Isabelle Genot’s PhD

The topic of my PhD relates to the origins and budgets of sulfur and water in the mantle wedge. I try to characterize sulfur sources coming from the slab and upper mantle in analyzing the isotopic composition (³²S, ³³S, ³⁴S, ³⁶S) of back-arc basin basalts. However, magmatic processes (e.g. degassing, sulfide segregation, hydrothermal sulfide assimilation) can affect sources sulfur isotopic composition and make glasses composition not representative of the primitive mantle composition. Water content and D/H ratio of basalts allow us to know if glasses lost water and potentially sulfur by degassing. Sulfide saturation following by segregation in silicated liquid where there are both reduced and oxidized sulfur can also lead to an evolution of liquid sulfur isotopic composition. Geochemical proxies can then be used to identify samples not affected by shallow processes. Unlike arc, back-arc basin basalts are sub-marine and hence less degassed. They also are less evolved, thus limiting sulfide saturation and segregation. However, sulfur can be dissolved under both reduced and oxidized forms, which has consequences, among others, on its solubility in the melt. As it is possible for now to chemically extract only S^2- and hence to analyze only reduced sulfur isotopic composition, an analytical part of my PhD concerns the development of a protocol enabling us to extract and analyze the isotopic composition of both S^2- and S⁶⁺.