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

We investigate the relationship between sulfur and oxygen fugacity at Erta Ale and Masaya volcanoes. Oxygen fugacity was assessed utilizing Fe 3+ /∑Fe and major element compositions measured in olivine‐hosted melt inclusions and matrix glasses. Erta Ale melts have Fe 3+ /∑Fe of 0.15–0.16, reflecting f O 2 of ΔQFM 0.0 ± 0.3, which is indistinguishable from f O 2 calculated from CO 2 /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 34 S gas − 0.5‰; δ 34 S scoria + 0.9‰) and Masaya (δ 34 S gas + 4.8‰; δ 34 S scoria + 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 6+ /∑S is <0.07 in Erta Ale melt inclusions compared to >0.67 in Masaya melt inclusions. No change is observed in Fe 3+ /∑Fe or S 6+ /∑S with extent of S degassing at Erta Ale, indicating negligible effect on f O 2 , and further suggesting that H 2 S is the dominant gas species exsolved from the S 2− ‐rich melt (i.e., no redistribution of electrons). High SO 2 /H 2 S observed in Erta Ale gas emissions is due to gas re‐equilibration at low pressure and fixed f O 2. 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 34 S oxidized sulfur through the subduction zone.