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. The theory presented shows how creeping flow redistributes large-scale passive anomalies of bulk and trace element composition as well as temperature. The term passive applies to heterogeneities that are transported by pure advection. It refers to those anomalies whose density and rheology are sufficiently close to average mantle that they can be held within the mantle for long periods of time without greatly perturbing the background circulation. Theoretical predictions of mixing time scales (the apparent lifetime of an anomaly) are compared directly with results of numerical experiments on mixing within Benard convection cells. Through this comparison, it is demonstrated that mixing by creeping flow can be described as a cascade process.