Model for the Closed-System Fractionation of a Dike Formed by Two Pulses of Dolerite Magma

Abstract

Elemental variations along a profile perpendicular to strike of an exceptionally wide (342-m) dolerite dike near Pageland, South Carolina, USA, were studied in order to determine the crystallization history of the dike. Although the modes show no systematic variation across the dike, the elemental concentrations generally exhibit overall trends about the approximate center of the dike with several superimposed subsidiary anomalies. The initial changes in concentration of the elements in the outer two meters of the dike margins are thought to represent a gradation from the chilled magma to the first differentiate. The other subsidiary inflections are interpreted as representing mixing of two magma pulses. A closed-system fractionation model for each pulse of magma can be tested by assuming the chemical composition of the final solid to be that of the final liquid and determining the composition of the “solids” (including entrapped magma) at various percentages of crystallization. The liquid line of descent can be back-calculated until the original magma composition is determined — simply a weighted average composition for that pulse. The calculated initial composition of each pulse and the composition of rocks from the dike margins compare quite favorably, indicating that the dike is the result of two magma pulses with essentially the same composition that have each undergone crystal fractionation as a closed system, with minor mixing at the boundaries between the two. It appears that the dike composition at the present level of erosion is representative of the overall composition.