Mantle overturn and thermochemical evolution of a non-plate tectonic mantle

Abstract

We use coupled thermomechanical and thermodynamic modelling to investigate the evolution of a heating mantle below a stagnant lithosphere that leads to overturn events as proposed for Venus. Eclogitized crust entrenched into the upper mantle accumulates in a gravitational trap at the lower-upper-mantle boundary, where basaltic compositions display an intermediate density between upper and lower mantle. While the convecting upper mantle remains at a stable mantle potential temperature, the lower mantle progressively heats. Over 900 Ma, the basalt-rich layer develops into a thermal boundary layer with an offset in potential temperatures as high as 300°C. Occasional plumes breaching the layer from below cause local increased magmatism at the surface but do not induce complete mantle overturn. Once the basalt-rich layer, however, reaches a critical thickness, so that the crust in its lowermost part experiences a transition into assemblages denser than lower mantle a runaway process is initiated. The entire crustal-rich layer is dragged into the lower mantle, switching the convective mode to whole mantle convection. Fertile, superheated lower mantle replaces the depleted cold upper mantle and magmatic production increases by orders of magnitude for ∼50 Ma. During this period, crustal production-rates are so high that not only a new surface in generated but also the delamination rate of lower crust into the upper mantle is increased dramatically with the hot low-viscosity crust now forming much larger delaminating bodies. About 50 Ma after the onset of the overturn, the magmatic production subsides and crust starts to accumulate at the lower-upper mantle boundary again. The overturn event creates a mantle that is chemically heterogeneous and intensely foliated at the model-resolution scale with intermingled crust, depleted and undepleted mantle. The phase-changed induced collapse of the basalt-layer and subsequent global overturn happen at mantle temperatures considerably lower than predicted by previous models for Venus.