the transition zone: slabs ’ purgatory cider, 2006 - group a garrett leahy, ved lekic, urska...
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The Transition Zone: Slabs’ Purgatory
CIDER, 2006 - Group A
Garrett Leahy, Ved Lekic, Urska Manners, Christine Reif, Joost van Summeren, Tai-Lin Tseng,
Magali Billen, Wang-Ping Chen, Adam Dziewonski
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Tonga Seismicity
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Predicted Slab Positions
Degree 45 and 24 spherical harmonic expansions of locations of slabs based on plate history reconstructions assuming no stagnation in transition zone.
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Tomographic Models
Harvard Berkeley
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Preliminary Conclusions
• Tomography reveals larger fast regions in the western Pacific transition zone.
• Deep earthquake stress axes show evidence of resistance to crossing the 660 km discontinuity.
• Structure below and above 660 km discontinuity has different spectral character.
• Implication: slabs stagnate in the transition zone for some length of time.
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A Simple Force Balance for slabs in the Transition Zone
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Fb =∫ gdxdz
x
z
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Constraints on and Clapeyron slopes
• Density contrasts– Seismic constraints– Lab experiments on mantle minerals/rocks– Lattice dynamics simulation
• Clapeyron slopes– Lab experiments on phase transformation– Calorimatric Calculations
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Summary Phase Transition Data
Seismic Constrains Calculations (Pyrolite)
Simulations (MgSiO3)
410 5% to 6% About 3%
660 7% to 9% 6% to 7% About 8%
Lab Experiments Calorimatric Calculation
dP/dT 410 (Mpa/K) to 2.5 to 4
dP/dT 660 (Mpa/K) to Mw+Pv
–3 to –1 About -3
dP/dT 660 (Mpa/K) Pyrolite -0.5
Density Contrast
Clapeyron Slope
For Clapeyron Slope of Olivine Polymorphs: Duffy, T., Synchrotron facilities and the study of the Earth's deep interior. Rep. Prog. Phys. 68 (2005) 1811-1859.
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Slab Thermal Anomaly
Gaussian Cross-slabProfile Exponential
DecreaseIn PeakAnomaly
Max. SlabDepth: 1000 km
Max. SlabDepth: 500 km
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Phase Transition Anomaly
Temperature AnomalyTransition Height (km)
410: = 3.0 MPa/K = 3-6%660: = -1.3 MPa/K = 7-9%
410: = 4.0 MPa/K = 4%
660: = -2 MPa/K
= 3%
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Effect of Dip on Sum of ThermalAnd Phase Change Forces
0 10 ---Dip (degrees)-- 80 90
Tot
al F
orce
(x
101
2 N
/m)
16
12
8
4
0
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Effect of Density Change at Phase Boundaries
Change in Density at 660 (%)
Cha
nge
in D
ensi
ty a
t 41
0 (%
)
6 6.5 7 7.5 8 8.5 9
6
5.
4
3
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Effect of Clapeyron Slope
Clapeyron Slope at 660 Mpa/KCla
peyr
on S
lope
at
410
Mpa
/K
-3 -2 -1 -0.5
5
4
3
2.5
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Effect of Shear Forces
Major slowing occurs upon entering lower mantle
Lower mantleviscosity greaterthan 1022 Pa scan strongly hinderSlab.
um=1019 Pastran = 1020 Pas
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Metastable Olivine
Growth Rate: G(T) =
A*k*T*exp[-H/(RT)](1-exp[G/(RT)]) k=exp(10) Growth constant A = 1e-3 Extrapolation parameter for
low T in slab.
Depth of Metastable Olivine in Slabz ~v*ln(1-f)/(-2*S*G)
v Slab velocityS = 1/d Grain boundary Surface
Area/Volumef = 0.95 Volume fraction of
wadsleyite at completion of transformation.
Cooler Temperature strongly inhibits transformation.
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What about a Metastable Olivine Wedge?
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Conclusions• Buoyancy from temperature can be order of magnitude
larger than other forces.– Need dynamic model of temperature.
• Extra buoyancy from 410 phase change may be much larger than resisting buoyancy from 660.
• Shear forces beneath 660 may significantly hinder slab sinking into lower mantle.
• If phase parameters at 410 and 660 are comparable, then a moderately high viscosity in lower mantle can hinder slab.
• If metastable olivine exists, it can “easily” stop slabs in the transition zone, especially for large grain size (~ cms)