MODELING THE MAJOR-ELEMENT EVOLUTION OF OLIVINE-HOSTED MELT INCLUSIONS

Abstract

This paper presents a detailed description of an approach for modeling the major element evolution of olivine-hosted melt inclusions. Numerical simulations constrained by experimental data on olivine/melt equilibrium and interdiffusion rates of Fe and Mg in olivine quantify the post-entrapment processes (crystallization, dissolution, Fe–Mg exchange with the host) that influence the major element compositions of included melts. Equilibrium at the olivine/melt interface is described by expressions for FeO and MgO partitioning calibrated using olivine–liquid pairs from the literature spanning temperatures of 1064 to 1950 °C and pressures of 1 bar to 70 kbar. The liquidus temperature and equilibrium olivine composition are calculated simultaneously for melt of a given composition using the partitioning expressions and olivine stoichiometry. Diffusion in the host olivine is modeled by the explicit finite-difference method. The main inputs for a simulation are the initial composition and size of the melt inclusion, the size of the host olivine, and the desired cooling path expressed as a constant cooling rate. The rate of olivine crystallization (dF/dT) within an inclusion is dynamically adjusted with each time step so that the observed Fe–Mg exchange coefficient (KDFe–Mg) at the inclusion/host interface matches the equilibrium value predicted by the partitioning equations. Application of the approach to model the major element evolution of an inclusion formed within the melting regime beneath an oceanic spreading center indicates that the composition of the included melt is significantly modified during transport to the surface along the mantle geotherm. Further, the compositional path followed by the included melt is dependent upon the capacity of the host olivine to maintain the inclusion at the pressure of entrapment.