Two‐stage evolution of the Earth's mantle inferred from numerical simulation of coupled magmatism‐mantle convection system with tectonic plates

Abstract

AbstractSelf‐consistent numerical models are developed for a coupled magmatism‐mantle convection system with tectonic plates in a two‐dimensional rectangular box to understand the Earth's mantle evolution. The mantle evolves in two stages owing to decaying internal and basal heating, provided that the lithosphere is mechanically strong enough to inhibit spontaneous formation of new subduction zones by ridge push force. On the earlier stage that continues for the first 1–2 Gyr, the deep mantle is strongly heated, and hot materials there frequently ascend to the surface as bursts. The mantle bursts cause vigorous magmatism and make the lithosphere move chaotically. The thermostat effect of the vigorous magmatism keeps the average temperature in the upper mantle below about 1800 K no matter how strongly the mantle is heated. As the heating rate of the mantle declines, however, the mantle evolves into the later stage where mantle bursts subside, rigid tectonic plates emerge to move rather steadily, and subducted basaltic crusts accumulate on the core‐mantle boundary to form compositionally dense piles. Hot plumes occasionally ascend from the basaltic piles to cause magmatism. It takes time on the order of one billion years for the slabs that sink into the lower mantle to return back to the upper mantle, and the long overturn time makes the thermal history of the upper mantle, which has been petrologically constrained for the Earth, distinct from that of the whole mantle. The long overturn time also makes water injected into the mantle by slabs distribute heterogeneously.