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

04/19/23 1

• 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.

04/19/23 2

Conclusions

How to infer the distribution of water from geophysical observations?

04/19/23 3

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

04/19/23 4

Seismic velocities are insensitive to water content.

5

Influence of water on seismic discontinuities

oli

oli

wad

wad

04/19/23

Topography of discontinuities is insensitive to water content (at high T).

6

electrical conductivity from geophysical studies

Kelbert et al. (2009)Tarits et al. (2004)Ichiki et al. (2006)Baba et al. (2010)

04/19/23

wadsleyite

Dai and Karato (2009b)

olivine, orthopyroxene, garnet, wadsleyite, ringwoodite

04/19/23 7

8

Sensitivity of electrical conductivity to T, Cw, fO2, Mg#

04/19/23

Electrical conductivity is sensitive to Cw, but not to other parameters.

Testing the model for the upper mantle

04/19/23 9

pyrolite (olivine+opx+pyrope), SIMS water calibration

[Dai and Karato (2009)]

Electrical conductivity and water in the mantle

04/19/23 10

Mineral physics model Geophysical model

04/19/23 11

XWater content is layered (+ lateral heterogeneity) Partial melting at ~ 410-km

What happens after 410-km melting?

04/19/23 12

a thick low velocity layer(due to complete wetting)

Most of the upper mantleis partially melted (with a small melt fraction).

04/19/23 13

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

04/19/23 14

410-km partial melting stabilizes the ocean mass.

No mid-mantle melting

With mid-mantle melting

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-kmNeed for experimental studies on lower mantle mineralsNeed for geophysical observations for the lower mantle

04/19/23

1604/19/23

1704/19/23

04/19/23 18

19

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)?

04/19/23

20

Influence of element partitioning

Fe H

wadsleyite

04/19/23

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

04/19/23 21

22

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

04/19/23

04/19/23 23