How does crust form at arcs? - a Multidisciplinary Study of the Lesser Antilles Volcanic Arc

陆壳的形成深刻地影响了生命的演化、大气、水圈和生物圈的化学以及资源的分布，因此了解它是如何形成的是一个首要的地球化学和地球物理问题。目前，可能在整个地球历史的大部分时间里，大多数新地壳都是在俯冲带上方产生的，因此俯冲带岩浆作用和地壳生长密切相关。然而，俯冲带产生的新的未成熟地壳的成分主要受玄武岩岩浆流量的控制，而大陆地壳的二氧化硅和不相容的微量元素比这(安山岩)更丰富。为了从弧形地壳变成真正的大陆地壳，需要在俯冲过程中(例如深厚堆积物的分化和剥离)或在俯冲之后(例如地壳内混合或碰撞期间深部地壳的清除)在弧形地壳中发生额外的过程。解开这些潜在重要机制的关键在于实际了解俯冲带(上板块)地壳的结构和组成。为此，我们提出了一种综合岩石学、岩石学实验、地球化学和地震学的新方法来描述大洋内岛弧(小安的列斯群岛)的地壳特征。地震约束与岩石学控制的集成是一种特别创新的方法，这是由于大量捕虏体收集和大量高质量地震数据的可用。为了清楚地解释这种整合将如何进行，我们提供了以下类比：假设你有几块拼图(包体)，想要找出整个拼图(在这种情况下，是小安的列斯弧壳的地球化学/矿物学剖面)是什么画面。拼图是这幅画的代表--所有缺失的都是与你已经拥有的类型相同的。如果你把拼图想象成一个x-y网格，那么岩石学实验，以及矿物成分的地质压力测量，可以用来告诉你拼图碎片占据了哪一行(捕虏体来自多深)。然后，地震数据可以被用作一个“模式”，告诉我们哪些部分在2维x-y网格中的位置。首先，这提供了地震和岩石学数据之间的验证(两种方法确定的深度需要一致)。其次，既然我们现在知道什么地震数据对应于哪种捕捉体类型，我们就可以使用地震数据来填充图片的其余部分，并建立沿弧形的地壳剖面。这些方法之间的互补性是关键；地震数据不能告诉我们确切的矿物组合，只能告诉我们允许的矿物组合。反过来，捕虏体只能提供下伏地壳的例子，并且只能通过将它们的地震性质(根据捕捉体的矿物学计算)与观测到的地震剖面联系起来，才能确认为代表给定的地壳深度。因此，通过将捕虏体数据、实验数据和地震数据结合在一起，我们将能够生成整个弧线上的地壳结构模型，这将为地壳是如何在俯冲带上方建立的提供新的线索，俯冲带是大陆地壳最终生成的关键阶段。该项目汇集了一个多学科和多机构的国际团队。最近在布里斯托尔进行的采样和实验活动使这项工作特别及时，它建立在丰富的现有数据和先前研究的基础上，直接与正在进行的实质性资助的研究线索相结合，因此所需资源仅占揭示约30x400公里弧形地壳段的结构和组成所需的大规模和旷日持久的地球物理分析活动成本的一小部分。

The creation of continental crust has profoundly influenced the evolution of life, the chemistry of the atmosphere, hydrosphere and biosphere and the distribution of resources, so knowing how it is made is a first-order geochemical and geophysical problem. At present, and probably throughout much of Earth history, most new crust is generated above subduction zones, so subduction zone magmatism and crustal growth are intimately linked. However, the composition of the new, immature crust generated at subduction zones is controlled by a largely basaltic magma flux, whereas the continental crust is more silica and incompatible trace element rich than this (~andesitic). Additional processes occurring in the arc crust during subduction (such as differentiation and delamination of deep cumulates) or subsequent to subduction (such as intra-crustal mixing or removal of deep crust during collision) are needed in order to go from arc crust to true continental crust. The key to unravelling these potentially important mechanisms lies in actually knowing the structure and composition of the (upper plate) crust at subduction zones. To that end, we propose a novel approach integrating petrology, petrological experiments, geochemistry and seismology to characterise the crust of an intra-oceanic island arc (The Lesser Antilles).The integration of seismic constraints with the petrological controls is a particularly innovative approach, made possible by the availability of an extensive xenolith collection, and a very large high-quality suite of seismic data. To explain clearly how this integration will be done, we offer the following analogy: Suppose you had a handful of jigsaw pieces (the xenoliths) and wanted to find out what picture was made by the whole jigsaw (in this case the geochemical/mineralogical profile of the Lesser Antilles Arc crust). The jigsaw pieces are representative of the picture - all the missing ones are the same types as the ones you already have. If you think of the jigsaw as an x-y grid, then the petrological experiments, along with geobarometry from mineral compositions, can be used to tell you what rows the jigsaw pieces occupy (how deep the xenoliths come from). The seismic data can then be used as a "pattern" telling us which pieces go where in a 2-dimensional x-y grid. Firstly this provides validation between the seismic and petrologic data (the depths determined by both methods need to agree). Secondly, since we now know what seismic data correspond to which xenolith type, we can then use the seismic data to fill in the rest of the picture and build an along-arc crustal section. The complementarity between these approaches is key; the seismic data cannot tell us the exact mineral assemblage, but only permitted combinations of minerals . The xenoliths, in turn, can only provide examples of the subjacent crust, and can only be confirmed as representative of a given crustal depth by tying to their seismic properties (calculated from the xenoliths' mineralogies) to the observed seismic profiles. So by drawing together the xenolith data, the experimental data and the seismic data we will be able to generate a model of the crustal structure along the entire arc, which will shed new light on just how crust is built above subduction zones, a critical stage in the ultimate generation of continental crust. The project brings together a multidisciplinary and multi-institutional international team.The recent sampling and experimental campaign at Bristol makes this effort particularly timely and it builds on a wealth of existing data and prior research, integrating directly with ongoing, substantively funded research strands The resource requested therefore represents a mere fraction of the cost of the large and protracted geophysical-analytical campaign that would otherwise be needed to reveal the structure and composition of a ~30x400km section of arc crust.