Water distribution in the Earth’s mantle Inferred from Electrical Conductivity
implications for the global water cycle
Shun-ichiro KaratoYale University
Department of Geology & Geophysics
New Haven, CT
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• 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.
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Conclusions
How to infer the distribution of water from geophysical observations?
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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.
seismic wave velocity versus water content
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Seismic velocities are insensitive to water content.
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Influence of water on seismic discontinuities
oli
oli
wad
wad
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Topography of discontinuities is insensitive to water content (at high T).
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electrical conductivity from geophysical studies
Kelbert et al. (2009)Tarits et al. (2004)Ichiki et al. (2006)Baba et al. (2010)
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wadsleyite
Dai and Karato (2009b)
olivine, orthopyroxene, garnet, wadsleyite, ringwoodite
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Sensitivity of electrical conductivity to T, Cw, fO2, Mg#
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Electrical conductivity is sensitive to Cw, but not to other parameters.
Testing the model for the upper mantle
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pyrolite (olivine+opx+pyrope), SIMS water calibration
[Dai and Karato (2009)]
Electrical conductivity and water in the mantle
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Mineral physics model Geophysical model
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XWater content is layered (+ lateral heterogeneity) Partial melting at ~ 410-km
What happens after 410-km melting?
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a thick low velocity layer(due to complete wetting)
Most of the upper mantleis partially melted (with a small melt fraction).
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thick low velocity regions above the 410-km (Tauzin et al. 2010)
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410-km partial melting stabilizes the ocean mass.
No mid-mantle melting
With mid-mantle melting
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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-kmNeed for experimental studies on lower mantle mineralsNeed for geophysical observations for the lower mantle
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Ito et al. (1983) Dixon et al. (2002)
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)?
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Influence of element partitioning
Fe H
wadsleyite
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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?
Water-temperature distribution from VP,S and MTZ thickness
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Water may affect seismological observations
• T-effect and water-effect on seismic wave velocities • T-effect and water-effect on the phase boundary
h
V
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