Zoning Pattern of Extremely Fractionated Plagioclase Crystallized from Simple System

Abstract

Under the condition of the maximum fractionation, chemical variations were numerically calculated for zoned plagioclase grains crystallized from liquids with various initial compositions in the Ab-An simple system. The thermodynamic data for calculating compositions of coexisting liquid and crystal at a given temperature were obtained from the phase diagrams of Bowen (1913), Lindsley (1966), and Yoder et. al. (1957). The zoning patterns thus obtained are shown graphically in Figs. 5 and 6, in which a radius of crystal grain is represented by the cube root of the value of percentage solidified. Grains crystallized from initially An-rich liquids have a narrow rim with considerable compositional variation arround a large and nearly homogeneous core, while those from Ab-rich liquids gradually change the composition from center to rim.