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Dynamics of decompression melting of upper mantle rocks
above hot spots under continental plates
V.S. Sobolev Institute of Geology
and Mineralogy SD RAS
Yu.V. Perepechko, K.E. Sorokin, V.N. Sharapov
Institute of Geology and Mineralogy SD RAS, Novosibirsk, Russia
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(Тзп 2100)
(Тзп 2200)
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(Тзп 2300)
(Тзп 2100)
(Тзп 2200)
Variation of melt volume
in the asthenosphere zone
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q, kg/m
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q, kg
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.
(Тзп 2300)
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t, Ma
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t, Ma
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(Тзп 2100)
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Variation of the
asthenosphere zone size
(Тзп 2200)
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(Тзп 2200)
(Тзп 2300)
LA, km
LA, km
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(Тзп 2100)
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(Тзп 2300)
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80 Ma0.5
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1
1.5
2
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3
3.5
4
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5
5.5
118 Ma
0.5
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1
1.5
2
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3
3.5
4
4.5
5
5.5
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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
f, %
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155 Ma
0.5
Основной
(Тзп 2100)
(Тзп 2200)
(Тзп 2300)
Variation of the mantle
melting degree in the
asthenosphere zone
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0.5
1
1.5
2
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3
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4
4.5
5
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t, Ma
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(Тзп 2100)
(Тзп 2200)
(Тзп 2300)
f, %
0.5
Основной
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t, Ma
Основной
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t, Ma
Melting evolution of asthenosphere zone over hot spot
Thicknesses of lithospheric plate - 150 km,
Hot spot size – 100 km
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 (THS) above the temperature on the upper and lower mantle border (Tb). The later was assigned as Тb =
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 Tb > 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 LHS = 400 km the decompression melting appears at a Tb >
1600°C and THS > 1900°C, size of asthenosphere zone ~700 km. When THS = 2000°C the maximum melting degree of the
primitive mantle is near 40%. An increase in the temperature at perovskite boundary to Tb = 1900°C the maximum degree of
melting could rich 100 % with the same size of decompression melting zone.
Thicknesses of lithospheric plate - 100 km,
Hot spot size – 400 km
We examined the sequence of the decompression melting above the hot spots having sizes (LHS) varying within LHS =
100 - 780 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 (LA) does not change substantially: LA=700 km when LHS = 100 km; LA= 800 km when LHS
= 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 Tb ≤ 1750°C with LHS =
100 km and maximum of THS = 2350°C. With an increase of Tb 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.
Supported by RFBR grant 12-05-00625.