Constraining the presence of amphibole and mica in metasomatized mantle sources through halogen partitioning experiments

Abstract

We present experimentally determined partition coefficients (Dmin/melt) for F and Cl between pargasite (amphibole), phlogopite (mica), clinopyroxene and basanitic melts. All experiments were performed at 1.4 GPa and temperatures between 1015 °C and 1250 °C to simulate fractionation of halogens during melting of metasomatized mantle. The water content of all starting materials was 16 wt% to promote crystallization of amphibole and mica. The halogen content of all starting mixtures was 0.45 wt% for F and 0.61 wt% for Cl. Our data show that F is compatible (D > 1) in pargasite and phlogopite, while Cl is strongly incompatible (D < 0.1) in both phases.The experimentally derived partition coefficients allow to quantitatively constrain the amounts of mica- and amphibole in metasomatized spinel-lherzolite sources from the observed halogen and rare earth element contents in derivative melts. Calculated F/Cl and F/Nd values of melts from different mantle sources, ranging from depleted to fertile and metasomatized mantle, are compared to those of melt inclusions from basalts from mid ocean ridges, ocean islands, and arc and intra-plate continental settings. Our results suggest that, depending on the tectonic setting, F/Cl and F/Nd of melt inclusions can be used to determine the presence and the amount of amphibole and mica in different mantle sources, and thus allow distinguishing between melting of nominally anhydrous and volatile-rich mantle sources.