Geophysical Anomalies in the Central Pacific Upper Mantle Implications for Water Transport by a Plume

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Geophysical Anomalies in the Central Pacific
Upper Mantle Implications for Water Transport
by a Plume

• Shun-ichiro Karato
• Yale University
• New Haven, CT, USA

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Outline

• Anomalies in the central Pacific upper mantle
• Seismic anisotropy
• Electrical conductivity
• Some mineral physics background
• Seismic anisotropy and water
• Electrical conductivity and water
• Conclusions
• Plume-asthenosphere interaction

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Upper mantle in the central Pacific has unusually
strong VSHgtVSV anisotropy.
Ekström and Dziewonski (1998)
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Upper mantle in the central Pacific has unusually
low conductivity.
Electrical conductivity, S/m

• Karato (2007)

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Cause for anomalies

• How do various laterally varying parameters
  affect seismic anisotropy and electrical
  conductivity?
• Mineral physics of anisotropy and conductivity
• Temperature X
• Major element chemistry X
• Partial melting X O
• Water (hydrogen) O

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Seismic anisotropy elastic anisotropy of
minerals LPO (lattice-preferred orientation)

• Temperature or major element chemistry (e.g.,
  Fe/Mg) does not change the elastic anisotropy so
  much.
• --gt A change in LPO should be a cause.

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LPO changes with physical/chemical conditions.

• LPO is determined by the dominant slip
  system(s).
• Conventional interpretation of anisotropy
  assumes one type of slip system (100(010)).
• If the dominant slip system changes, LPO will
  change (fabric transition), then the nature of
  seismic anisotropy will change.
• How could the dominant slip system change with
  physical/chemical conditions?

olivine
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LPO is controlled by the relative strength of
slip systems. Deformation with b 001 slip
systems is more enhanced by water than
deformation with b 100 slip systems.
Could fabric transition occur at higher water
content? Karato (1995) --gtsimple shear
deformation experiments (change in the slip
direction) at higher water fugacity (0.3
GPa--gt15 GPa)
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Olivine LPO
Conventional interpretation of seismic anisotropy
is based on the limited observations and assumed
A-type fabric. But lab studies in my group have
shown that other LPOs dominate under different
conditions.
Karato et al. (2007)
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Influence of water content and stress on
deformation fabrics of olivine (at T1500-1600 K
(asthenospheric temperature))
Seismic anisotropy in the asthenosphere, plume
roots is likely caused by A-, or E- or C-type
olivine fabrics depending on the water content.
E- or C-type fabric.In the typical
asthenosphere, dominant fabric is likely E-type.
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A-type olivine fabric causes strong VSHgtVSV
anisotropy. E-type olivine fabric causes weak
VSHgtVSV anisotropy.
c
E-type
b
a
b
A-type
c
a
Karato (2007)
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Influence of water (hydrogen) on electrical
conductivity in olivine

• A large amount of hydrogen can be dissolved in
  olivine.
• Hydrogen diffusion is fast.
• Hydrogen may enhance conductivity?(Karato, 1990)

Electrical conductivity, S/m
Wang et al. (2006)
Water content
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Conductivity in normal asthenosphere can be
explained by a typical water content (0.01
wt).Conductivity in the central Pacific
corresponds to dry olivine.
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• Anomalies can be attributed to the dry
  asthenosphere in the central Pacific.
• Why is the asthenosphere of the central Pacific
  dry?
• What is the role of the Hawaii plume on
  modifying the properties of the central Pacific
  asthenosphere?

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Roles of a plume to modify the composition of the
asthenosphere

• Plume enriched (undepleted) more water
• direct mixing does not explain dry
  asthenosphere.
• plumewet hot-gt deep melting-gt depleted
  materials

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deep melting in a plume
(from Hirschmann (2006))
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In a plume column, melting occurs in the deep
asthenosphere, providing depleted (dry)
materials to the asthenosphere.
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A plume will feed depleted (dry) materials to
the asthenosphere due to deep melting --gt cause
for geophysical anomalies in the central Pacific?
Karato et al. (2007)
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Conclusions

• Both seismic anisotropy and electrical
  conductivity in the central Pacific are
  anomalous.
• These anomalies can be attributed to a low water
  (hydrogen) content in the asthenosphere in this
  region.
• A plume supplies depleted (dry) materials to
  the asthenosphere due to deep melting.

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A three-dimensional fabric diagram of olivine
Karato et al. (2007)
B
A
Couvy et al. (2004)
C
C
E
E
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At low stress and high T, A-, E- or C-type
olivine fabrics will be important.