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

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

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

seismic wave velocity versus water content Seismic velocities are insensitive to water content. 4/23/2017

Influence of water on seismic discontinuities wad oli wad oli Topography of discontinuities is insensitive to water content (at high T). 4/23/2017

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

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

Sensitivity of electrical conductivity to T, Cw, fO2, Mg#  Electrical conductivity is sensitive to Cw, but not to other parameters. 4/23/2017

Testing the model for the upper mantle pyrolite (olivine+opx+pyrope), SIMS water calibration [Dai and Karato (2009)] 4/23/2017

Electrical conductivity and water in the mantle Geophysical model Mineral physics model 4/23/2017

X Water content is layered (+ lateral heterogeneity)  Partial melting at ~ 410-km 4/23/2017

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) 4/23/2017

thick low velocity regions above the 410-km (Tauzin et al. 2010) 4/23/2017

 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

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

4/23/2017

4/23/2017

4/23/2017

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

Influence of element partitioning H Fe wadsleyite 4/23/2017

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

Water may affect seismological observations h V T-effect and water-effect on seismic wave velocities T-effect and water-effect on the phase boundary 4/23/2017

4/23/2017