Effect of fluorine on near-liquidus phase equilibria of an Fe–Mg rich basalt

Abstract

Volatile species (H2O, CO2, F, Cl, etc.) have important effects on the formation and crystallization history of basaltic magmas. Here, we have experimentally investigated the effects of F on phase equilibria of Fe–Mg-rich basalt. Our results show that fluorine has large effects on the liquidus temperature and the chemistry of crystallizing minerals. Compared to the F-free system, addition of ~2wt.% F moves the olivine-pigeonite liquidus point down ~2kbar and 95°C (from 12kbar, 1375°C to 10kbar, 1280°C). With increasing fluorine concentrations, KdMineral-MeltFe−Mg dramatically increases for both pyroxene and olivine, suggesting that fluorine in basaltic magmas complexes primarily with MgO. Complexing with MgO in the melt decreases its MgO activity, and forces the crystallizing minerals to greater Fe/Mg, and so increases KdMineral-MeltFe−Mg. Models of basalt generation, where the magma is fluorine-rich, need to include the effect of not only water but fluorine on liquidus depression and minerals crystallizing/melting. Our results suggest that fluorine may significantly aid in the petrogenesis of silica-poor, alkali-rich magmas in the Earth and Mars.