Title: Geophysical Anomalies in the Central Pacific Upper Mantle Implications for Water Transport by a Plume
1Geophysical Anomalies in the Central Pacific
Upper Mantle Implications for Water Transport
by a Plume
- Shun-ichiro Karato
- Yale University
- New Haven, CT, USA
2Outline
- 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
3Upper mantle in the central Pacific has unusually
strong VSHgtVSV anisotropy.
Ekström and Dziewonski (1998)
4Upper mantle in the central Pacific has unusually
low conductivity.
Electrical conductivity, S/m
5Cause 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
6Seismic 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.
7LPO 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
8LPO 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)
9Olivine 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)
10Influence 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.
11A-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)
12Influence 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
13Conductivity in normal asthenosphere can be
explained by a typical water content (0.01
wt).Conductivity in the central Pacific
corresponds to dry olivine.
14- 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?
15Roles 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
16deep melting in a plume
(from Hirschmann (2006))
17In a plume column, melting occurs in the deep
asthenosphere, providing depleted (dry)
materials to the asthenosphere.
18A 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)
19Conclusions
- 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.
20A three-dimensional fabric diagram of olivine
Karato et al. (2007)
B
A
Couvy et al. (2004)
C
C
E
E
21At low stress and high T, A-, E- or C-type
olivine fabrics will be important.