Basaltic accumulation instability and chaotic plate motion in the earliest mantle inferred from numerical experiments

摘要

A series of self-consistent numerical models of mantle convection with magmatism and moving plates are presented to clarify the dynamics of the strongly heated mantle of the earliest Earth. Broad hot accumulations of subducted basaltic crusts develop on the core-mantle boundary and remain there for geologic time only when the internal heating rate is below a threshold. Above the threshold, the basaltic accumulations become unstable. Instead, the thermal buoyancy induced by the strong internal heating repeatedly causes a massive upwelling, or burst, of hot materials from deep mantle. The mantle burst induces vigorous magmatic activity on the surface. The magmatic activity, in turn, damages the lithosphere and disintegrates it into smaller fragments. In contrast to the well-ordered plate motion observed below the threshold, the lithospheric fragments chaotically move and new subduction zones sporadically develop. The mantle burst and chaotic motion of lithospheric fragments effectively stir the mantle, and the mantle remains chemically rather homogeneous in spite of the chemical differentiation due to the vigorous magmatism. The Earth's mantle convection has probably changed its regime from one dominated by frequent mantle bursts and chaotic motion of lithospheric fragments to one dominated by stable basaltic accumulations and well-ordered plate motion, as the internal heat source decays. The suggested regime transition fits in with the Earth's tectonic evolution inferred from geologic observations of the continents of various ages.