Experimental determination of the melting phase relations of subducted sediment - a case study in the Lesser Antilles

俯冲沉积物熔融相关系的实验测定——以小安的列斯群岛为例

基本信息

批准号：
NE/G016615/1
负责人：
Jonathan Blundy
金额：
$ 44.86万
依托单位：
University of Bristol
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FG016615%2F1
关键词：
Experimental determination melting phase relations

项目摘要

One of the most striking pieces of evidence for modern plate tectonic was the discovery that the ocean floor is spreading, forming new oceanic lithospheric plate and giving rise to drifting continents. The fact that oceanic lithosphere forms at mid-ocean ridges requires that elsewhere oceanic lithosphere must be transferred back into the deep Earth. This happens at subduction zones, where dense oceanic lithosphere sinks below over-riding oceanic or continental lithosphere. One process that is ultimately related to subduction zones is the formation of volcanic arcs. The most prominent example is subduction of the Pacific Oceanic plate below the north American plate in the east, below the Aleutians in the north and below Japan in the northwest, forming the so called 'Ring of Fire'. Volcanoes above subduction zones are characteristically explosive, as exemplified by the 1980 eruption of Mount St. Helens (USA) or the 1883 eruption of Krakatoa (Indonesia). Such large eruptions can have a profound effect on local populations and global climate. Subduction zones give rise to volcanism because fluids and melts are released from the subducting lithospheric slab as it becomes gets heated up while sinking through Earth's mantle. These fluids and melts are released from different portions of the slab, namely from the part that represents the oceanic crust, from its sedimentary cover (e.g. shales, deep marine oozes, clays) and from the underlying lithospheric mantle, variably serpentinised by hydrothermal activity at mid-ocean ridges. These chemically buoyant fluids and melts interact with the overlying column of mantle peridotite, eventually triggering melting by lowering the melting point. Water-bearing basaltic magmas so-produced ascend into the crust, differentiate to more silicic compositions and eventually give rise to explosive volcanism on the over-riding plate. While geologists broadly know that such processes must happen in subduction zones, its details remain poorly understood. Geochemical data on volcanic rocks has proven to be a useful tracer of plate tectonic processes in general. Magmas erupted in different plate tectonic settings have characteristic geochemical 'flavours'. For example, enrichment in light rare earth elements, uranium and thorium, coupled with depletion in the high field-strength elements characterises lavas from subduction zones, supporting the involvement of fluids from the subducting slab. Although the process is conceptually simple, the details remain elusive, most notably the temperature to which the subducting slab is subjected at depth, the nature of the extracted fluids and the chemistry of the residual materials recycled into the deep mantle. In order to be able to study subduction zone processes in more detail, the conditions where fluids and melts are generated in subduction zones must be reproduced in laboratory experiments. Traditionally such experiments focus on the volumetrically dominant basaltic and serpentinised portions of the slab, with scant experimental data on the diverse (and trace element-rich) subducted sediment. Our pilot study on high-pressure melting of red clay with variable amounts of water highlights the important role that accessory phases rich in certain trace elements play in controlling the chemistry of the fluids and melts released from the slab. The temperature dependence of the stability of these minerals (notably rutile, monazite, ilmenite and apatite) means that the chemistry of erupted arc magmas has unrealised potential as a precise geothermometer of conditions in the underlying subduction zone. We aim to conduct further experiments on red clay and other oceanic sediments. Chemical data from the West Indies will be used as a field example against which geochemical characteristics of the experimental results will be compared. Involvement of the Project Partner will enable our results to be extended to the Tonga-Kermedec arc in the SW Pacific.

现代板块构造最引人注目的证据之一是发现海底正在扩张，形成新的大洋岩石圈板块并产生漂移的大陆。海洋岩石圈在洋中脊形成的事实要求其他地方的海洋岩石圈必须转移回地球深处。这种情况发生在俯冲带，即稠密的海洋岩石圈沉入上方的海洋或大陆岩石圈之下。最终与俯冲带相关的一个过程是火山弧的形成。最突出的例子是太平洋板块在东部俯冲到北美板块下方，在北部俯冲到阿留申群岛下方，在西北部俯冲到日本下方，形成所谓的“火环”。俯冲带上方的火山具有典型的爆炸性，1980 年圣海伦斯火山（美国）喷发或 1883 年喀拉喀托火山（印度尼西亚）喷发就是例证。如此大规模的喷发会对当地人口和全球气候产生深远的影响。俯冲带会引发火山活动，因为当俯冲岩石圈板片在地幔下沉时变得受热时，流体和熔体会从俯冲岩石圈板片中释放出来。这些流体和熔体从板块的不同部分释放出来，即从代表洋壳的部分、从其沉积盖层（例如页岩、深海软泥、粘土）和从下面的岩石圈地幔释放出来，这些岩石圈地幔因热液活动而不同程度地蛇纹石化。大洋中脊。这些化学浮力流体和熔体与上覆的地幔橄榄岩柱相互作用，最终通过降低熔点引发熔化。由此产生的含水玄武岩岩浆上升到地壳中，分化成更多的硅质成分，并最终在上覆板块上引起爆炸性火山活动。尽管地质学家普遍知道此类过程必定发生在俯冲带，但对其细节仍知之甚少。火山岩的地球化学数据已被证明是板块构造过程的有用示踪剂。在不同板块构造背景下喷发的岩浆具有特有的地球化学“味道”。例如，轻稀土元素、铀和钍的富集，加上高场强元素的损耗，是俯冲带熔岩的特征，支持来自俯冲板片的流体的参与。尽管这个过程在概念上很简单，但细节仍然难以捉摸，最值得注意的是俯冲板片在深处所承受的温度、提取的流体的性质以及回收到深部地幔的残留材料的化学成分。为了能够更详细地研究俯冲带过程，必须在实验室实验中重现俯冲带中产生流体和熔体的条件。传统上，此类实验主要集中在板块的体积占主导地位的玄武岩和蛇纹石部分，而关于各种（且富含微量元素）俯冲沉积物的实验数据很少。我们对用不同量的水高压熔化红粘土的初步研究强调了富含某些微量元素的辅助相在控制从板坯中释放的流体和熔体的化学性质方面发挥的重要作用。这些矿物（特别是金红石、独居石、钛铁矿和磷灰石）稳定性的温度依赖性意味着喷发的弧岩浆的化学成分具有作为下方俯冲带条件的精确地温计的尚未实现的潜力。我们的目标是对红粘土和其他海洋沉积物进行进一步的实验。来自西印度群岛的化学数据将用作现场示例，与实验结果的地球化学特征进行比较。项目合作伙伴的参与将使我们的成果扩展到西南太平洋的汤加-凯梅德克弧。