Intermittent layered convection in a model mantle with an endothermic phase change at 670 km

Abstract

IT is not yet known whether convection in the Earth's mantle occurs in cells that extend throughout the entire depth of the mantle, or if the upper and lower mantle convect in independent layers; both possibilities seem to be consistent with geochemical and seismologi-cal observations^1-4. It is generally accepted, however, that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change, spinel → perovskite + MgO. Here we investigate the results of including this phase change in a model of mantle convection. For realism and to eliminate scaling problems, our calculations use commonly accepted values of geophysical parameters. For a Clapeyron slope y = −2 × 10⁶, a value close to experimental results and theoretical expectations, we observe local intermittent mixing between the upper and lower mantle. Such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole-mantle and layered convection.