A Revised Picture Of Deep-Earth Sulfur

A study of sulfur in fluids at high temperatures and pressures suggests that this geochemically ubiquitous element may exist deep in Earth in the form S₃ ^–, rather than the presumed sulfide (H₂S, HS^–, and S ^2–) and sulfate (HSO₄ ^– and SO₄ ^2–) species (Science, DOI: 10.1126/science.1199911). If true, this finding would change the picture of sulfur-rich fluid chemistry that drives the migration of elements such as gold, copper, and platinum from deep fluids to the surface to form ore deposits. It also implies that sulfur isotopic fractionation models used to date geologic samples, which are based on the assumption that sulfides and sulfates are the dominant species, may need to be revised. Gleb S. Pokrovski of France’s Paul Sabatier University and Leonid S. Dubrovinsky of Germany’s University of Bayreuth used a diamond anvil cell to subject sulfur-water solutions to a range of temperature and pressure conditions—up to 450 °C and 3.5 gigapascals. They found that at moderate temperatures sulfides and sulfates do predominate. However, at higher temperatures, like those encountered in Earth’s magma, S₃ ^– emerges as the dominant species.