the transition zone: slabs ’ purgatory cider, 2006 - group a garrett leahy, ved lekic, urska...
TRANSCRIPT
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
Tonga Seismicity
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.
Tomographic Models
Harvard Berkeley
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.
A Simple Force Balance for slabs in the Transition Zone
Fb =∫ gdxdz
x
z
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
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.
Slab Thermal Anomaly
Gaussian Cross-slabProfile Exponential
DecreaseIn PeakAnomaly
Max. SlabDepth: 1000 km
Max. SlabDepth: 500 km
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%
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
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
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
Effect of Shear Forces
Major slowing occurs upon entering lower mantle
Lower mantleviscosity greaterthan 1022 Pa scan strongly hinderSlab.
um=1019 Pastran = 1020 Pas
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.
What about a Metastable Olivine Wedge?
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)