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
5/28/2016
1
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.
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How to infer the distribution of water from
geophysical observations?
sensitivity to:
T
C (major) C (water)
Seismic velocity
X
Seismic discontinuity
Seismic Q
Seismic anisotropy
Electrical conductivity
d
resolution of geophysical
observations
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.
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seismic wave velocity versus water content
S
e
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Influence of water on seismic discontinuities
wad
oli
wad
oli
Topography of discontinuities is insensitive to water content (at high T).
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electrical conductivity from geophysical studies
Kelbert et al. (2009)
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Tarits et al. (2004)
Ichiki et al. (2006)
Baba et al. (2010)
6
olivine, orthopyroxene, garnet, wadsleyite, ringwoodite
wadsleyite
Dai and Karato (2009b)
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Sensitivity of electrical conductivity to T, Cw,
fO2, Mg#
 Electrical conductivity is sensitive to Cw, but not to other parameters.
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Testing the model for the upper mantle
pyrolite (olivine+opx+pyrope), SIMS water calibration
[Dai and Karato (2009)]
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Electrical conductivity and water in the mantle
Mineral physics model
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G
e
o
p
h
y
s
i
c
a
l
m
o
d
e
l
10
Water content is layered (+ lateral heterogeneity)
 Partial melting at ~ 410-km
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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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thick low velocity regions above the 410-km (Tauzin et al. 2010)
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No mid-mantle melting
dX ocean
dt
=
1
t1
X mantle - R =
mantle
dX mantle
1
=
X
dt
t1
ocean
total
=X
à X
1
t1
(X
total
)
- X ocean - R
+R
- t 1R : ocean mass is sensitive to regassing rate
With mid-mantle melting
dX ocean
dt
dX UM
dt
=
XUM
t1
+
X MTZ
t2
-R
UM
= - Xt 1 + b R + Y
dX MTZ
dt
= - X t 2 + (1 - b ) R - Y
MTZ
à
X ocean = X total - xt 1R - z X l
with
x = 1-
if z =
t 2 -t1
t 2 -t 3
t 3 t 2 -t1
t1 t 2 -t 3
(1 - b )
and z =
t 2 -t1
t 2 -t 3
>0, then x < 1
à ocean mass is less sensitive to regassing rate
 410-km partial melting stabilizes the ocean mass.
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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-km
 Need for experimental studies on lower mantle minerals
 Need 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
H
Fe
wadsleyite
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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?
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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
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