Dynamics of melting in the oceanic upper mantle

Convection in the upper mantle has been investigated in the Bossinesq appro ximation with due regard to four solid-phase transitions above the "perovsk ite" transition at a depth of 670-700 km for systems of two types: (1) with out basaltic crust (some slowly spreading ridges) and (2) with basaltic cru st 7 to 20 km thick. Numerical calculations have been carried out on the ba sis of the method of control volume for a 2D region, 700 x (2500- 5000) km( 2) in size. Development of melting and its extent were estimated on the bas is of parameterization of water-free lherzolite according to McKenzie and N iu and Batiza. Mantle rocks melt in systems with initial temperatures excee ding 500 degreesC at a depth of 100 km, 800 degreesC at 300 km, and 1700 de greesC at the upper-lower mantle boundary. At a temperature at the lower bo undary below 2000 degreesC, melting proceeds at depths of 24-290 km with th e degrees of melting of similar to0.05-0.8 15-20 Ma after the beginning of convection and exists for about 20-70 Ma. At a temperature higher than 2000 degreesC, the magmatic system passes through two stages: nonstationary, wi th the degree of melting of about 0.5-0.6 at depths of similar to 30-290 km , and quasi-stationary, with the degree of melting of 0.06-0.4 at depths of similar to 60-100 km. The horizontal extent of melting zones varies from 1 50 to 1000 km. A periodical structure forms in the melting zone, and the ma ximum of melting degree considerable shifts toward the upper boundary of th e upper mantle. The velocity of mantle matter near the upper boundary reach es similar to 14-25 cm/year before melting, then gradually decreases to 3-9 cm/year by the climax of the magmatic system and to 1.5-2 cm/year by its d egeneration. There is no linear relationship between the degree of melting and velocity of the Earth's crust over ascending flows. The thermal flow ne ar the surface of the Earth's crust reaches similar to 60-145 mW/m(2) at ma ximum melting and decreases to similar to 40-60 mW/m(2) on degeneration of the magmatic system.