Systematics and energetics of trace-element partitioning between olivine and silicate melts: Implications for the nature of mineral/melt partitioning

Abstract

Olivines are structurally simple and provide an ideal phase with which to examine what controls the variation in partition coefficients between different elements. Electron probe analysis and secondary ion mass spectrometry have yielded high-precision in situ measurements of partition coefficients for alkaline earths, transition metals and rare-earth elements for olivine-glass pairs produced in experiments. Partition coefficients for these elements range from <10 −5 to >1. The variation in divalent partition coefficients can be accurately modelled in terms of the strain energy associated with the expansion of the oxygen octahedra to accommodate a large cation, calculated using the olivine bulk modulus taken from the literature. No deviations from Henry's law are observed for trivalent cations between concentrations of 0.05 ppm to >1%, thus local charge balance for the partitioning of trivalent cations into olivine is maintained by a coupled REE, Mg −1-Al,Si −1 substitution rather than by the creation of vacancies or interstitials. The bulk modulus required to model the trivalent cations is much larger than that for the divalent cations and probably reflects the local decrease in compressibility of the oxygen lattice near sites where Si has been replaced by the larger Al ion. The ability to calculate the partition coefficients for these elements demonstrates that the substitution mechanism for the very incompatible cations is indistinguishable from that for Mg or Cr, and is now well understood. The partition coefficients between clinopyroxene, orthopyroxene, garnet or amphibole and silicate melts exhibit similar dependencies on ionic radius and charge. This similarity suggests that mineral/melt partitioning generally occurs by substitution onto crystallographic sites in crystalline phases and that the partitioning of trivalent cations is charge balanced by a coupled substitution of Al for Si.