DISTRIBUTION OF TRACE ELEMENTS (REE, SR, BA, Y, TI, ZR, HF, NB, PB) BETWEEN DACITIC MELT, PLAGIOCLASE, ORTHOPYROXENE, AND CLINOPYROXENE: EVIDENCE FROM SILICATE MELT INCLUSIONS

Partitioning behavior between silicate melt, clinopyroxene, orthopyroxene, and plagioclase for ti, mn, sr, y, zr, nb, ba, la, nd, sm, eu, gd, dy, ho, yb, hf, and pb have been examined using laser ablation-inductively coupled plasma-mass spectrometry (la-icp-ms) of melt inclusions. Samples were selected from the 1988 eruption of white island, new zealand because petrographic evidence of the three mineral phases and the silicate melt suggests that all phases are in equilibrium. All three phenocrysts are found as mineral inclusions within each of the other phases, and are also often present in growth-zone assemblages with melt inclusions present as well. Major and trace element compositions of melt inclusions do not vary based upon the host mineral phase, suggesting that boundary layer effects are not important for these samples. Partition coefficients were calculated from the host and melt inclusion compositions and results were compared to the literature values for similar magma compositions. Most partition coefficients overlap those from the literature, but the white island data are consistently at the lower end of published values. Rapid crystal growth-rates, crystal zonation, or the presence of accessory mineral inclusions in phenocrysts might account for the elevated partition coefficients reported in the literature. This study strongly supports the melt inclusion-mineral (mim) technique as a viable method of determining partition coefficients.