The Transition Zone: Slabs ’ Purgatory

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

x z Fb =∫gdxdz 

Constraints on Dr 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

Seismic Constrains Calculations (Pyrolite) Simulations (MgSiO3) Dr410 5% to 6% About 3% Dr660 7% to 9% 6% to 7% About 8% Lab Experiments Calorimatric Calculation dP/dT 410 (Mpa/K) atob 2.5 to 4 dP/dT 660 (Mpa/K) gto Mw+Pv –3 to –1 About -3 dP/dT 660 (Mpa/K) Pyrolite -0.5 Summary Phase Transition Data 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-slab Profile Exponential Decrease In Peak Anomaly Max. Slab Depth: 500 km Max. Slab Depth: 1000 km

Phase Transition Anomaly 410:  = 3.0 MPa/K  = 3-6% 660:  = -1.3 MPa/K  = 7-9% Temperature Anomaly Transition Height (km) • 410: • = 4.0 MPa/K  = 4% • 660: • = -2 MPa/K  = 3%

Effect of Dip on Sum of Thermal And Phase Change Forces 16 12 8 4 0 Total Force (x 1012 N/m) 0 10 ---Dip (degrees)-- 80 90

Effect of Density Change at Phase Boundaries 6 5. 4 3 Change in Density at 410 (%) 6 6.5 7 7.5 8 8.5 9 Change in Density at 660 (%)

Effect of Clapeyron Slope 5 4 3 2.5 Clapeyron Slope at 410 Mpa/K -3 -2 -1 -0.5 Clapeyron Slope at 660 Mpa/K

Effect of Shear Forces um=1019 Pas tran = 1020 Pas Major slowing occurs upon entering lower mantle Lower mantle viscosity greater than 1022 Pa s can strongly hinder Slab.

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 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.

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)

The Transition Zone: Slabs ’ Purgatory