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

2. P,S, and Q Tomography - Tonga Arc
Velocity tomography shows anomalies relative to average model [Conder and Wiens, 2006];
Q tomography shows log(Q) from new tomographic Inversion of data from Roth et al [1999]

3. 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 !

4. 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 ?

5. Modeling the effect of melt removal from mantle peridotite (Schutt & Lesher 2006)

6. Experiments – 1/Q and VS at high pressure and temperature
Figure from Ian Jackson, ANU

7. Jackson et al., 2004

9. Extrapolation in grain size

10. 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 ~ 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

11. 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

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

13. 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

14. Melt Geometry from x-ray synchrotron microtomography
Melt forms interconnected
network even at low porosity
From
Zhu et al.,
[2011]

15. 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

16. 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 to 0.25 wt % H2O in the mantle source

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

19. 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]

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

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

22. Observations cannot be matched with temperature and water effects (Wei et al., 2016)
Model (Harmon & Blackman, 2010)
Sv Velocity
P-wave attenuation
Geodynamic model results (Harmon & Blackman, 2010) are converted to velocity and attenuation using Jackson et al (2010) for temperature and Karato (2012) for water content
Upper mantle water alone cannot explain the large velocity and attenuation anomalies
Low velocity, high attenuation regions result from the effect of melt