Léa Courtonne’s PhD

Origins and isotopic compositions of sulfur in arc magmas and their link with their redox state

The redox state of magmas, defined by their oxygen fugacity (fO₂), strongly influences the behavior of elements, particularly sulfur. Arc magmas, which are more oxidized than mid-ocean ridge basalts, can dissolve up to 15,000 ppm of sulfur, compared to around 1,000 ppm in ridge basalts. This difference is due to their higher fO₂, but precisely determining the initial fO₂ remains challenging because of the interactions between redox evolution and sulfur degassing.

Arc magmas contain more sulfur than can be supplied by the depleted mantle, suggesting a contribution from the subducting slab, which is enriched in heavy sulfur isotopes (³⁴S). Two major hypotheses are being explored: First, sulfur degassing could alter the redox state of the magma, depending on the initial sulfur speciation (H₂S vs. SO₂) and pressure conditions. Second, unexpected isotopic disequilibria between sulfide and sulfate (up to 30‰) have been observed, far beyond theoretical equilibrium values. These disequilibria may be linked to late-stage processes, such as rapid cooling or late degassing, calling into question traditional interpretations of fO₂, particularly in silica-rich magmas.

This PhD project aims to clarify these questions by combining isotopic geochemical analyses of sulfur and advanced techniques like XANES spectroscopy. The goal is to better understand the links between redox state, degassing, and sulfur speciation, and to assess the impact of late-stage processes on magmatic signatures.