Partitioning coefficients between olivine and silicate melts

Abstract

Variation of Nernst partition coefficients ( D) between olivine and silicate melts cannot be neglected when modeling partial melting and fractional crystallization. Published natural and experimental olivine/liquid D data were examined for covariation with pressure, temperature, olivine forsterite content, and melt SiO 2, H 2O, MgO and MgO/MgO + FeO total. Values of olivine/liquid D generally increase with decreasing temperature and melt MgO content, and with increasing melt SiO 2 content, but generally show poor correlations with other variables. Multi-element olivine/liquid D profiles calculated from regressions of D REE–Sc–Y vs. melt MgO content are compared to results of the Lattice Strain Model to link melt MgO and: D 0 (the strain compensated partition coefficient), E M 3+ (Young's Modulus), and r 0 (the size of the M site). Ln D 0 varies linearly with Ln MgO in the melt; E M 3+ varies linearly with melt MgO, with a dog-leg at ca. 1.5% MgO; and r 0 remains constant at 0.807 Å. These equations are then used to calculate olivine/liquid D for these elements using the Lattice Strain Model. These empirical parameterizations of olivine/liquid D variations yield results comparable to experimental or natural partitioning data, and can easily be integrated into existing trace element modeling algorithms. The olivine/liquid D data suggest that basaltic melts in equilibrium with pure olivine may acquire small negative Ta–Hf–Zr–Ti anomalies, but that negative Nb anomalies are unlikely to develop. Misfits between results of the Lattice Strain Model and most light rare earth and large ion lithophile partitioning data suggest that kinetic effects may limit the lower value of D for extremely incompatible elements in natural situations characterized by high cooling/crystallization rates.