Effects of multiple phase transitions in a three-dimensional spherical model of convection in Earth's mantle
Numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three-dimensional flow pattern, with cylindrical features and linear features that behave differently in their ability to penetrate the 670-km discontinuity. The dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth, resulting in frequent "avalanches" of upper mantle material into the lower mantle. The effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670-km phase change, resulting in smaller and more frequent avalanches, and a wider range of morphologies. Large quantities of avalanched cold material accumulate above the coremantle boundary (CMB), resulting in a region of strongly depressed mean temperature at the base of the mantle. The 670-km phase change has a strong effect on the temperature field, with three-distinct regions being visible: (1) the upper mantle, containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum; (2) the midmantle, containing quasi-cylindrical avalanche conduits and characterized by a low amplitude, broad spectrum; and (3) the deep mantle, containing large pools of cold, avalanched material and characterized by a high amplitude, ultra-red (i.e., long wavelength) spectrum. The effect on the velocity field is very different. Flow penetration across the 670-km phase change is strongly wavelength-dependent, with easy penetration at long wavelengths but strong inhibition at short wavelengths. Thus, when comparing numerical models with long wavelength seismic tomography, diagnostics based on the density field, such as the radial correlation function, are much more sensitive to the effects of phase transitions than those based on the velocity field. The amplitude of the geoid is not significantly affected by the partial layering, because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670-km discontinuity. Avalanches are associated with geoid lows. However, a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in Earth.
© 1994 American Geophysical Union. Received September 20, 1993; revised March 14, 1994; accepted March 31, 1994. We thank D.A. Yuen and W.R. Peltier for preprints and useful discussions S. D. King and W.S. Kiefer for constructive reviews, and W. M. Kaula for providing us with planetary gravity coefficients. This research was performed in part using the Intel Touchstone Delta operated by Caltech on behalf of the Concurrent Supercomputing Consortium. Access to this facility was provided by Caltech. This work was supported by NSF grant EAR9017893 and by the Institute of Geophysics and Planetary Physics at Los Alamos. Division of Geological and Planetary Sciences contribution 5331. Paper number 94JB00853.
Published - jgrb9616.pdf