QUANTITATIVE RELATIONS IN THE FRACTIONATION OF TRACE ELEMENTS DURING THE CRYSTALLIZATION DIFFERENTIATION OF MAGMATIC MELTS

Abstract

As follows from experience in the simulation of the geochemical structures of differentiated complexes of mafic and ultramafic rocks, intrusive magma always contains intratelluric solid phases (there are no natural magmas overheated above their liquidus temperatures), and the solidification and development of differentiated intrusions is associated with the onset of sedimentation convection of the melt containing suspended crystals, which are formed near the roof of the magmatic chamber. The percentage of suspended crystals can be fairly significant (from a few to a few dozen percent), and this imposes certain constraints onto the action of fractionation laws. The point is that crystals suspended in melt can be regarded as occurring in equilibrium with it, which results in the violation of one of the conditions of Rayleigh fractionation, namely, the postulate about the infinitely small mass of the fractionated solid phase in equilibrium with the melt at any given moment of time. Because of this, a more realistic model for this natural process should involve the assumption of the finite mass of the solid phase in equilibrium with the melt. This should modify relations between the degree of solidification and the degree of fractionation of the crystallizing system.