The oxidation state of sulfur in magmatic fluids

Sulfur compounds in volcanic gases are responsible for the global cooling after explosive eruptions and they probably controlled the early evolution of the Earth's atmosphere. We have therefore studied the oxidation state of sulfur in aqueous fluids under the pressure and temperature conditions and oxygen fugacities typical for magma chambers (0.5-3 kbar, 650-950 °C, Ni-NiO to Re-ReO2 buffer conditions). Sulfur speciation was determined by Raman spectroscopy of quenched fluids trapped as inclusions in quartz. Our results show that sulfur in hydrothermal fluids and volcanic gases is much more oxidized than previously thought and in particular, some explosive eruptions may release a significant fraction of sulfur as SO3 or its hydrated forms. In the pressure range from 500 to 2000 bar, the equilibrium constant K1 of the reaction 2H2S + 3O2 = 2SO2 + 2H2O in aqueous fluids can be described by lnK1 = −(57.1 ± 7.1) + (173,480 ± 7592)T− 1, where T is temperature in Kelvin. The equilibrium constant K2 for the reaction SO2 + ½O2 = SO3 in aqueous fluids, where SO3 may include hydrated forms, such as H2SO4, was found to be strongly pressure dependent, with lnK2 = −(5.2 ± 5.7) + (19,243 ± 5993)T− 1 at 1500 bar; lnK2 = −(11.1 ± 1.3) + (25,383 ± 1371)T− 1 at 2000 bar and lnK2 = −(22.1 ± 2.2) + (37,082 ± 2248)T− 1 at 2500 bar. Our data imply that volcanoes may directly inject hexavalent sulfur in the form of H2SO4 into the atmosphere, not only on Earth, but possibly also on Venus and on Mars, when it was still tectonically active. Remote measurements from satellites may have underestimated the sulfur yield of some recent eruptions. Moreover, the mechanisms of the interaction of volcanic gases with the stratosphere need to be reconsidered.

Research Highlights► Sulfur in volcanic gases is much more oxidized than previously thought. ► The sulfur yield from previous explosive eruptions has been underestimated. ► The mechanisms for the formation of sulfate aerosols need to be reconsidered. ► Sulfate on Mars may be the direct product of volcanic degassing.