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。这些特性并非没有联系;生命的进化在很大程度上依赖于水，水通过板块构造循环被泵到地球周围。而板块构造及其生成我们赖以生存的大陆的能力也取决于水的存在。俯冲带是板块构造系统中最重要的“阀门”。它们形成于地壳板块下沉到地幔的地方。在这里，水，沿着其他挥发物，如二氧化碳和硫，返回到深处的内部。然而，回归并不是批发。当下沉的板块受到热和压力的作用时，大部分进入的挥发物被“发汗”并添加到上覆的地幔中，在那里它导致熔化。这些融化物为俯冲带的火山提供了燃料，俯冲带的特点是具有危险的爆炸性。当考虑到板块相互摩擦引发的地震以及随之而来的海啸和山体滑坡时，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，与岩浆一起流动的液体混合物负责将有价值的金属矿床输送和定位到地壳中，爆炸性火山分布的细灰产生营养丰富的肥沃土壤。在俯冲带中循环挥发物的重要性不言而喻。然而，我们仍然不知道它是如何工作的，以及传递到俯冲带的挥发物的预算是多少，与那些再循环到岩石圈，水圈和大气中的挥发物相比，与那些被隔离回地幔深处的挥发物相比。需要回答的问题包括：挥发分如何影响岩浆的类型和数量？他们如何控制火山的位置，如皮纳图博火山和蒙特塞拉特火山的位置，以及它们的爆炸性？挥发分如何决定矿床的形成位置？挥发物是如何调节俯冲带的孕震行为的--是否有像日本和苏门答腊岛那样的大型“逆冲断层”地震，或者滑动是否不那么猛烈？我们的重点区域是小安的列斯群岛弧，这是一个特殊的情况，因为它是仅有的两个大西洋俯冲带之一。在缓慢扩张的大西洋中脊的板块形成过程中，产生了一个比广泛研究的太平洋板块更普遍的水合板块。此外，横向变化的能力，以携带水在板块和沉积物俯冲下安的列斯群岛弧是一个可能的罪魁祸首弧的高度可变的风格和强度的地震和火山活动。通过绘制沿着弧线的结构差异，我们将能够查明可变水输入的影响。我们计划使用新的地震方法，辅以地球化学分析和集成使用数值模型，以确定和量化的挥发物在下行板块，它们在深度释放，以及它们是如何从俯冲板块通过地幔楔输送到弧。我们将使用一套独特的岩石，这些岩石来自地壳深处，在火山中被携带上来，以帮助我们了解岩浆是如何演变的，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其他俯冲带推断的水道进行比较，这一大型研究项目一方面将以大量的资源、数据、样本、专门知识和以往研究的成果为“后盾”，这些研究将提供极高的资金价值，另一方面将特别注重社会效益;为自然灾害规划提供信息，并更好地了解经济存款的形成方式和地点。

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.