A BIFURCATION IN THE COUPLED MAGMATISM-MANTLE CONVECTION SYSTEM AND ITS IMPLICATIONS FOR THE EVOLUTION OF THE EARTHS UPPER-MANTLE

A numerical model is presented for the coupled magmatism-mantle convec tion system in the upper mantle to study how the coupled system contro ls the thermochemical state of the upper mantle depending upon the int ernal or basal heating rate. The solid-state convection in the upper m antle is modeled as a convection of a binary eutectic material with co nstant viscosity in a two-dimensional rectangular box uniformly heated from the bottom boundary or by an internal heat source. The density o f the material depends on the composition and melt-content as well as temperature. Magmatism is modeled as a permeable flow of the melt gene rated by a pressure-release partial melting of the material; the perme able flow is driven by the buoyancy of the melt. There are two branche s in the thermochemical state controlled by the coupled system for bot h of the basal and internal heating cases. On one branch called the TC -branch, the solid-state convection occurs dominantly as a thermal con vection, the box remains chemically homogeneous as a whole, and magmat ism occurs only slightly at most. The TC-branch is stable only when th e heating rate is lower than a threshold; a bifurcation occurs on the TC-branch at the threshold and the thermochemical state falls on anoth er branch called the CS-branch above the threshold. An episodic magmat ism actively occurs, a chemically stratified structure develops well i n spite of the homogenizing effect of convective stirring, the tempera ture becomes as high as the solidus temperature at depth, and the conv ection is strongly affected by the buoyancy of melt and the chemical b uoyancy that accompanies the chemical stratification on the CS-branch. The CS-branch is stable even at heating rates slightly lower than the threshold; a hysteresis in the thermochemical state occurs between th e two branches as the heating rate changes around the threshold. The o bservations of tectosphere suggest that the thermochemical state on th e CS-branch occurred in the upper mantle in the Archean and early Prot erozoic when the mantle was strongly heated by radioactive elements an d that the thermochemical state jumped to the TC-branch at the end of the early Proterozoic. (C) 1997 Elsevier Science B.V.