Characterising devolatilisation beneath the Mariana Forearc

构成地球板块的地壳有两种；海洋和大陆，前者薄而密，位于世界海洋之下；后者更厚，浮力更强，构成了大陆。海洋地壳形成于被称为洋中脊的不同板块边界，即海底火山链。随着新形成的地壳年龄的增长，并远离海脊，它冷却，密度增大，下沉。在大洋和大陆地壳之间出现会聚板块边界的地方，密度较大的大洋地壳下沉并俯冲到大陆地壳之下。在两个大洋板块汇聚的地方，较老、密度较大的板块下沉和俯冲。这样的边界形成了地球上最深的海沟，例如马里亚纳海沟（最深11033米）。俯冲带是地壳物质进入地幔的主要区域，因此在板块构造理论中代表了基本的再循环过程。这些辐合边缘的地质过程控制着地震活动性、深部成矿作用和伴生火山作用，以及海洋、地壳和地幔之间的地球化学循环。这些边缘通常会产生大的海啸性地震，因此了解控制俯冲界面地震发生的物理性质是阐明板块边界行为的关键。沉积物和地壳岩石在俯冲过程中经历压力和温度的变化，矿物质变得不稳定，并分解成新的、密度更大的矿物质，在此过程中释放出浮力强的流体，上升到地震形成核的断层上方，与上覆的地幔岩石相互作用。俯冲板块在开始融化之前所经历的化学和物理变化控制着这些板块边界的地震行为、地球化学循环和岩浆活动。为了研究这些过程，人们研究了俯冲带中隆起到现今大陆地壳上的老块，但对这些海洋地壳板块向大陆转移过程中发生的化学和物理变化知之甚少，而且对形成压力和温度的估计也存在很大误差。在现场研究这些过程是困难的，因为它们发生在地壳深处和地球上最深的一些水域。已经成功地钻探了被厚沉积物桩覆盖的俯冲带，但厚沉积物桩模糊了深部流体的特征，因此无法研究深俯冲带的过程。在马里亚纳辐合边缘，太平洋板块俯冲到马里亚纳板块之下，那里的沉积物覆盖层很少。马里亚纳板块有断裂，这些断裂与泥火山有关。泥火山起源于从俯冲板块释放出来的流体通道，这些流体携带着该板块的物质，并在化学上改变了其上的地幔。这为采集俯冲过程中产生的流体和俯冲板块的物质提供了独特的机会，而无需钻深。我们将使用IODP 366远征队在马里亚纳前弧泥火山钻探的样本来分析俯冲过程中产生的水。先前的研究表明，随着板块的深入，流体化学会发生变化。我们将测量流体中不同化学示踪剂的同位素比率，以记录这些过程的演化和下向岩石的变质转化，以及流体在上覆地幔中促进的化学反应。这些信息将与对随流体移动的岩石的分析相结合，以量化哪些被带入更深的俯冲带，哪些被释放到上面的地壳和海洋中。这些分析将有助于约束控制地震形成核的物质强度的反应。

There are two types of crust that make up the plates of the World; oceanic and continental, the former is thin and dense and underlies the oceans of the world; and the latter is considerably thicker and more buoyant and makes up the continents. Oceanic crust forms at divergent plate boundaries known as mid ocean ridges, chains of underwater volcanoes. As the newly formed crust ages and moves further from the ridge it cools, becomes denser and sinks. Where a convergent plate boundary occurs between oceanic and continental crust the denser oceanic crust sinks and subducts beneath the continental crust. Where convergence occurs between two oceanic plates, the older, denser plate sinks and subducts. Such boundaries are responsible for the deepest oceanic trenches on Earth, for example the Mariana Trench (deepest point 11,033 m).Subduction zones are the principal areas on Earth where crustal materials are transported into the mantle and therefore represent the fundamental recycling process in plate tectonic theory. Geological processes at these convergent margins control seismicity, plutonism and associated volcanism, and geochemical cycling between the ocean, crust and mantle. These margins commonly generate large, tsunamigenic earthquakes, thus understanding the physical properties controlling seismogenesis at the subduction interface is key to elucidating plate boundary behaviour. Sediments and crustal rocks experience changing pressures and temperatures as they subduct, minerals become unstable and breakdown to form new, denser minerals, and in doing so release fluids that are buoyant and rise into the faults above where earthquakes nucleate and interact with overlying mantle rocks. The chemical and physical changes that the subducting plate undergoes prior to the onset of melting controls earthquake behaviour, geochemical cycling and magmatism at these plate boundaries.To study these processes old pieces of subduction zones that have been uplifted onto the present day continental crust have been studied, but the chemical and physical changes that occurred during the transport of these sections of oceanic crust onto the continents are poorly constrained, and there are large errors on estimated pressures and temperatures of formation. Studying these processes in situ is difficult because they occur deep in the crust and in the some of deepest stretches of water on Earth. Subduction zones that are covered by thick piles of accreted sediments have been successfully drilled, but the thick sediment piles obscures signatures of deep fluids and so the deep subduction zone processes cannot be studied.At the Mariana convergent margin, the Pacific Plate subducts under the Mariana Plate where there is little sediment overburden. The Mariana Plate is faulted and these faults are associated with mud volcanoes. The mud volcanoes originate from the passage of fluids liberated from the subducting plate and carry material from this plate and chemically altered overlying mantle with them. This provides a unique opportunity to sample fluids generated during subduction and materials from the subducting plate without having to drill deep.We will use samples from the IODP Expedition 366 drilling into mud volcanoes in the Mariana forearc to analyse waters generated during subduction. Previous work shows that fluid chemistry changes as the plate moves deeper. We will measure the isotopic ratios of different chemical tracers in the fluids to document the evolution of these processes and metamorphic transformations in the downgoing rocks and the chemical reactions the fluids facilitate in the overlying mantle. This information will be combined with analyses of rocks that are transported with the fluids to quantify what is carried into the deeper subduction zone and what is liberated to the crust and ocean above. These analyses will help to constrain the reactions controlling the strength of materials where earthquakes nucleate.