New and larger sanidine/melt partition coefficients for Ba and Sr as determined by ion microprobe analyses of melt inclusions and their sanidine host crystals

Six melt inclusions and their sanidine host crystals from the Bishop Tuff were analyzed by ion microprobe for the concentrations of Ba and Sr. Host crystals were analyzed 30 [mu]m away from the melt inclusions. The analyses yield an average sanidine/melt-inclusion partition coefficient for Ba of 28.4 [+-] 4.1 and for Sr of 11.35 [+-] 0.74. These values are significantly larger than those produced by analyses of mineral separates and matrix glasses, which largely fall between 4 and 7 for Ba and 1 and 4 for Sr. The new results are further confirmed by the concentration ratios of these two elements in the rims of the late Bishop sanidine phenocrysts and late Bishop matrix glasses. The new findings are considered closer to the real phenocryst/metal partition coefficients than those produced by bulk analyses of crystal separates and matrix glass, because ion microprobe analysis eliminates or at least greatly reduces the error introduced by the existence of chemical zonations and impurities. Melt inclusions are better representatives of the melt in equilibrium with the phenocrysts than the matrix glass, because matrix glass is subject to various chemical changes after formation of the phenocrysts. Modifications of chemical concentrations in melt inclusions by boundarymore » layer buildup during melt inclusion formation has been shown to be negligible for most elements. These new and considerably larger sanidine/melt partition coefficients for Ba and Sr shed new light on the debate over origin of high-silica rhyolites with extremely low concentrations of Ba and Sr. Using these new partition coefficients, only about 40% fractional crystallization is required to produce a rhyolitic magma with 2 ppm Ba from a magma that originally had 2,000 ppm Ba.« less