Dynamics of decompression melting of upper mantle rocks above hot spots under continental plates Yu.V. Perepechko, K.E. Sorokin, V.N. Sharapov Institute of Geology and Mineralogy SD RAS, Novosibirsk, Russia The numeric simulation of the decompression melting under the hot spots was accomplished under the following conditions. The initial temperature of the temperatures within the crust mantle section was postulated. The thickness of the metasomatized lithospheric mantle is determined by the mantle rheology and by the position of upper boundary of asthenosphere. The upper and lower boundaries were postulated to be not permeable and the condition for adhesion and the distribution of temperature (1400÷2050°C). Conditions on the lateral boundaries imitated infinity of the layer. Sizes and the distribution of lateral points, their symmetry, and maximum temperature varied between the thermodynamic condition for existences of perovskite-majorite transition and its excess above the temperature the transition. Problem was solved numerically a cell-vertex finite volume method for thermohydrodynamic problems. For increasing the convergence of iterative process, the method of lower relaxation with the different value of the parameter of relaxation for each equation was used. The method of through calculation was used for the increase in the computing rate for the two-layered upper mantle - lithosphere system (700 × 2100÷4900 km). The time step for the study of the asthenosphere dynamics composed 0.15÷0.65 Ma. We examined the sequence of the decompression melting above the hot spots having sizes (L HS ) varying within L HS = km at a boundary temperature changing from 1850°C to 2100°C with the thickness of lithosphere 100 km. It is shown that the size of asthenolens (L A ) does not change substantially: L A =700 km when L HS = 100 km; L A = 800 km when L HS = 780 km. With the presence of asymmetry of the large hot spots, the region of advection is developed above the hotspot maximum with the formation of asymmetrical cell. The Influence the thicknesses of lithospheric plate on appearance and evolution of asthenolens above the hotspots were investigated for the model stepped profile for the T b ≤ 1750°C with L HS = 100 km and maximum of T HS = 2350°C. With an increase of T b the difference of the asthenolens horizontal sizes beneath the lithospheric steps, it is levelled with the retention of a certain difference to the melting degrees and time of the melting appearance a top of the hot spot. The following factors controlling the sizes and the degree of melting of the convecting upper mantle, are shown: a) the initial temperature distribution along the section of upper mantle, b) sizes and the symmetry of hot spot, c) temperature excess within the hot spot (T HS ) above the temperature on the upper and lower mantle border (T b ). The later was assigned as Т b = 1500÷2000°С with the 5÷15 % deviation but no exceed 2350°С. It is found, that the appearance of decompression melting with the presence of hot spot initiate the melting of the primitive mantle with T b > 1600°C. In this case, the influence of the initial heating of upper mantle on the dimensions of asthenosphere zones with a constant size of the hot spot is influencing mainly to the degree of decompression melting. Thus, with the sizes of L HS = 400 km the decompression melting appears at a T b > 1600°C and T HS > 1900°C, size of asthenosphere zone ~700 km. When T HS = 2000°C the maximum melting degree of the primitive mantle is near 40%. An increase in the temperature at perovskite boundary to T b = 1900°C the maximum degree of melting could rich 100 % with the same size of decompression melting zone. Supported by RFBR grant Phase transitions in crust and mantle F1 ‑ Melting of granite F2 ‑ Melting of depleted metasomatised ultrabasite F3 ‑ Melting of lherzolite F4 ‑ Garnet lherzolite–spinel and plagioclase lherzolite F5 ‑ Olivine–β-spinel F6 ‑ β-spinel–γ-spinel F7 ‑ Spinel–perovskite 80 Ma 118 Ma 155 Ma Melting evolution of asthenosphere zone over hot spot Thicknesses of lithospheric plate km, Hot spot size – 400 km Thicknesses of lithospheric plate km, Hot spot size – 100 km Variation of the mantle melting degree in the asthenosphere zone Variation of the asthenosphere zone size Variation of melt volume in the asthenosphere zone V.S. Sobolev Institute of Geology and Mineralogy SD RAS