SYSTEMATICS AND ENERGETICS OF TRACE-ELEMENT PARTITIONING BETWEEN OLIVINE AND SILICATE MELTS - IMPLICATIONS FOR THE NATURE OF MINERAL MELT PARTITIONING

Olivines are structurally simple and provide an ideal phase with which to examine what controls the variation in partition coefficients betw een different elements. Electron probe analysis and secondary ion mass spectrometry have yielded high-precision in situ measurements of part ition coefficients for alkaline earths, transition metals and rare-ear th elements for olivine-glass pairs produced in experiments. Partition coefficients for these elements range from <10(-5) to >1. The variati on in divalent partition coefficients can be accurately modelled in te rms of the strain energy associated with the expansion of the oxygen o ctahedra to accommodate a large cation, calculated using the olivine b ulk 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 ca tions into olivine is maintained by a coupled REE, Mg-1-Al,Si-1 substi tution rather than by the creation of vacancies or interstitials. The bulk modulus required to model the trivalent cations is much larger th an that for the divalent cations and probably reflects the local decre ase in compressibility of the oxygen lattice near sites where Si has b een replaced by the larger Al ion. The ability to calculate the partit ion 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 coeffici ents between clinopyroxene, orthopyroxene, garnet or amphibole and sil icate melts exhibit similar dependencies on ionic radius and charge. T his similarity suggests that mineral/melt partitioning generally occur s by substitution onto crystallographic sites in crystalline phases an d that the partitioning of trivalent cations is charge balanced by a c oupled substitution of Al for Si.