Sulfur degassing at Erta Ale (Ethiopia) and Masaya (Nicaragua) volcanoes: Implications for degassing processes and oxygen fugacities of basaltic systems. (2nd October 2013)

• Record Type: Journal Article
• Authors: de, J. M.; Fischer, T. P.; Sharp, Z. D.; King, P. L.; Wilke, M.; Botcharnikov, R. E.; Cottrell, E.; Zelenski, M.; Marty, B.; Klimm, K.; Rivard, C.; Ayalew, D.; Ramirez, C.; Kelley, K. A.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 14:Number 10(2013)
• Page Start: 4076
• Page End: 4108
• Publication Date: 2013-10-02
• Subjects: Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/ggge.20255
• Languages: English
• ISSNs: 1525-2027

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <p>[1] We investigate the relationship between sulfur and oxygen fugacity at Erta Ale and Masaya volcanoes. Oxygen fugacity was assessed utilizing Fe<sup>3+</sup>/∑Fe and major element compositions measured in olivine‐hosted melt inclusions and matrix glasses. Erta Ale melts have Fe<sup>3+</sup>/∑Fe of 0.15–0.16, reflecting <italic>f</italic>O<sub>2</sub> of ΔQFM 0.0 ± 0.3, which is indistinguishable from <italic>f</italic>O<sub>2</sub> calculated from CO<sub>2</sub>/CO ratios in high‐temperature gases. Masaya is more oxidized at ΔQFM +1.7 ± 0.4, typical of arc settings. Sulfur isotope compositions of gases and scoria at Erta Ale (δ<sup>34</sup>S<sub>gas</sub> − 0.5‰; δ<sup>34</sup>S<sub>scoria</sub> + 0.9‰) and Masaya (δ<sup>34</sup>S<sub>gas</sub> + 4.8‰; δ<sup>34</sup>S<sub>scoria</sub> + 7.4‰) reflect distinct sulfur sources, as well as isotopic fractionation during degassing (equilibrium and kinetic fractionation effects). Sulfur speciation in melts plays an important role in isotope fractionation during degassing and S<sup>6+</sup>/∑S is <0.07 in Erta Ale melt inclusions compared to >0.67 in Masaya melt inclusions. No change is observed in Fe<sup>3+</sup>/∑Fe or S<sup>6+</sup>/∑S with extent of S degassing at Erta Ale, indicating negligible effect on <italic>f</italic>O<sub>2</sub>, and further suggesting that H<sub>2</sub>S is the dominant gas species exsolved from the S<sup>2−</sup>‐rich melt (i.e., no redistribution of electrons). High SO<sub>2</sub>/H<sub>2</sub>S observed in Erta Ale gas emissions is due to gas re‐equilibration at low pressure and fixed <italic>f</italic>O<sub>2.</sub> Sulfur budget considerations indicate that the majority of S injected into the systems is emitted as gas, which is therefore representative of the magmatic S isotope composition. The composition of the Masaya gas plume (+4.8‰) cannot be explained by fractionation effects but rather reflects recycling of high δ<sup>34</sup>S oxidized sulfur through the subduction zone.</p> </abstract>