HORNBLENDE-MELT TRACE-ELEMENT PARTITIONING MEASURED BY ION MICROPROBE

Trace-element abundances were measured in situ by ion microprobe in fi ve samples of hornblende and melt ranging from basaltic andesite to hi gh-silica rhyolite. Except for one sample, for which quench overgrowth or disequilibrium is suspected, the abundance ratios show systematic inter-element and inter-sample variations, and probably approach true partition coefficients. Apparent partition coefficients are reported f or La, Ce, Nd, Sm, Dy, Er, Yb, Sc, Ti, V, Cr, Sr, Y and Zr. Rare-earth elements (REE) and Y form smooth convex-upward partitioning patterns that rise to higher D-values and become increasingly convex in more ev olved samples. Apparent partition coefficients for REE, Y, Ti, V and C r can be parametrized as functions of the distribution of Ca between h ornblende and melt, giving expressions to predict hornblende-melt trac e-element partitioning values. These expressions are used to show that heavy REE-enriched hornblende/whole-rock REE abundance patterns in gr anitoids may result from partial re-equilibrium of hornblende and late -stage residual liquids rather than from anomalous partitioning values .