HR: 14:00h
AN: V33F-02 INVITED    [Abstracts]
TI: Controls on the Solubility of Sulfur in Anhydrous and Hydrous Silicate Melts Saturated with Sulfide Liquid, Pyrrhotite or Anhydrite: Empirical Models and Applications
AU: * Li, C
EM: cli@indiana.edu
AF: Department of Geological Sciences, Indiana University, 1001 East Tenth Street, Bloomington, IN 47405, United States
AU: Ripley, E M
EM: ripley@indiana.edu
AF: Department of Geological Sciences, Indiana University, 1001 East Tenth Street, Bloomington, IN 47405, United States
AB: The solubility of S in silicate melts depends on P, T, composition and oxidation state. Available experimental results indicate that under reducing conditions S is mainly dissolved as S2- whereas under oxidizing conditions S is mainly dissolved as S6+ in silicate melts. Under reducing conditions excess S in Fe-bearing silicate melts occurs as sulfide liquid at high T and as pyrrhotite at low T. In Ca-bearing systems under oxidizing conditions excess S is present as anhydrite. At sulfide saturation the composition of silicate melt is controlled by fO2/fS2 and the volume of sulfide liquid. Such interdependency allows the construction of an empirical equation that relates S solubility to P, T and silicate melt composition using available experimental results of sulfide-saturated systems. This empirical equation can be used as a yardstick to measure deviation of S solubility in anhydrite-saturated systems and hydrous systems saturated with pyrrhotite or anhydrite. A comparison between the predicted values from the empirical equation and the results of available experiments reveals that S solubility in anhydrite- saturated, anhydrous silicate melts is one order of magnitude higher than that in sulfide-saturated silicate melts. Such a large difference is also present in hydrous systems. The solubility of S in hydrous melts saturated with pyrrhotite may be as much as 3 times the predicted values. In contrast, the solubility of S in hydrous melts saturated with anhydrite is one order of magnitude higher than the predicted value. The experiments of Luhr (1990) indicate that the positive deviation of S solubility in H2O-saturated dacitic magma is positively correlated with CaO and H2O concentrations when anhydrite is present and negatively correlated with H2O concentration when pyrrhotite is present. The empirical equation, when considered in conjunction with the effects of oxidation and hydration on S solubility, can be used to evaluate the consequences of magma differentiation. The fate of chalcophile elements during magma degassing can be evaluated based on available experimental results. Regardless of whether or not magma is saturated with sulfide or anhydrite, degassing is an effective mechanism to transfer S from magma to a vapor because of the high vapor/magma D value for S. However, mass transfer of chalcophile elements from magma to a vapor during degassing is more complicated. If magma is not saturated with sulfide liquid, degassing can rapidly transfer chalcophile elements such as Os, Pd, Au and Cu from magma to a vapor because these elements partition more strongly into vapor than magma. Degassing is unlikely to be an effective mechanism to transfer chalcophile elements from sulfide liquid to a vapor because these elements favor sulfide liquid over a vapor. In pyrrhotite-saturated systems Cu and Au may be transferred from magma to a vapor if the vapor/pyrrhotite ratio is high. The transfer of chalcophile elements from magma to a vapor is possible in anhydrite-saturated systems because anhydrite does not concentrate PGE, Au and Cu from coexisting magma.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting