Path of Fractional Differentiation of Basaltic Magmas and Fractionation Diagram

Abstract

Variation of chemical composition during magmatic differentiation is quantitatively calculated based upon assumptions of ideal solution and strong fractionation. The distribution coefficient k defined by [concentration in solidified rock/concentration in residual melt] is an essential parameter. The results are illustrated in a form of fractionation diagram which shows the variations of concentration versus solidification stage. (1) A model path for a simple crystallization differentiation of the Skaergaard-like magma is obtained and plotted in a FeO+Fe₂O₃-MgO-Na₂O+K₂O triangular diagram (Fig. 3). A model path for a contamination differentiation is also obtained in Fig. 4. (2) Based upon experimentally determined phase diagrams of FeO-Fe₂O3-MgO-SiO₂ system, a path for simple crystallization differentiation is traced. These model paths are useful to estimate physical conditions prevailing the differentiation of H. Kuno’s hyperthenic and pigeonitic rock series.