  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.
