Minor element effects of combined fractional partial melting and crystallization

Abstract

Formation of igneous rocks generally comprises generation of a magma by partial melting, ascension of this magma and its crystallization. A model is constructed and treated mathematically, in which both partial melting and crystallization are assumed to follow Rayleigh fractionation conditions and crystallization is supposed to proceed in a magma chamber continuously recharged by the products of partial melting. The model comprises DePaolo's [1] continuous magma recharge fractional crystallization model as a special case. If the timing of partial melting and of crystallization (in terms of change of the ratio of residual to original amount of transforming material) is of the same order of magnitude, concentration patterns of minor elements in crystallization products resemble those of products of fractional partial melting the more closely, the higher the degree of recharge of the magma chamber is and the more the solid/liquid distribution coefficients deviate from unity. If partial melting proceeds slowly in comparison to crystallization, concentration patterns in crystallization products resemble those of fractional crystallization products the more closely, the lower the degree of recharge is and the less the distribution coefficients deviate from unity. At low degree of solidification of the magma concentration patterns qualitatively correspond to fractional crystallization, while during the last stages of crystallization the distribution of minor element concentrations is preferentially controlled by partial melting. Application of the model allows a new understanding of the sequence mafic → sialic → mafic frequently observed in global, regional and local magmatic processes, particularly in the sequence of preorogenic, synorogenic and postorogenic magmatic rocks.