Partitioning of Cl and F between fluid and hydrous phonolitic melt of Mt. Vesuvius at ~ 850–1000 °C and 200 MPa

The solubilities of Cl- and/or F-bearing fluids in phonolitic melts of Vesuvius volcano have been determined experimentally at temperatures of 845–865 °C and 1000 °C, and pressure of 200 MPa. At 845–865 °C, the maximum Cl concentration in the melt (mCl) coexisting with Cl-rich (~ 35.5 wt.%; flCl) and F-free fluid is about 0.5 wt.%. An addition of F to the system increases the Cl content of the melt up to 0.7 wt.% at ~ 39 wt.% Cl and ~ 13 wt.% F in the fluid(s). The relationship between mCl and flCl indicates that low-density vapor and dense brine phases might have coexisted in some experiments. The relationship between mCl and flCl at 1000 °C is similar to that determined at 845–865 °C, implying that variations of temperature in the range 850–1000 °C have no significant effect on the partitioning of Cl between fluid and phonolitic melt.The concentration of F in phonolitic melt coexisting with F-rich fluid(s) reaches at least 0.9 wt.% at 845–865 °C and 1.3 wt.% at 1000 °C. The comparison of data from experiments conducted in Au and Pt capsules indicates that F may react with Au, which influences the quality of the experimental results. The partition coefficients of F between bulk fluid and melt (fl/mDF) are higher than 1 in all experiments, indicating that fluorine is preferentially incorporated into the fluid phase in phonolitic systems, in contrast to fl/mDF values determined in rhyolitic systems. The influence of Cl on fl/mDF is not detectable at the conditions of our experiments. The experimental data imply that Mt. Somma–Vesuvius phonolitic magmas from the eruptions between A.D. 79 and A.D. 1944 may have coexisted with Cl-rich and F-rich fluid phase(s) prior to eruption.