MAGMATIC DIFFERENTIATION EXAMINED WITH A NUMERICAL MODEL CONSIDERING MULTICOMPONENT THERMODYNAMICS AND MOMENTUM, ENERGY AND SPECIES TRANSPORT

Abstract

Magmatic differentiation processes in a cooling magma body were examined using a numerical model considering multicomponent thermodynamics and momentum, energy and species transport. The model accounts for melt transport induced by its density variation, resulting primarily from solid–liquid phase change. The equilibrium mineral assemblages, their mass fractions and their chemical compositions are determined by multicomponent thermodynamic models, and these parameters are linked with the calculations of velocity, pressure, temperature and species fields at each iteration and time step. For simplicity, solid phases are assumed to be stationary, and only olivine and plagioclase that are the earliest crystallization phases in common basaltic magmas are considered as fractionating phases. Application of the model to natural magmatic system shows that crystallization occurs selectively along the chilled boundaries soon after the cooling, and the magma body is separated into high-crystallinity mush zones and mostly crystal-free main magma. Because of the melt exchange between the mush zones and the main magma by compositional convection, the main magma evolves progressively in composition and the spatial chemical heterogeneity is grown with time. The melt segregated from the sidewall mush zone is accumulated below the roof mush zone, forming compositional stratification in the upper part of the main magma. On the other hand, the fractionated melt from the floor mush zone principally mixes with the overlying ambient magma in the middle and lower parts of the main magma body. This study shows that the direct treatment of nonlinear coupling among momentum, energy and species transport provides useful information of the thermal and chemical evolution of magma chambers as a function of time and space.