Numerical experiments on the simple models of coupled mantle magmatism-mantle convection system were made to study the thermal and chemical evolution of the Earth's mantle. Mantle convection is modeled by the convection of a two-component Newtonian constant-viscosity fluid in an internally heated two-dimensional rectangular region. The density of the convecting fluid is assumed to depend on its chemical composition as well as its temperature. The two components of the convecting fluid are assumed to constitute an eutectic system. Mantle magmatism (i.e., magma segregation in the mantle) is modeled by differentiation of the eutectic mixture from the convecting fluid. This chemical differentiation is assumed to occur when the temperature T increases to the solidus temperature Ts (assumed to increase linearly with depth) owing to convective heat transport. Taking into account the change in density distribution due to this chemical differentiation, we solve the basic equations for convection numerically as an initial value problem.

We find that how the mantle evolves depends strongly on two quantities: H (internal heating rate) and r*≡(&rgr;m-&rgr;&egr;)/Δ&rgr;(&rgr;m is the density of undifferentiated mantle material at 0¿C, &rgr;e is the density of the eutectic mixture at 0¿C, and Δ&rgr;(>0) is a measure of density change in the mantle due to thermal expansion): r* represents the magnitude of the dependence of density on chemical composition. (1) When -4≲r≲*-2 and HHc, Tav increases with time, while q continuse to be nearly constant. Magmatism occurs to produce a residue, which occupies the uppermost part of mantle, throughout the evolutionary history of the mantle. (3) When -1H*c, Tav increases with time, while q decreases with time. Mantle magmatism occurs to produce a residue, which accumulates at the bottom of mantle, throughout the evolutionary history of the mantle.

We also find that mantle magmatism occurs episodically when r*>1 and r*≲-2, while mantle magmatism occurs continuously when -1<r*=%1. From an estimate of r* appropriate for the Earth's mantle and known features of the Archean continental crusts, we conclude that the type 1 mantle evolution occurs beneath the continental crusts; many features of the Archean continental crusts fit in with the framework of the type 1 mantle evolution. ¿ American Geophysical Union 1988