Reaction Between Ultramafic Rock and Fractionating Basaltic Magma I. Phase Relations, the Origin of Calc-alkaline Magma Series, and the Formation of Discordant Dunite

Abstract

This paper presents results of modelling reaction between peridotite and fractionating tholeiitic basalt in simple and complex silicate systems. Reactions are outlined in appropriate binary and ternary silicate systems. In these simple systems, the result of reactions between ‘basalt’ and ‘peridotite’ may be treated as a combination of Fe-Mg exchange and mass transfer reactions at constant Fe/Mg. Fe-Mg exchange in ternary and higher-order systems is nearly isenthalpic, and involves a slight decrease in magma mass at constant temperature. Mass transfer reactions, typically involving dissolution of orthopyroxene and consequent crystallization of olivine, are also nearly isenthalpic in ternary and higher-order silicate systems, and produce a slight increase in the magma mass at constant temperature. The combined reactions are essentially isenthalpic and produce a slight increase in magma mass under conditions of constant temperature or constant enthalpy.