OSCILLATION PHENOMENA AT MAGMATIC CRYSTALLIZATION: ROLE OF DEVOLATILIZATION

The two component haplogranite melt crystallization model is proposed. The new model takes into account the thermodynamic interaction with fluid components (H₂O, NaF) and combine it with non-equilibrium model of H₂O loss-and-gain by melt during crystallization. The numerical application is developed for simplified working system: alb – qtz – NaF – H₂O. The process of crystallization takes place in dyke belonging to upper part crystallizing granite system. The model of fluid release is based on the scheme which includes the input of dissolved H₂O by magma convection, the accumulation of H₂O in melt due to crystallization, the reversible transfer of H₂O between magmatic melt and vapor (fluid) phase, the autocatalytic irreversible stage of this transfer and the vapor buoyancy escape out of system. The autocatalysis of devolatilization is conditioned by the pair collisions of vapor (fluid) bubbles. Mineral crystallization and the buoyancy process have the “viscous” activation energy (450 kJ/mole) and the kinetics of bubbles forming has the diffusion activation energy (60 kJ/mole). The calculations reveal the wide region of kinetic parameters with oscillation regime of melt degassing. The changes of H₂O concentration, the temperature and the rates of mineral crystallization get the oscillation character. The variation in H₂O concentration dissolved in melt produces a swing motion of eutectic position in phase diagram and causes rhythmic crystallization of mineral components. At the initial water content in melt 4.4% and at fluorine content 4-6% the oscillation of H2O concentration are in the range 1.6-2.8%. For 4% of F in melt the temperature oscillation focused around of 740 ⁰C, and for 6% of F – around of 690 ⁰C. A comparison of the model patterns with the natural samples of line rocks from Etyka and Orlovka (Transbaikalia) shows a principal relevance of model.