Thermochemical regime of the early mantle inferred from numerical models of the coupled magmatism‐mantle convection system with the solid‐solid phase transitions at depths around 660 km

Abstract

A numerical model is presented for the coupled magmatism‐mantle convection system in the entire mantle to study how the coupled system controls the thermochemical state of the mantle under the influence of the solid‐solid phase transitions at depths around 660 km depending on the internal heating rate. The solid‐state convection in the mantle is modeled by a convection of a binary eutectic material with constant viscosity in a two‐dimensional square box uniformly heated by an internal heat source. One of the end‐members of the material stands for an olivine‐rich material in the uppermost mantle and is transformed into its high‐pressure phase at depths greater than a threshold around 660 km. The phase boundary has a negative Clausius‐Clapeyron slope. Another end‐member stands for a garnet‐rich material in the uppermost mantle and is gradually transformed into its high‐pressure phase with increasing depth in a depth range around 660 km. The density of the material depends on its composition, phase, melt content, and temperature. Magmatism is modeled by a permeable flow of melt generated by a pressure release partial melting of the material. The permeable flow is driven by the buoyancy of the melt. There are two regimes in the thermochemical state of the mantle regardless of the strength of the barrier against mass exchange between the upper mantle and the lower mantle due to the solid‐solid phase transitions. On one regime called the TC branch, magmatism occurs only mildly at most, the mantle remains chemically homogeneous as a whole, and the solid‐state convection occurs dominantly as a thermal convection. The convective circulation is mantle‐wide or layered depending on the strength of the barrier due to the solid‐solid phase transitions. The TC branch is stable only when the internal heating rate is lower than a threshold. A bifurcation occurs on the TC branch at the threshold, and the thermochemical state falls on another regime called the CS branch above the threshold. An episodic magmatism actively occurs to make the mantle chemically stratified with the upper mantle largely occupied by olivine‐rich residual materials and the deeper part of the lower mantle occupied by magmatic products of basaltic composition on the CS branch. The solid‐state convection occurs as a whole mantle convection when the barrier is weak, while it occurs as a layered convection punctuated by flushing events that induce particularly vigorous magmatic activities when the barrier due to the phase transition of the garnet‐rich material is sufficiently strong. The overall features of the thermochemical state on the CS branch mesh in many observations from the Archean and early Proterozoic continents.