backarc - .01 to 0.25 wt % H2O in the mantle source

Qualitative description of the effect of parameters on seismic observables Wiens and Smith, 2003

Shear Velocity Structure for 100 Myr-old Oceanic Crust

P,S, and Q Tomography - Tonga Arc Velocity tomography shows anomalies relative to average model [Conder and Wiens, 2005]; Q tomography shows log(Q) from new tomographic Inversion of data from Roth et al [1999]

Geodynamic Modeling of Tomographic Velocities Temperature Model P velocity calculated from temperature model S velocity calculated from temperature model

Modeling Attenuation Structure Calculated Q model (temperature effect only) Temperature model Q tomography