HR: 10:55h
AN: V42B-03 INVITED     [Abstracts]
TI: Volatile Solubilities in Mt. Somma-Vesuvius Phonolite Melt and New Insights on Degassing of Sulfur, Chlorine, and Water
AU: * Webster, J D
EM: jdw@amnh.org
AF: Dept. of Earth and Planetary Sciences, AMNH, Central Park West at 79th St., New York, NY 10024, United States
AU: Sintoni, M F
EM: sintoni@amnh.org
AF: Dipartimento di Scienze della Terra, Univ. di Napoli Federico II, Via Mezzocannone 8, Napoli, 80134, Italy
AU: De Vivo, B
EM: bdevivo@unina.it
AF: Dipartimento di Scienze della Terra, Univ. di Napoli Federico II, Via Mezzocannone 8, Napoli, 80134, Italy
AU: Lima, A
EM: alima@unina.it
AF: Dipartimento di Scienze della Terra, Univ. di Napoli Federico II, Via Mezzocannone 8, Napoli, 80134, Italy
AB: To better understand volatile exsolution, degassing, and eruptive processes in subduction-related magmas, we have conducted thirty H2O plus S plus Cl solubility experiments with phonolite melt at 905 to 1000 deg. C, 200 MPa, and relatively oxidizing conditions. The experiments include an 8000-year old Mt. Somma-Vesuvius phonolite, distilled H2O, NaCl, KCl, and CaSO4, and they involve a new method of constraining the concentration of S in the run-product fluids. Unlike prior S-solubility experiments, the S concentration in fluid is determined as proportional to the mass loss of the anhydrite crystals in the starting charges of the experiments. This method provides accurate S contents of fluids. The H2O, Cl, and S concentrations of the phonolitic glasses of our experiments range from 4 to 8, 0.38 to 0.84, and 0.01 to 0.19 wt.%, respectively. Sulfur solubility increases with increasing CaO and FeO (total iron) in melt, decreasing Cl and K2O in melt, decreasing Cl in fluid(s), and with increasing oxygen fugacity values greater than NNO. Chlorine solubility in melt increases with decreasing S content of melt and decreasing S and H2O in the coexisting fluid(s). Water solubility in melt shows no systematic variation with melt composition, but varies strongly with the composition of fluids. The partition coefficients (wt.% of X in fluid[s]/wt.% of X in phonolitic melt) range from 40 to > 200 for S and from 12 to 87 for Cl. At pressure-temperature-oxygen fugacity conditions similar to those of this study, these partition coefficients are equivalent to those determined previously for natural equilibria involving andesite melt plus Cl-free, S-bearing aqueous fluid (Scaillet and Pichavant, 2003) and experimental equilibria with andesite melt plus S-free, Cl-bearing aqueous fluid (Webster et al., 1999), respectively. Our research also shows that these partition coefficients for S and Cl are inversely proportional to one another. Silicate melt inclusions in pyroxene phenocrysts from Mt. Somma-Vesuvius plinian and interplinian eruptive materials show strong positive correlations between S and Cl; Mt. Somma-Vesuvius magmas apparently contained from 0.01-0.3 wt.% S and 0.01 to 1 wt.% Cl. Our new partition coefficients are useful for interpreting the abundances of H2O, S, and Cl in Somma-Vesuvius magmas as they exsolved S- and Cl-enriched, saline fluids prior to eruption. (Scaillet, B., Pichavant, M., 2003, Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes. In: Volcanic Degassing. Oppenheimer, C., Pyle, D.M., and Barclay, J, (eds). Geol. Soc. Lond. Spec. Pubs. 213, pp. 23-52 AND Webster, J.D., Kinzler, R.J., and Mathez, E.A., 1999, Chloride and water solubility in basalt and andesite liquids and implications for magmatic degassing. Geochim. Cosmochim. Acta 63, 729-738)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8411 Thermodynamics (0766, 1011, 3611)
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly