Stirring in three‐dimensional mantle convection models and implications for geochemistry: 2. Heavy tracers

Three‐dimensional, spherical models of subduction and stirring of oceanic crust, taking account of the excess density of subducted oceanic crust in the mantle, confirm the main results from previous two‐dimensional models. Thus we find that the excess density of subducted crust significantly increases the ages of ocean island basalt (OIB) samples, assumed to come from the bottom 100 km of the mantle, but has little influence on the age of mid‐ocean ridge basalt (MORB) samples from the top of the mantle. The degree of processing of the mantle through melting zones is also little affected, and the Earth's mantle is still about 98% processed after 4.5 Ga. The latter results indicate that the processing time is still the main parameter controlling mantle processing and apparent MORB source ages, even with heavy tracers. However, the mean age of model OIB samples is significantly larger than observed apparent isotopic ages. Simplified estimates indicate that this discrepancy can be resolved by assuming that a population of ancient tracers in the OIB sampling zone, evident in the numerical models, is only marginally sampled by plumes because they are concentrated in denser accumulations at the bottom of the compositional boundary layer.