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幔。地幔的氧化作用可引起地幔环流和结构的时空变化。地幔氧逸度（fO 2）的演变可能是对氧化还原敏感元素转移的响应。硫在洋壳中相对丰富，可以氧化地幔至QFM+2，与弧岩浆和弧下地幔捕虏体中观察到的QFM至QFM+2的fO 2一致。然而，在板状流体的氧化还原特性上几乎没有科学共识。最近的研究表明，板状流体主要由还原的（H2S，HS-）或氧化的（SO 42-，HSO 4-，HSO 3-）S物种。高压（HP）变质岩从俯冲带折返的研究是必要的，直接监测板片流体中的S的形态。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，磷灰石提供了一个独特的机会，以跟踪在高压变质作用下的流体-岩石相互作用的S形态。在本研究中，我们将联合收割机在硅酸盐矿物中磷灰石S ~（6+）/S ~（6+）的原位分析与Fe ~（3+）/Fe ~（3+）的原位测量相结合。这些数据将被用来监测硫铁氧化还原对在渐进交代过程中的演变。含硫化物的岩石将从希腊锡罗斯岛挖掘出的HP化石中收集。锡罗斯混杂岩中的块体在其榴辉岩核（保存了从闪长岩到峰期变质作用）和蛇纹岩混杂岩基质之间显示出各种交代反应带。这些反应带可能提供了板片流体的氧化还原特征的记录，这些流体在折返过程中沿沿着板幔界面穿过混杂岩基质。磷灰石颗粒的初步微X射线吸收近边结构（µ-XANES）分析显示，在最外面的石榴石片岩反应区中，S6+/δ S = 0.80，而来自内部石榴石-辉石-石榴石反应区的磷灰石颗粒显示S6+/δ S = 0.54。这些数据是一致的榴辉岩块由含硫板流体的渐进外向内氧化。初步观察共沉淀硫化物和富Fe 3+矿物的反应区建议S-还原Fe-氧化反应过程中交代。我们的初步数据表明，通过连接的高压岩石中的S和Fe的氧化还原收支，它将有可能直接监测板流体的氧化还原特性。预期的数据是至关重要的，以确定是否板片派生的S是能够提升弧岩浆和弧下地幔fO 2。因此，该项目将代表在连接地球表面和地幔氧化还原过程方面迈出的重要一步。

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.