Olivine-Liquid Equilibria: Temperature, Pressure and Composition Dependence of the Crystal/Liquid Cation Partition Coefficients for Mg, Fe2+, Ca and Mn

Abstract

From the results of 747 experiments on dry basic and ultrabasic magmas and related synthetic systems equations have been derived which predict the olivine/liquid cation partition coefficients for Mg (correlation coefficient r = 0·996), Fe2+ (r = 0·993), Ca (r = 0·79) and Mn (r = 0·70) as a function of temperature, pressure and liquid composition. The ratio of the partition coefficients for Mg and Fe2+ (KD) varies from 0·25 to 0·38. For any given magma composition liquidus olivines are slightly more iron-rich at high pressures than they would be at low pressures. The Mg and Fe2+ partition coefficient equations may be used as geothermometers which are accurate to better than ±1 per cent if pressures can be estimated by independent methods. Minor element partition thermometers (Ca and Mn) are too sensitive to analytical errors, or to departures from equilibrium, to prove reliable. Temperatures may also be obtained from a geothermometer based on the concept of olivine saturation. This is independent of olivine composition and can be used where these is evidence of disequilibrium between olivine and host liquid. In such a situation the errors indicated by the Mg and Fe2+ partition thermometers are asymmetric about the true temperature except when the equilibrium olivine composition is Fo50. These geothermometers are sufficiently sensitive to demonstrate magma mixing. The partition coefficient equations may be used to model equilibrium crystallization of olivine even though the partition coefficients, and the ratio of the partition coefficients (KD). are continuously changing. Fractional crystallization of olivine may be approximated by sequential removal of small amounts of olivine formed by equilibrium crystallization.