Résumé
Oxygen fugacity (fO (sub 2) ) of magmatic and hydrothermal systems influences, for instance, crystallization and degassing processes as well as metal solubilities in melts and fluids. Apatite is a ubiquitous mineral in magmatic and hydrothermal environments that can record and preserve volatile zonation. It can contain several thousand mu g/g of the redox sensitive element sulfur (S), making S-in-apatite a potential fO (sub 2) sensor. Despite the polyvalent properties of S (e.g., S (super 2-) , S (super 4+) , S (super 6+) ), the oxidation state and incorporation mechanisms of S in the apatite structure are poorly understood. In this study, the oxidation state of S-in-apatite as a function of fO (sub 2) is investigated using X-ray absorption near-edge structures (XANES) spectroscopy at the S K-edge. Apatites crystallized from lamproitic melts at 1000 degrees C, 300 MPa and over a broad range of fO (sub 2) and sulfur fugacities (fS (sub 2) ) were measured. Peaks corresponding to S (super 6+) ( approximately 2482 eV), S (super 4+) ( approximately 2478 eV) and S (super 2-) ( approximately 2470 eV) were identified in apatite. The integrated S (super 6+) /STotal (S (sub Total) =S (super 6+) +S (super 4+) +S (super 2-) ) peak area ratios show a distinct positive correlation with fO (sub 2) , increasing from approximately 0.17 at FMQ+0 to approximately 0.96 at FMQ+3. Ab-initio calculations were performed to further understand the energetics and geometry of incorporation of S (super 6+) , S (super 4+) and S (super 2-) into the apatite (F-, Cl-, OH-) end-members. The results confirm that apatite can contain three different oxidations states of S (S (super 6+) , S (super 4+) , S (super 2-) ) as a function of fO (sub 2) . This makes apatite probably the first geologically relevant mineral to incorporate reduced (S (super 2-) ), intermediate (S (super 4+) ), and oxidized (S (super 6+) ) S in variable proportions. We emphasize that the strong dependence of the S oxidation state in apatite as a function of fO (sub 2) is also coupled with changing S content of apatite and co-existing melt (i.e., with changing fS (sub 2) ), resulting in a complex correlation between [1] apatite-melt (or fluid) partitioning, [2] redox conditions and [3] the melt and/or fluid composition, making the application of previously determined S apatite-melt partitioning coefficient debatable. Upon calibration over a range of geologically relevant T-P-X-fO (sub 2) -fS (sub 2) , S-in-apatite can serve as a powerful oxybarometer to quantify fO (sub 2) .