Mixing of passive heterogeneities by mantle convection.

The kinematic processes by which thermal convection can mix together large-scale mantle heterogeneities are described theoretically and with a series of numerical experiments. We have analyzed the time evolution of a large-scale passive, diffusive scalar anomaly subject to two models of mantle flow: 1) turbulent flow, in which material elements are elongated exponentially with time, and 2) laminar flow, in which material elements are elongated according to a power law function of time. Mixing times from the numerical experiments agree most closely with predictions from the laminar theory. The theory and the numerical experiments indicate: 1) Mixing by creeping flow in the mantle is a cascade phenomenon. The time scale for mixing thermal anomalies is short, in the range 75-200 Ma, depending on the initial scale of the anomaly. The time scale for mixing large-scale compositional anomalies is far longer, approximately the age of the Earth. 2) Mixing of compositional anomalies tends to produce a laminated upper mantle, in which ancient heterogeneity is stretched into thin subhorizontal layers. -from Authors