Establishing apatite as a redox sensor in subducting slabs

• Applicant: Professor Dr. Horst Marschall
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2021 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464606040

In subduction zones altered and oxidized oceanic crust is introduced to Earth’s mantle. Oxidation of the mantle may produce spatio-temporal variations in mantle circulation and structure. Mantle oxygen fugacity (fO2) may evolve in response to a transfer of redox sensitive elements. Sulfur is relatively abundant in oceanic crust and may oxidize the mantle up to QFM+2, consistent with fO2 of QFM to QFM+2 observed in arc magmas and subarc mantle xenoliths. However, there is little scientific consensus on the redox characteristics of slab fluids. Recent studies propose slab fluids dominated by either reduced (H2S, HS-) or oxidized (SO42-, HSO4-, HSO3-) S species. Studies of high-pressure (HP) metamorphic rocks exhumed from subduction zones are required which directly monitor the speciation of S in slab fluids. Apatite is a common accessory phase in HP rocks and may incorporate S6+, S4+, S2-, and S-. Thus, apatite provides a unique opportunity to track S speciation during fluid-rock interaction under HP metamorphism. In our study, we will combine in situ apatite S6+/ΣS analysis with in situ measurements of Fe3+/ΣFe in silicate minerals. These data will be used to monitor how the S-Fe redox pair evolves during progressive metasomatism. Sulfide-bearing rocks will be collected from the exhumed HP terrane on the island of Syros, Greece. Blocks in the melange on Syros display various metasomatic reaction zones between their eclogitic cores, which preserve prograde to peak metamorphism, and the serpentinite mélange matrix. These reaction zones may provide a record of the redox characteristics of the slab fluids which passed through the mélange matrix during exhumation along the slab-mantle interface. Preliminary micro- X-ray Absorption Near Edge Structure (µ-XANES) analyses of apatite grains show S6+/ΣS = 0.80 in the outermost chlorite schist reaction zones, whereas apatite grains from an inner garnet-pyroxene-chlorite reaction zones exhibit S6+/ΣS = 0.54. These data are consistent with progressive outwards-in oxidation of eclogite blocks by S-bearing slab fluids. Preliminary observations of co-precipitated sulfides and Fe3+-rich minerals in the reaction zones suggest S-reducing Fe-oxidizing reactions during metasomatism. Our preliminary data demonstrate that by linking the S and Fe redox budget in HP rocks, it will be possible to directly monitor the redox characteristics of slab fluids. The anticipated data are critical to establishing whether slab-derived S is capable of elevating arc magma and subarc mantle fO2. Thus, this project will represent a significant step forward in linking Earth’s surface and mantle redox processes.

• DFG Programme: Research Grants