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

Abstract

Self-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.

Evolution of the interior of Mercury influenced by coupled magmatism-mantle convection system and heat flux from the core

“…First, the aspect ratio of the convecting vessel λ = 4 implies that the global scale lateral heterogeneity of Mercury's crust and mantle [ Ernst et al ., ; Peplowski et al ., ; Charlier et al ., ; Weider et al ., , ; Denevi et al ., ], which is addressed in earlier numerical studies [e.g., Roberts and Barnouin , ; Michel et al ., ], is beyond the scope of this study. Earlier numerical models of the Earth [ Ogawa , ], however, show that the effects of magmatism on mantle dynamics, which is the issue addressed here, do not sensitively depend on λ when λ ≥ 4, and I chose λ = 4 for a computational reason. Second, a shallower mantle and no‐slip boundary condition at the CMB may be more appropriate, if a solid layer of FeS develops at the top of the core [ Smith et al ., ].…”

Supercontinent Cycle and Thermochemical Structure in the Mantle: Inference from Two-Dimensional Numerical Simulations of Mantle Convection

“…However, considering their incompatible nature, the heat-producing elements are most likely to be distributed non-uniformly in the real mantle, by their fractionation into magmatic products and recycling into the mantle through subduction. In particular, assuming that the origin of chemically-dense materials in the lowermost mantle is ancient magmatic products such as subducted basalts (e.g., [38]), the heat-producing elements must be concentrated much more highly in the dense materials than in the ambient (depleted) mantle. That is, if distributed highly non-uniformly in the mantle, the internal heating is expected to locally heat up the chemically-dense materials, as well as to alter the style of overall convection.…”

“…The range of B c employed in this study includes the values of B c chosen in [36] for the materials forming LLSVPs (B c = 0.8). In addition, the value of the density difference between MORB (mid-ocean ridge basalt) and the surrounding materials in the lowermost mantle is estimated to be about 100 kg/m 3 [37], which roughly corresponds to the values of B c in the range of 0.9 ≤ B c ≤ 1.2 [38].…”

“…Taken together, at a certain time instance in the Earth's evolution, there might occur a state where a continental lithosphere covers a third of the Earth's surface and a layer of chemically-dense material is formed at the base of the mantle. In other words, our present calculations consider the temporal evolutions of the mantle and (super)continents only in a later stage of Earth's history where the vigor of magmatism subsides (e.g., [38]). …”

“…In other words, the occurrence of the supercontinent cycle for sufficiently large B c is not due to the change in the vigor of ascending plumes, but due to the anchoring of plumes by dense materials. In order to study in much more detail how the chemical heterogeneity of dense materials behaves in the lowermost mantle during the supercontinent cycle, we show in Figures 8 and 9 the temporal evolutions of flow patterns and thermochemical states for Case 4 (B c = 1.125) where the density difference between two components is comparable with that between MORB and the surrounding materials in the lowermost mantle [36][37][38]. In Figure 8, we show by color the temporal evolution as a function of the horizontal coordinate θ of the fraction c of dense Component B averaged over a d/16 (=181.25 km) depth just above the bottom surface.…”