Fe²⁺–Mg fractionation between orthopyroxene and spinel: experimental calibration in the system FeO–MgO–Al₂O₃–Cr₂O₃–SiO₂, and applications

Abstract

We have determined the equilibrium Fe²⁺–Mg fractionation between orthopyroxene and spinel in the ferromagnesium system at 0.9–1.4 GPa, 850–1,250 °C, and also as a function of the Cr/Al ratio of spinel at 1.24 GPa, 1,000 °C. At each P–T condition, the equilibrium value of the distribution coefficient, K_D(Fe–Mg), was constrained by experiments with crystalline starting mixtures, and approaching from both higher and lower initial values. The experimental data have been cast, within a thermodynamic framework, in the form of a geothermometer in the system FeO–MgO–Al₂O₃–Cr₂O₃–SiO₂ (FMACrS). Using the data of O'Neill and Wall (1987) on the thermodynamic properties of Fe³⁺ and Ti⁴⁺ bearing spinels, we extended the thermometric formulation to account for the effect of these components. However, practical application of the extended formulation is beset with the problem of accurate determination of Fe³⁺ content of natural minerals. Using published data, the thermometric formulation in the FMACrS system has been applied to a number of natural assemblages that have small Fe³⁺ content. The retrieved temperatures are generally higher, on the average by ~60 °C, than those obtained from the olivine-spinel Fe²⁺–Mg exchange thermometer of O'Neill and Wall, as modified by Ballhaus et al. (1991), but are more compatible with the original temperature estimates by the authors of the publications. The smaller Fe²⁺–Mg interdiffusion coefficient, D(Fe–Mg), in orthopyroxene compared with those in both olivine and spinel is expected to yield higher temperatures from orthopyroxene–spinel than from olivine–spinel thermometry.