HR: 0800h
AN: V41D-0795    [Abstracts]
TI: Sulfur Speciation and Oxygen Fugacity in Primitive Magmas From the Trans-Mexican Volcanic Belt
AU: * Vigouroux, N
EM: nvigouro@sfu.ca
AF: Dept. of Earth Sciences, Simon Fraser University 8888 University Drive, Burnaby, BC V5A 1S6, Canada
AU: Wallace, P J
EM: pwallace@uoregon.edu
AF: Dept. of Geological Sciences, 1273 University of Oregon, Eugene, OR 97403, United States
AU: Johnson, E R
EM: ejohns10@uoregon.edu
AF: Dept. of Geological Sciences, 1273 University of Oregon, Eugene, OR 97403, United States
AB: Sulfur dissolves in silicate melts in both reduced (S^-2) and oxidized (S⁺⁶) forms, and the ratio of species depends on the oxygen fugacity of the melt. The more oxidized the magma, the more sulfur is disolved as sulfate and the higher the overall S solubility (e.g., Luhr, 1990). We have measured the speciation of S in olivine- hosted melt inclusions from 11 mafic cinder cones and 1 maar in the Trans-Mexican Volcanic Belt using the S Kα wavelength shift method (Carroll and Rutherford, 1988). Four of the cinder cones are from the Colima Graben and are high-Mg silica-undersaturated potassic rocks. The other cinder cones are from the Michoacan- Guanajuato Volcanic Field (MGVF) and include calc-alkaline high-Mg basalts and basaltic andesites. The maar, also in the MGVF, erupted alkali basalt. A basaltic andesite from the Colima Graben was also analyzed due to its transitional chemical composition between the potassic magmas and the MGVF calc-alkaline magmas. For the calc-alkaline melt inclusions, S⁺⁶ accounts for 65-90% of total dissolved S. Similarly, the potassic and basaltic andesite melt inclusions have 60-90% and 78-94% of total S as S⁺⁶ respectively. In contrast the alkali basalt melt inclusions have lower values, with 50-60% of total S dissolved as S⁺⁶. Some cinder cones exhibit nearly the entire range of melt S speciation whereas others have restricted ranges, but there does not appear to be a correlation between melt degassing (based on H₂O contents of the melt inclusions) and either oxygen fugacity or S speciation. Based on the average value of melt inclusions from each cone and the relationship of S speciation to oxygen fugacity (Wallace and Carmichael, 1994), the calc-alkaline cinder cones, including the basaltic andesite, have oxygen fugacity values of NNO+0.9 to +1.3. The potassic cinder cones have similar values of NNO+0.9 to +1.3. The alkali basalt averages NNO+0.5. The values have 1 standard deviation uncertainties of ±0.2 log units for the calc-alkaline basalts, ±0.4 for the potassic cones, ±0.5 for the basaltic andesite and ±0.08 for the alkali basalt, which emphasizes the distinction of the alkali basalt with respect to the other magma types. The potassic and calc-alkaline magmas appear to have similar oxygen fugacities that are distinctly higher than the alkali basalt. This distinction cannot be explained by compositional differences but appears to be related to the mantle source of the magmas and the presence or absence of a subduction derived H₂O-rich component.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting