1. Department of Geology & Geophysics
Water distribution in the Earth’s mantle Inferred from Electrical Conductivity implications for the global water cycle
Shun-ichiro Karato
Yale University
Department of Geology & Geophysics
New Haven, CT
4/23/2017

2. Conclusions
Electrical conductivity is a useful sensor for the water content in the mantle.
Water content is both radially and laterally heterogeneous.
A large contrast in water content between the upper mantle and the transition zone suggests partial melting at ~410-km.
 Most of the upper mantle is partially melted (melt fraction is small and does not affect properties except for seismic wave velocities in the deep upper mantle).
 Partial melting at 410-km stabilizes the ocean mass.
4/23/2017

3. How to infer the distribution of water from geophysical observations?X X X ? * * X X
*: mostly for the upper mantle
Properties involving thermally activated processes are sensitive to water content.
Lab studies are more complete for electrical conductivity than for Q and LPO.
4/23/2017

4. seismic wave velocity versus water content
Seismic velocities are insensitive to water content.
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5. Influence of water on seismic discontinuities
wad
oli
wad
oli
Topography of discontinuities is insensitive to water content (at high T).
4/23/2017

6. electrical conductivity from geophysical studies
Tarits et al. (2004)
Ichiki et al. (2006)
Baba et al. (2010)
Kelbert et al. (2009)
4/23/2017

7. olivine, orthopyroxene, garnet, wadsleyite, ringwoodite
Dai and Karato (2009b)
4/23/2017

8. Sensitivity of electrical conductivity to T, Cw, fO2, Mg#
 Electrical conductivity is sensitive to Cw, but not to other parameters.
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9. Testing the model for the upper mantle
pyrolite (olivine+opx+pyrope), SIMS water calibration
[Dai and Karato (2009)]
4/23/2017

10. Electrical conductivity and water in the mantle
Geophysical model
Mineral physics model
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11. X Water content is layered (+ lateral heterogeneity)
 Partial melting at ~ 410-km
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12. What happens after 410-km melting?
Most of the upper mantle
is partially melted (with a
small melt fraction).
a thick low velocity layer
(due to complete wetting)
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13. thick low velocity regions above the 410-km (Tauzin et al. 2010)
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14.  410-km partial melting stabilizes the ocean mass.
No mid-mantle melting
With mid-mantle melting
 410-km partial melting stabilizes the ocean mass.
4/23/2017

15. conclusions
Water content (Cw) in the transition zone/upper mantle can be mapped from electrical conductivity observations.
Mantle water content is layered.
~0.01 wt% for the upper mantle, ~0.1 wt% for the transition zone
partial melting at 410-km
a majority of the upper mantle is partially melted.
a thick low velocity layer above 410-km
Ocean mass is buffered by partial melting at 410-km
Need for experimental studies on lower mantle minerals
Need for geophysical observations for the lower mantle
4/23/2017

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19. MORB source region (asthenosphere): well constrained (~0.01 wt%)
Dixon et al. (2002)
Ito et al. (1983)
MORB source region (asthenosphere): well constrained (~0.01 wt%)
OIB source regions: water-rich (FOZO) (~0.1 wt%)
How are they distributed?
localized?
global (layered)?
4/23/2017

20. Influence of element partitioningH Fe
wadsleyite
4/23/2017

21. Water-temperature distribution from VP,S and MTZ thickness
Meier et al. (2009)
puzzling results <-- due to insensitivity of seismological properties to water content?
<-- radial heterogeneity in water content?
<-- influence of kinetics on phase boundary topography?
4/23/2017

22. Water may affect seismological observationsh V
T-effect and water-effect on seismic wave velocities
T-effect and water-effect on the phase boundary
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