Establishing apatite and iron-bearing silicates as redox sensors in subducting slabs

Subduction zones tectonically link Earth’s oxidized surface with its reduced deep interior. Recent studies propose that fluids driven from the subducting plate contain either reduced or oxidized S species. These fluids rise through the overlying mantle to generate arc magmas; however, it is unclear how these fluids influence arc magma redox. We propose to use S and Fe XANES analysis of apatite and Fe-silicate minerals, respectively, in exhumed subduction zone rocks. These rocks underwent intense fluid-rock interaction at depths of >80 km. The S and Fe speciation in the minerals record the redox characteristics of the fluids: If these fluids contain oxidized S, the S and Fe in the rock will be oxidized as the fluid:rock ratio increases. From these data we can identify the S species carried in the fluids. Our results will be of major interest to the Earth Science community and have profound implications for the cycling of volatiles and their impact on Earth’s climate and atmosphere.