Stabilization of S3O4 at high pressure: implications for the sulfur-excess paradox

The amount of sulfur in SO2 discharged in volcanic eruptions exceeds that available for degassing from the erupted magma. This geological conundrum, known as the sulfur excess, has been the subject of considerable interests but remains an open question. Here, in a systematic computational investigation of sulfur-oxygen compounds under pressure, a hitherto unknown S3O4 compound containing a mixture of sulfur oxidation states +II and +IV is predicted to be stable at pressures above 79 GPa. We speculate that S3O4 may be produced via redox reactions involving subducted S-bearing minerals (e.g., sulfates and sulfides) with iron and goethite under high-pressure conditions of the deep lower mantle, decomposing to SO2 and S at shallow depths. S3O4 may thus be a key intermediate in promoting decomposition of sulfates to release SO2, offering an alternative source of excess sulfur released during explosive eruptions. These findings provide a possible resolution of the excess sulfur degassing paradox and a viable mechanism for the exchange of S between Earth's surface and the lower mantle in the deep sulfur cycle. (c) 2022 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

Automated summary

A hitherto unknown sulfur-oxygen compound, S3O4, is found to be stable under high-pressure conditions. This compound is speculated to be produced via redox reactions involving subducted sulfur-bearing minerals and iron, and decomposes into sulfur dioxide and sulfur at shallow depths. These findings may provide a resolution to the excess sulfur degassing paradox during volcanic eruptions and offer a viable mechanism for sulfur exchange between Earth's surface and the lower mantle.