Amphibole-silicate melt halogen partition coefficients: cryptic amphibole fractionation and chlorine to fluorine ratios in arc magmas

Abstract

In this study 14 experiments were performed in a cold seal pressure vessel at conditions of 850–975 °C and 150–200 MPa to understand Cl and F partitioning between amphibole and dacitic to rhyolitic melts in arc settings. The starting compositions for the experiments were chosen to replicate basaltic andesite samples taken from Shiveluch volcano and a more evolved andesite. We find that F is moderately compatible in amphibole for crustal conditions, with D_F^amph/melt = 4.5–11, and Cl is incompatible, with D_Cl^amph/melt = 0.11–0.25. This adds to the growing experimental database for halogen partitioning between amphibole and melt, but some incongruity remains between mantle condition and crustal condition studies. We performed forward crystallization modeling on the effects of amphibole crystallization on arc volcanic halogen contents and find that the high Cl/F ratios observed in some olivine hosted melt inclusions at arcs may be explainable by the uptake of F into amphibole, providing further evidence for cryptic amphibole crystallization.