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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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F b = ∫ 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 ConstrainsCalculations (Pyrolite) Simulations (MgSiO3) 410 5% to 6%About 3% 660 7% to 9%6% to 7%About 8% Lab ExperimentsCalorimatric Calculation dP/dT 410 (Mpa/K) to 2.5 to 4 dP/dT 660 (Mpa/K) to Mw+Pv –3 to –1About -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-slab Profile Exponential Decrease In Peak Anomaly Max. Slab Depth: 1000 km Max. Slab Depth: 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 Thermal And Phase Change Forces 0 10 ---Dip (degrees)-- 80 90 Total Force (x 10 12 N/m) 16 12 8 4 0

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Effect of Density Change at Phase Boundaries Change in Density at 660 (%) Change in Density at 410 (%) 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/K Clapeyron Slope 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 mantle viscosity greater than 10 22 Pa s can strongly hinder Slab. um =10 19 Pas tran = 10 20 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-3Extrapolation parameter for low T in slab. Depth of Metastable Olivine in Slab z ~v*ln(1-f)/(-2*S*G) v Slab velocity S = 1/d Grain boundary Surface Area/Volume f = 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)

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