(Currently under construction)

From a recent AGU meeting abstract:

Mantle mixing revisited: Effects of temperature- and pressure-dependent viscosity on exchange of material between the upper and lower mantle in 2 dimensions

Donna L. Hunt and Louise H. Kellogg

Using a 2-D finite element model of mantle convection containing several hundred thousand tracer particles to represent different mantle reservoirs, we examine the effects of pressure- and temperature-dependent viscosity on rates of mixing. We simulate the processes of addition of chemical heterogeneities (for instance, at subduction zones) and their destruction (by dispersion and by melting at mid-ocean ridges). Particles are introduced at downwellings and destroyed when they are so thoroughly dispersed that it would be impossible to measure their presence in the geochemical signature of mid-ocean ridges or oceanic islands. A large number of factors influence the flow pattern and thus the rate at which heterogeneities are dispersed by convection. Among these are the presence or lack of compositional stratification in the mantle, the motion of the overriding plates, how effective upwelling plumes are at disrupting plate-driven flow, how strongly the viscosity depends on temperature, and how much the viscosity increases with depth. We concentrate on the latter two factors, and determine how both the residence time of heterogeneities, and the exchange rate between the upper and lower mantle, vary with viscosity. Using a fractal and grid method to characterize the resulting distribution of heterogeneities in space and time, we compare mixing and development of heterogeneities for convection with uniform viscosity, flow with strongly temperature-dependent viscosity, flow with pressure-dependent viscosity, and flow with both. We also examine the effect of continuous increase of viscosity with depth versus an abrupt jump in viscosity between the upper and lower mantle. <\BODY>