FEATURES OF MELTING IN THE THERMOCHEMICAL PLUME CONDUIT AND HEAT AND MASS TRANSFER DURING CRYSTALLIZATION DIFFERENTIATION OF BASALTIC MELT IN A MUSHROOM-SHAPED PLUME HEAD

The number Ka=N/N₁ is used to evaluate the thermal power of a plume; N is the thermal power transferred from the plume base to its conduit, and N₁ is the thermal power transferred from the plume conduit into the surrounding mantle. At the relative thermal power 1.9<Ka<10, after eruption of the melt from the plume conduit to the surface, melting occurs in the crustal block above the plume roof, resulting in the formation of a mushroom-shaped head of the plume. A thermochemical plume originates at the core-mantle boundary and ascends (melts up) to the surface. Based on laboratory and theoretical modeling data, we present the flow structure of melt in the conduit and the head of the thermochemical plume. The features of melting in the plume conduit are elucidated on the basis of the phase diagram of the CaO-MgO-Al₂O₃-SiO₂ model system. The two upper convection cells of the plume conduit relate to the region of basic and ultrabasic compositions. Our study shows that melting in these cells proceeds according to monovariant equilibria of eutectic type L=Cpx+Opx+An+Sp and L=Fo+An+Cpx+Opx. In case of the CaO–MgO–Al₂O₃–SiO₂–Na₂O system, crystallization differentiation proceeds as separation of plagioclase crystals. Separation of plagioclase crystals enriched in anorthite component leads to enrichment of the residual melt in silica and alkaline components. Assuming the initial basaltic melt, we calculated the compositional changes in the melt, which are powered by the heat and mass transfer processes in the mushroom-shaped plume head. The calculations were performed in two stages: (1) after settling of refractory minerals; (2) after settling of plagioclase in the melt resulting from the first stage. In the second stage, the melt contains 88.5 % of plagioclase component. The calculations were performed for melt temperature T_melt=1410 °C and pressure P=2.6 kbar and 6.3 kbar. The calculated weight contents of oxides, the normative compositions for solid phase, and the oxide content and normative composition for the residual melt were tabulated. The SiO₂ content in the residual melt amounts to 59.6–62.3 % and corresponds to the crustal SiO₂ content.

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