Volatile Recycling at the Lesser Antilles Arc: Processes and Consequences

地球在太阳系中是独一无二的，因为它有丰富的液态水、板块构造和生命。这些属性并非互不关联；生命的进化在很大程度上依赖于水，而水是通过板块构造循环输送到地球各地的。反过来，板块构造及其产生我们赖以生存的大陆的能力也取决于水的存在。俯冲带是板块构造系统中最重要的“阀门”。它们形成于地壳板块沉入地幔的地方。在这里，水，连同其他挥发物，如二氧化碳和硫，被送回深处的内部。然而，这种回归并不是大规模的。由于下沉的板块受到热量和压力的影响，大部分进入的挥发物被“蒸发掉”，并添加到上覆的地幔中，在那里它导致融化。这些融化物为俯冲带的火山提供了食物，这些火山具有危险的爆炸性特征。考虑到板块相互摩擦而引发的地震以及随之而来的海啸和山体滑坡，很明显，俯冲带是地球上最危险的地方。然而，这些地区也有好处：在俯冲带的岩浆中流动的液体混合物负责将有价值的金属沉积物运送到地壳中，并将其安置在地壳中，而火山爆发所散布的细火山灰产生了营养丰富、肥沃的土壤。通过俯冲带循环挥发物的重要性是不言而喻的。然而，我们仍然不知道它是如何工作的，也不知道送到俯冲带的挥发物的预算是多少，与那些被回收到岩石圈、水圈和大气中的挥发物相比，那些被隔离回地幔深处的挥发物的预算是多少。我们提出了一种创新的多学科实验来跟踪俯冲带的挥发物。要回答的问题包括：挥发物如何影响岩浆的类型和数量？他们如何控制皮纳图博火山和蒙特塞拉特火山等火山的位置以及它们的爆炸性？挥发物如何决定矿床的形成？挥发物是如何调节俯冲带的发震行为的——是否存在像日本和苏门答腊那样的大型“巨型逆冲”地震，或者滑动是否不那么剧烈？我们的重点区域是小安的列斯岛弧，这是一个特例，因为它是仅有的两个大西洋俯冲带之一。缓慢扩张的大西洋中脊的板块形成过程产生了一个比被广泛研究的太平洋中更为普遍的水合板块。此外，板块携带水的横向变化能力和沉积物俯冲到安的列斯岛弧下方，可能是造成该岛弧地震和火山活动风格和强度高度变化的罪魁祸首。通过绘制沿弧线的结构差异图，我们将能够查明不同水输入的影响。我们计划使用新的地震方法，辅以地球化学分析，并结合数值模型来确定和量化挥发物在下沉板块中的位置，它们在深度释放的位置，以及它们是如何从俯冲板块通过地幔楔向弧输送的。我们将利用一组独特的岩石，这些岩石来自地壳深处，它们被火山带了上来，以帮助我们了解岩浆是如何演化的，以及是什么让它们浓缩了矿石金属。绘制的水道将与地震和火山活动以及在其他俯冲带推断出的活动进行比较。一方面，这个大型研究项目将被大量的资源“封死”；以往研究的数据、样本、专业知识和结果将提供卓越的物有所值，另一方面特别关注社会效益；为自然灾害规划提供信息，更好地了解经济沉积物的形成方式和位置。

The Earth is unique in our solar system in having abundant liquid water, plate tectonics and life. These properties are not unconnected; The evolution of life has depended heavily on water, and water is pumped around the planet by the plate tectonic cycle. Plate tectonics in turn, and its capacity to generate the very continents on which we live, also depends on the existence of water.Subduction zones are the most important "valve" in the plate tectonic system. They form where tectonic plates sink back into the mantle. Here water, along with other volatiles such as carbon dioxide and sulphur, are returned to the deep interior. However, the return is not wholesale. As the sinking plate is subjected to heat and pressure, a large fraction of the incoming volatiles is "sweated off" and added to the overlying mantle where it causes melting. These melts feed volcanoes at subduction zones which are characteristically dangerously explosive. When considered with the earthquakes triggered by the plates scraping past each other and the consequent tsunamis and landslides, it is clear that subduction zones are the most hazardous places on Earth. Yet, these regions also have benefits: the cocktail of fluids travelling with magmas at subduction zones is responsible for transporting and emplacing valuable metal deposits into the crust, and the fine ash distributed by the explosive volcanoes produces nutrient-rich, fertile soils.The importance of cycling volatiles through subduction zones is self-evident. However we still don't really know how it works and what the budgets are of volatiles delivered to the subduction zone, versus those recycled into the lithosphere, hydrosphere and atmosphere compared with those sequestered back into the deep mantle.We propose an innovative multidisciplinary experiment to track volatiles at a subduction zone. Questions to be answered include: How do volatiles influence the types and amounts of magmas generated? How do they control where volcanoes, such as Mt Pinatubo and Montserrat are located and how explosive they are? How do volatiles dictate where ore deposits are formed? How do volatiles mediate the seismogenic behaviour of subduction zones - whether there are large "megathrust" earthquakes like Japan and Sumatra or whether slip is less violent?Our focus area is the Lesser Antilles Arc, which is a special case, because it is one of only two Atlantic subduction zones. Plate formation processes at the slowly-spreading mid-Atlantic ridge produce a much more pervasively hydrated plate than those in the extensively studied Pacific. Furthermore, a laterally varying capacity to carry water in the plate and sediments subducting below the Antillean arc are a likely culprit for the arc's highly variable style and intensity of seismic and volcanic activity. By mapping structural differences along the arc we will be able to pinpoint the effects of variable water input. We plan to use novel seismic approaches complemented by geochemical analyses and integrated using numerical models to identify and quantify where volatiles are in the downgoing plate, where they are released at depth, and how they are transported from the subducting plate through the mantle wedge to the arc. We will use a unique suite of rocks from deep in the crust which have been carried up in volcanoes to help us understand how magmas evolve, and what allows them to concentrate ore metals. Mapped water pathways will be compared with seismic and volcanic activity, as well as with those inferred at other subduction zones.This large research project will be "bookended' on the one hand by an enormous amount of resource; data, samples, expertise and results from previous studies that will provide excellent value for money, and on the other hand a special focus on the societal benefits; informing natural hazard planning, and a better appreciation of how and where economic deposits form.