The partitioning behavior of Cl, S, and H 2O in aqueous vapor- ± saline-liquid saturated phonolitic and trachytic melts at 200 MPa

Abstract

Hydrothermal experiments were conducted with molten Mt. Somma–Vesuvius phonolite and mixed-composition fluids comprised of O–H–Cl–S-cations to determine the solubilities and melt-fluid(s) partitioning behavior of the dominant volatile components at 896–1022 °C, 200 MPa, and f O2 of NNO +0.54 to NNO + 1.6. The final melt compositions ranged from phonolitic to trachytic due to component exchange between the melt and fluid(s) and limited crystallization of plagioclase and iron–titanium oxides. The Cl concentration of the fluids was measured directly and confirmed by mass-balance calculation, and the S concentration of the fluids was determined by mass-balance computations involving a new technique based on the mass loss of CaSO 4 crystals which served as the primary S source in the starting experimental charges. These experiments determine the apparent molal solubility of CaSO 4 in NaCl-, KCl-, and CaCl 2-enriched fluids to be: [ m C a S O 4 = 71.52 ( X ( N a , K ) C l C a S O 4 − freefluids ) − 1.61 ] at 200 MPa and ca . 920 °C . The concentration of Cl in these melts increases with temperature and the Cl, CaO, and FeO concentrations of the system; Cl concentration decreases with increasing S in the system. The fluid(s)/melt partition coefficients for Cl, D Cl, range from ca. 3–50 and vary with the Cl S, and FeO concentrations of the system and the molar (Na 2O/Na 2O + K 2O) of the melt. The concentration of S in the melts increases with temperature, the S content of the system, the CaO content of the melt, the (S 6+/S total) melt, and with f O2. The fluid/melt partition coefficients for S, D S, range from 2 to nearly 1100, but most range from 50–300. D S increases with increasing S in the system just as D Cl increases with increasing Cl in the system, and D S increases with the Larson Index of the melt. The concentration of H 2O in these melts increases significantly with increasing Cl, S, and cations in the fluids. These new partition coefficients have been integrated with the results of Raleigh fractionation modeling of Mt. Somma–Vesuvius magmas [that underwent plinian to subplinian eruptions in ca. 3550 a.B.P. (Avellino), AD 79 (Pompeii), and AD 472 (Pollena)] to compute the compositions of magmatic fluids exsolved at 200 MPa. The magmas range from phonotephritic to phonolitic compositions. The modeled Somma–Vesuvius fluids contain from 3 to ca. 50 wt.% Cl and 4–17 wt.% S with 1 to 4 wt.% fluid(s) in the evolving magmas. These concentrations vary strongly with the bulk mass of fluid(s) exsolved and the stage of magma evolution as expressed by the Larson Index. The mass (S/Cl) ratio of these fluids was calculated for 1 wt.% fluid(s) in the magmas, and it ranges from ca. 0.4–4.7 for these modeled conditions. These modeled values are strikingly consistent with the (S/Cl) of gases from numerous subduction-related volcanoes.