EarthRef.org Reference Database (ERR) -- Ogawa 2000

Numerical models are presented for the thermal and chemical evolution of the mantle when it is controlled by both magmatism and mantle convection. The mantle is modeled as a fluid with temperature and pressure-dependent viscosity in a two-dimensional square box heated by incompatible radioactive elements that decay with time. Thr influence of phase transitions at 660 km between the upper and lower mantle is also included. Magmatism is modeled as permeable flow of magma produced upon pressure release partial melting of the convecting mantle materials. It is shown that the mantle may exist in one of two regimes. At first, when the internal heating rate is higher than 3 threshold, the mantle is on a regime where episodic, but active, magmatism takes place to make the mantle chemically stratified with the upper mantle occupied by magma residue (harzburgite) and the deeper part of the lower mantle occupied by basaltic materials. There is a chemical discontinuity along the 660 km phase boundary. Mantle convection occurs as a layered convection punctuated by flushing events and is sluggish except at the time of flushing events owing to the compositional buoyancy fi om the chemical stratification. Thr flushing events induce vigorous magmatic activity. As the internal heating rate falls below the threshold owing to the decay of radioactive elements, the thermal and chemical state jumps to another regime. In this regime, magma does not erupt, and mantle convection occurs as layered thermal convection. In spite of the stirring due to the thermal convection, the chemical discontinuity and a portion of the chemical stratification formed in the earlier regime remains for billions of years. The numerical models suggest that (a) mantle convection was much less active than expected from the strong internal healing, and chemically distinct reservoirs are formed in the early Earth, and once formed (b), the reservoirs have survived convective stirring for billions of years. (C) 2000 Elsevier Science B.V. All rights reserved.