3D spherical geodynamic modeling through time Scott D. King Department of Geosciences Virginia Tech, Blacksburg USA Session 21f: Chemical Geodynamics Through Time

Unique Model Features cooling core boundary conditioncooling core boundary condition radiogenic decay (non-constant internal heating)radiogenic decay (non-constant internal heating) hot mantle initial conditionhot mantle initial condition mobile surface boundary conditionmobile surface boundary condition weak lithosphere + asthenosphere (Höink et al., 2008)weak lithosphere + asthenosphere (Höink et al., 2008) imposed plate velocities (present day)imposed plate velocities (present day) yield-stress formulationyield-stress formulation

typical thermal history curve From Schubert, Turcotte and Olson, 2001 Figure 13.1

3D spherical convection Bousinessq, which means:Bousinessq, which means: constant coef thermal expansion, Cp, densityconstant coef thermal expansion, Cp, density no adiabatic heating/gradientno adiabatic heating/gradient no viscous shear heatingno viscous shear heating rheology strong function of temperature (Hirth and Kohlstedt, 2003)rheology strong function of temperature (Hirth and Kohlstedt, 2003) factor of 30 increase in viscosity at 660factor of 30 increase in viscosity at 660 start from moderately hot mantle with a short wavelength perturbationstart from moderately hot mantle with a short wavelength perturbation

decreasing CMB temperature 70 K/Gyr taken from Davies (1999) constant heat source, constant CMB temp constant heat source, decreasing CMB temp

decaying HPE taken from Turcotte and Schubert constant heat source, constant CMB temp decay HPE source, decreasing CMB temp

Add Asthenosphere asthenosphere no asthenosphere

RMS Velocities no asthenosphereasthenosphere

Radial average temp no asthenosphereasthenosphere

Temperature Anomalies no asthenosphereasthenosphere

What is happening? asthenosphere addition of asthenosphere creates long-wavelength flowaddition of asthenosphere creates long-wavelength flow seen by before (Zhong and Zuber, 2001; Roberts and Zhong, 2006)seen by before (Zhong and Zuber, 2001; Roberts and Zhong, 2006) the asthenosphere effectively decouples the mantle from the stagnant lidthe asthenosphere effectively decouples the mantle from the stagnant lid

Mobilize lithosphere asthenosphere no asthenosphere mobile lid

dla4 Temperature Anomalies stagnant lidmobile lid dla4 asthenosphere

Long Wavelength Short Wavelength Torsvik et al. 2006

imposed plate motion present day plate motions temperatures at 300 km depth

Temperature at 650 km at 2.7 Ga Farallon slab hemisphere Western Pacific hemisphere

Yield-Stress Plate Formulation

Compare Thermal History

Nearly Decoupled upper/lower mantle

Conclusions The asthenosphere plays a major role in mantle evolution and flow.The asthenosphere plays a major role in mantle evolution and flow. Decreasing CMB temperature with time plays almost no role.Decreasing CMB temperature with time plays almost no role. If 90% of surface heat flow is from radiogenic heating and/or secular cooling, you have to do a lot to the 10% (CMB) to get a noticeable effect on the mantleIf 90% of surface heat flow is from radiogenic heating and/or secular cooling, you have to do a lot to the 10% (CMB) to get a noticeable effect on the mantle Must consider a mobile lid or asthenosphere – with a stagnant lid the mantle gets/stays too hot.Must consider a mobile lid or asthenosphere – with a stagnant lid the mantle gets/stays too hot. When did plate tectonics begin?When did plate tectonics begin? How mobile was the lithosphere before plate tectonics?How mobile was the lithosphere before plate tectonics?