Tracing volatile cycling during progressive subduction in the Mariana Forearc

板块边界，即构造板块相互滑动的地方，是地球上发生最大和最迅速的地质变化的主要区域，包括灾难性的地震和火山爆发。在一种边界，即俯冲带，较冷、密度较大的海洋构造板块俯冲到较热、浮力较大的板块之下，进入下面的地幔。这一过程控制着板块构造旋回，推动着每2 - 2.7亿年一次的海底复刻。在这个过程中，地球表面的物质被运送到地幔，地幔几乎构成了地球的全部体积。当这些物质被拉入地幔时，它会引起周围地幔岩石的变化，促进融化，从而推动像安第斯山脉这样的弧形火山活动。在俯冲带之上的火山活动，以及在海洋扩张中心形成新地壳的地方，是地幔物质返回地球表面的主要途径。社会越来越关注二氧化碳水平的上升及其对全球气候的影响。事实上，对全球不同碳库的碳转移的研究是研究地球系统调节气候能力的前沿。这些控制作用的时间尺度取决于地球上不同碳库之间的循环速率和大小。这些过程的平衡以及地表和地球深处之间的转移控制了地球历史上大气成分的演变，并缓和了气候。据估计，地球的地幔和地核保存了地球上90%的碳，但地球系统将碳锁在地幔中的能力尚不清楚。我们可以测量出俯冲带上方火山喷发出的碳量，这个值大约是最初被拉入地幔的碳量的一半。碳可能在到达弧状火山下面的熔化带之前就从俯冲板块中逸出，在某些情况下，这些碳在俯冲板块正上方的地幔中以碳酸盐矿物的形式稳定下来。这一点的证据来自于最初由与地幔岩石相同的物质构成的岩石，这些岩石现在已经100%碳化了。当一个构造板块俯冲到另一个板块之下时，温度和压力的增加导致孔隙空间中的海水被挤出，随后含有水的矿物质被分解。这些水被排出到俯冲板块滑过的地幔岩石中。地幔与地球表面远未达到平衡，因此这些水的释放推动了蛇纹石作用（地幔岩石的水化作用），形成了一种新的矿物，称为蛇纹石，它比其他地幔岩石密度小。密度差异驱使这种新形成的岩石运动，驱使它向上移动，从海底的泥火山喷发出来。蛇纹石化过程产生的能量可能驱动深层微生物生命，而像这里研究的环境可能是地球上生命最初开始的地方。目前地球上约有62000公里的俯冲带，俯冲板的温度和俯冲角度是多变的；这些因素控制着在水下发生化学反应的条件。这项研究将调查从俯冲板块释放碳和水的条件，并通过太平洋马里亚纳前弧俯冲带上方的现代泥火山取样。通过研究有利于俯冲板块上方地幔中碳储存的条件，并量化俯冲板块的碳释放，我们将确定这一过程的全球意义，并最终确定其在全球碳收支中的作用。

Plate boundaries, where tectonic plates slide past each other, are the principal regions on Earth where the greatest and most rapid geological changes occur, including catastrophic earthquakes and volcanic eruptions. At one type of boundary, subduction zones, the colder, denser oceanic tectonic plate plunges beneath the hotter more buoyant plate into the mantle below. This process controls the plate tectonic cycle and drives the resurfacing of the ocean floor every ~200-270 million years. During this process materials from the Earth's surface are transported to the mantle, which makes up almost all of the Earth's volume. As this material is pulled into the mantle it induces changes in the surrounding mantle rocks, facilitating melting that drives volcanism in arcs, such as the Andes. Volcanism above subduction zones, and where new crust is formed at ocean spreading centres is the main route for material in the mantle to return to the Earth's surface. Society is increasingly concerned about rising levels of CO2 and its effect on global climate. Indeed, the study of transfers of carbon from different global reservoirs is at the forefront of investigations into the ability of the Earth System to regulate climate. The timescales over which these controls operate depend upon the rate of cycling between, and the size of different reservoirs of carbon on Earth. The balance of these processes and transfers between the surface and the deep Earth has controlled the evolution of the composition of our atmosphere and moderated climate over Earth's history. The mantle and core of the Earth are estimated to hold ~90% of Earth's carbon, but the ability of the Earth System to lock away carbon in the mantle is not well understood. We can measure how much carbon is coming out of volcanoes above subduction zones, and this value is about half of what is initially pulled down into the mantle. Carbon may escape from the subducting plate before it reaches the zone of melting below arc volcanoes, in some cases this carbon is stabilised as carbonate minerals in the mantle directly above the subducting plate. Evidence for this comes from rocks that were originally made of the same material as mantle rocks that have now become 100% carbonated.As one tectonic plate subducts beneath the other the resulting temperature and pressure increase causes seawater in pore spaces to be squeezed out followed by the breakdown of minerals that contain water. This water is expelled into the mantle rocks that the subducting plate slides through. The mantle is far from equilibrium with the Earth's surface such that the release of these waters drive serpentinisation (the hydration of mantle rocks), which forms a new group of minerals called serpentine which is less dense than the other mantle rocks. The density discrepancy drives movement of this newly formed rock, driving it to move upwards where it erupts from mud volcanoes on the seafloor. The process of serpentinisation generates energy that may drive deep microbial life, and environments like the one studied here may be where life first began on Earth. Presently there are ~62,000 km of subduction zone on Earth, and there is variability in temperature of the subducting slab, and angle at which they subduct; these factors control the conditions under which chemical reactions occur at depth. This study will investigate the conditions where carbon and water are released from the subducting plate and sampled through modern day mud volcanoes above a subduction zone in the Mariana Forearc in the Pacific Ocean. By investigating the conditions that favour carbon storage in the mantle above subducting plates, and quantifying releases of carbon from the subducting slab, we will identify the global significance of this process and ultimately its role in the global carbon budget.