Can large increases in viscosity and thermal conductivity preserve large-scale heterogeneity in the mantle?

Abstract

Listen

Translate article

Recent studies in mineral physics suggest that the major component minerals undergo substantial changes in properties at the pressure and temperature conditions that pertain at the lower mantle. The associated affect on mantle viscosity and thermal conductivity could influence mantle flow. Several studies explore the effect of viscosity variations on mantle mixing, but none have yet examined the combined effects of pressure on both viscosity and thermal conductivity. Here, we explore the dynamical influence of variations in thermal conductivity and viscosity suggested by recent experiments [Badro, J., Fiquet, G., Guyot, F., Rueff, J., Struzhkin, V.V., Vanko, G., Monaco, G., 2003. Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity. Science, 300, 789–791; Badro, J., Rueff, J., Vanko, G., Monaco, G., Fiquet, G., Guyot, F., 2004. Electronic transitions in perovskite: possible nonconvection layers in the lower mantle. Science 305, 383–386] as a mechanism for generating mantle heterogeneity. We performed a series of 2-D computations using the finite element thermal convection code ConMan [King, S.D., Raefsky, A., Hager, B.H., 1990. ConMan—vectorizing a finite element code for incompressible 2-dimensional convection in the Earth's mantle. Phys. Earth Planet. Inter. 59, 195–207], where the viscosity and/or thermal conductivity increases at the vertical mid-point of either internally or basally heated models. Because the effect of pressure on material properties is poorly characterized, we explored a wide range of values. Increasing either viscosity or thermal conductivity alone or both viscosity and thermal conductivity together by a factor of up to 150 changes the thermal profile and dynamics, with increases in either or both tending to increase the wavelength of structures and the temperature in the lowermost mantle. Passive tracers show efficient transfer of mass across the transition depth, with no layering. The configurational entropy of the tracer distributions increases with time and indicates relatively efficient mixing. If geochemical constraints such as neon, xenon, and neodymium isotopes require long-lived isolated mantle reservoirs, a mechanism other than varying viscosity and thermal conductivity must be invoked; intrinsic density contrasts may be required.