Sulfur is an important element in igneous systems due to its impact on magma redox, its role in the formation of economically valuable ore deposits, and the influence of catastrophic volcanogenic sulfur degassing on global climate. 

The mobility and geochemical behavior of sulfur in magmas is complex due to its multi-valent (from S^2- to S⁶⁺) and multi-phase (solid, immiscible liquid, gaseous, dissolved ions) nature. Sulfur behavior is closely linked with the evolution of oxygen fugacity (fO₂) in magmas; the record of fO₂ evolution is often difficult to extract from rock records, particularly for intrusive systems that undergo cyclical magmatic processes and crystallize to the solidus. 

We apply a novel method of measuring sulfur isotopic ratios via secondary ion mass spectrometry (SIMS) in zoned apatite crystals that we interpret as a record of open-system magmatic processes. 

These findings have implications for the coupled sulfur and fO₂ evolution of granitic plutons and suggest that the in-situ measurement of sulfur isotopic ratios in apatite is a powerful new tool for evaluating coupled redox and sulfur behavior in a wide range of terrestrial and extraterrestrial magmatic systems.