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

弧形地壳是如何形成的？

基本信息

批准号：
NE/K004328/1
负责人：
Colin Macpherson
金额：
$ 42.57万
依托单位：
Durham University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2013
资助国家：
英国
起止时间：
2013 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FK004328%2F1
关键词：
How does crust form arcs

项目摘要

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.

大陆地壳的形成深刻地影响了生命的演化、大气圈、水圈和生物圈的化学组成以及资源的分布，因此了解它是如何形成的是一个一级地球化学和地球物理问题。目前，可能在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生成的，因此俯冲带岩浆活动和地壳生长是密切相关的。然而，在俯冲带产生的新的，未成熟的地壳的组成是由一个很大的玄武岩岩浆流量控制，而大陆地壳是更多的硅和不相容的微量元素比这（~安山岩）丰富。为了从弧壳变成真正的大陆壳，还需要在俯冲过程中（如深堆积层的分异和分层）或俯冲之后（如地壳内混合或碰撞过程中深部地壳的移动）在弧壳中发生其他过程。解开这些潜在的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，我们提出了一种新的方法，结合岩石学，岩石学实验，地球化学和地震学来解释地壳的洋内岛弧（小安的列斯群岛）。地震约束与岩石学控制的集成是一个特别创新的方法，使广泛的捕虏体收集的可用性，和一个非常大的高质量的地震数据套件。为了清楚地解释这种整合是如何进行的，我们提供了以下类比：假设你有一把拼图（捕虏体），想知道整个拼图（在这种情况下是小安的列斯岛弧地壳的地球化学/矿物学剖面）构成了什么样的图像。拼图是代表图片-所有失踪的是相同的类型，你已经有。如果你把拼图想象成一个x-y网格，那么岩石学实验，沿着矿物成分的地质压力测量，可以用来告诉你拼图块占据了什么行（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二维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，既然我们现在知道了哪种捕虏体对应哪种地震数据，那么我们就可以用地震数据来填充图片的其余部分，并建立一个沿弧地壳剖面。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，而只能告诉我们矿物的组合。捕虏体只能提供次地壳的例子，并且只能通过将其地震特性（根据捕虏体的矿物学计算）与观察到的地震剖面联系起来，才能被确认为给定地壳深度的代表。因此，通过收集捕虏体数据、实验数据和地震数据，我们将能够生成一个沿着整个弧形的地壳结构模型，这将为地壳如何在俯冲带之上形成提供新的线索，俯冲带是大陆地壳最终形成的关键阶段。该项目汇集了一个多学科和多机构的国际团队。最近在布里斯托进行的采样和实验活动使这一努力特别及时，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，因此，所要求的资源只是大规模和长期的地球物理研究费用的一小部分，分析活动，否则将需要揭示弧壳的约30 x400公里的结构和组成部分。