Establishing apatite as a redox sensor in subducting slabs

在俯冲带，蚀变和氧化的洋壳被引入地球地幔。地幔的氧化作用可能导致地幔环流和结构的时空变化。地幔氧逸度(FO2)可能随着氧化还原敏感元素的转移而演化。硫在洋壳中相对丰富，可能将地幔氧化至QFm+2，与在弧状岩浆和弧下地幔包体中观察到的QFm至QFm+2的fO2一致。然而，关于板坯流体的氧化还原特性，几乎没有科学上的共识。最近的研究表明，板坯流体以还原的(H2S，HS-)或氧化的(SO42-，HSO4-，HSO3-)S物种为主。从俯冲带折返出来的高压变质岩的研究直接监测了S在板条流体中的赋存状态。磷灰石是高压岩石中常见的副相，可能含有S6+、S4+、S2-和S-。因此，磷灰石提供了在高压变质作用下流体-岩石相互作用过程中追踪S物种形成的独特机会。在我们的研究中，我们将结合现场磷灰石S6+/ΣS分析和现场测量硅酸盐矿物中的Fe~(3+)/ΣFe。这些数据将被用来监测S-铁氧化还原对在递进交代过程中的演化。含硫化物的岩石将从希腊锡罗斯岛上挖掘出来的惠普地体中采集到。Syros混杂岩块在其榴辉岩核和蛇纹岩混杂基质之间显示了各种交代反应带，这些榴辉岩核保存了变质作用的进行到峰期。这些反应带可能记录了板岩流体在沿板岩-地幔界面折返过程中穿过混杂基质的氧化还原特征。磷灰石颗粒的微区X射线吸收近边结构(µ-XANES)初步分析表明，最外层绿泥石片岩反应带的S6+/ΣS=0.80，而石榴石-辉石-绿泥石内部反应带的磷灰石颗粒S6+/ΣS=0.54。这些数据与含S板条流体对榴辉岩块的自外向内逐步氧化作用相吻合。反应带中共沉淀硫化物和富Fe3+矿物的初步观察表明，交代过程中S还原了铁氧化反应。我们的初步数据表明，通过将高压岩石中的S和铁氧化还原收支联系起来，将有可能直接监测板条流体的氧化还原特征。这些预期的数据对于确定板片派生的S是否有能力抬升弧岩浆和弧下地幔氧含量至关重要。因此，该项目将代表着在连接地球表面和地幔氧化还原过程方面向前迈出的重要一步。

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.