Three-Dimensional Instabilities of Mantle Convection with Multiple Phase Transitions

Abstract

The effects of multiple phase transitions on mantle convection are investigated by numerical simulations that are based on three-dimensional models. These simulations show that cold sheets of mantle material collide at junctions, merge, and form a strong downflow that is stopped temporarily by the transition zone. The accumulated cold material gives rise to a strong gravitational instability that causes the cold mass to sink rapidly into the lower mantle. This process promotes a massive exchange between the lower and upper mantles and triggers a global instability in the adjacent plume system. This mechanism may be cyclic in nature and may be linked to the generation of superplumes.

Recent Seismological Studies on Upper Mantle Structure.

“…Broad low-velocity regions in the southern Pacific and the southern Africa may be interpreted as upwelling hot plumes generated by temperature disturbances due to cold remnant slabs. His hypothesis is consistent with the results of three-dimensional numerical simulations of mantle convection with the 660-km phase boundary (e.g., Honda et al, 1993).…”

Model for Convective Cooling of Mantle with Phase Changes: Effects of Aspect Ratios and Initial Conditions.

“…They showed that the flow through the phase boundary is intermittent in character; flow is impeded temporarily on the phase boundary and, later, it leaks through catastrophically because of the mass accumulation on the phase boundary. Their findings are confirmed by various recent modeling studies, such as axisymmetric geometry (Machetel and Weber, 1991;Peltier and Solheim, 1992), 2-D Cartesian geometry (Liu et al, 1991;Weinstein, 1993), 3-D Cartesian geometry (Honda et al, 1993) and 3-D spherical geometry . This type of intermittent flow behavior is called "flushing" or "avalanche" in recent 3-D studies (Honda et al, 1993;Tackley et al, 1993).…”

Interaction of the Upwelling Plume with the Phase and Chemical Boundaries. (2). Effects of the Pressure-Dependent Viscosity.

“…Recent numerical studies with more realistic geometries (three dimensionality, spherical geometry or a large aspect ratio) and parameters (high Rayleigh number) showed that a Clapeyron slope has a greater effect than that of the model with the small aspect ratio and the low Rayleigh number (Liu et al, 1991;Machetel and Weber, 1991;Zhao et al, 1992;Peltier and Solheim, 1992;Honda et al, 1993;Tackley et al, 1993). Therefore, our conclusion that plume penetration is more difficult than slab penetration does not change at least for the 670 km discontinuity consisting of a phase change, if we use more realistic geometries and parameters.…”

“…The interaction of the thermal convection with a chemical boundary was investigated by experimental and/or numerical studies (Richter and McKenzie, 1981;Olson, 1984;Kellogg, 1991). Many numerical studies have been performed to investigate the mantle convection with a single phase transition (Christensen and Yuen, 1985;Machetel and Weber, 1991;Tackley et al, 1993) or multiple phase transitions (Liu et al,1991;Zhao et al, 1992;Peltier and Solheim, 1992;Honda et al, 1993). In these studies, the models of the mantle with the constant viscosity or the depth-dependent viscosity were used.…”

“…Increase of the viscosity, therefore, should be expected to have an important effect to encourage the plume to ascend through the 670 km boundary. The depth (pressure)-dependent viscosity is employed in some previous studies (Honda et al, 1993;Tackley et al, 1993). They concentrated their effort to show that the effect of the phase boundary with the pressure-dependent viscosity results in the large-scale lateral heterogeneities in the mantle given by tomographic studies (e.g., Su and Dziewonski , 1992).…”