Stabilization of S3O4 at High Pressure-Implications for the Sulfur Excess Paradox

TL;DR

This study predicts a new sulfur-oxygen compound, S3O4, stable at high pressures, which could explain the excess sulfur observed in volcanic eruptions and impact the understanding of Earth's deep sulfur cycle.

Contribution

The paper introduces a previously unknown high-pressure sulfur compound, S3O4, and proposes its role in sulfur cycling and volcanic sulfur emissions.

Findings

S3O4 is stable above 79 GPa.

S3O4 can decompose into SO2 and S at shallow depths.

S3O4 offers a mechanism for sulfur excess in eruptions.

Abstract

The geological conundrum of sulfur excess refers to the finding that predicted amounts of sulfur, in the form of SO2, discharged in volcanic eruptions much exceeds the sulfur available for degassing from the erupted magma. Exploring the source of the excess sulfur has been the subject of considerable interest. Here, from 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 emerges and is predicted to be stabilized above a pressure of 79 GPa. We predict that S3O4 can be produced via multiple redox reactions involving subducted S-bearing minerals (e.g., sulfates and sulfides) at high pressure conditions relevant to the deep lower mantle, and conversely be decomposed into SO2 and S at shallow depths of Earth. Therefore, S3O4 can be considered as a key intermediate compound to promote the decomposition of sulfates to release SO2, which offers an alternative source of the excess sulfur released during explosive eruptions. These findings provide a possible resolution to the geological paradox of excess sulfur degassing and a viable mechanism for the understanding of S exchange between surface and the lower mantle for the deep sulfur cycle.