Volatile Recycling at the Lesser Antilles Arc: Processes and Consequences

地球在我们的太阳系中是独一无二的，拥有丰富的液态水、板块构造和生命。这些特性并不是没有联系的；生命的进化在很大程度上依赖于水，而水是通过板块构造旋回在地球周围泵送的。反过来，板块构造及其形成我们赖以生存的大陆的能力也取决于水的存在。俯冲带是板块构造体系中最重要的“阀门”。它们形成于构造板块下沉回地幔的地方。在这里，水与二氧化碳和硫磺等其他挥发物一起被返回到深处。然而，回报并不是批发的。由于下沉板块受到热和压力的影响，进入的挥发物中的一大部分被“汗水排出”，并被添加到上复的地幔中，在那里它会导致融化。这些熔体为俯冲带的火山提供了能量，这些俯冲带具有典型的危险爆炸性。考虑到板块相互擦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：在俯冲带与岩浆一起流动的流体混合物负责将有价值的金属矿藏输送和转移到地壳中，火山爆发分布的细灰产生营养丰富的肥沃土壤。挥发物在俯冲带中循环的重要性不言而喻。然而，我们仍然不知道它是如何工作的，也不知道传递到俯冲带的挥发物的预算是多少，与回收到岩石圈、水圈和大气中的挥发物相比，回收到岩石圈、水圈和大气中的挥发物与滞留在地幔深处的挥发物相比。我们提出了一个创新的多学科实验来追踪俯冲带的挥发物。有待回答的问题包括：挥发分如何影响岩浆生成的类型和数量？它们如何控制皮纳图博山和蒙特塞拉特等火山的位置以及它们的爆炸性？挥发分如何决定矿床形成的地点？挥发分如何影响俯冲带的孕震行为--是否存在像日本和苏门答腊岛那样的大型“大逆冲”地震，或者滑动是否不那么剧烈？我们关注的区域是小安的列斯弧区，这是一个特例，因为它是大西洋上仅有的两个俯冲带之一。在缓慢展开的大西洋中部海脊的板块形成过程产生了比被广泛研究的太平洋板块更具渗透性的水合板块。此外，板块携带水的横向变化能力以及安的列斯弧下俯冲的沉积物可能是安的列斯弧形地震和火山活动形式和强度高度不同的罪魁祸首。通过绘制弧线上的结构差异图，我们将能够准确地确定不同的水输入的影响。我们计划使用新的地震方法，辅之以地球化学分析，并利用数值模型进行整合，以识别和量化挥发物在下行板块中的位置，它们在深处释放的位置，以及它们是如何从俯冲板块通过地幔楔形运往弧区的。我们将使用一套独特的地壳深处的岩石，这些岩石已经被火山携带起来，以帮助我们了解岩浆是如何演化的，以及是什么使它们能够浓缩矿石金属。绘制的水道将与地震和火山活动以及在其他俯冲区推断的水道进行比较。这一大型研究项目一方面将被大量的资源、数据、样本、专业知识和以前研究的结果作为书本，提供极好的金钱价值，另一方面将特别关注社会效益；为自然灾害规划提供信息，并更好地了解经济矿床是如何形成的以及在哪里形成的。

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.