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

The amount of sulfur in SO 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 SO compound containing a mixture of sulfur oxidation states +II and +IV is predicted to be stable at pressures above 79 GPa. We speculate that SO 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 SO and S at shallow depths. SO may thus be a key intermediate in promoting decomposition of sulfates to release SO, 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.