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

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，并形成漂移的大陆。海洋岩石圈在大洋中脊形成的事实要求，在其他地方，海洋岩石圈必须转移回地球深处。这发生在俯冲带，在那里致密的海洋岩石圈下沉到覆盖大洋或大陆的岩石圈之下。最终与俯冲带有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东低于北美板块，北低于阿留申山脉，西北低于日本，形成了所谓的“火环”。俯冲带上方的火山具有典型的爆炸性，例如1980年美国圣海伦斯火山的喷发或1883年印度尼西亚喀拉喀托火山的喷发。如此大规模的喷发可能会对当地人口和全球气候产生深远影响。俯冲带导致火山活动，因为流体和熔体从俯冲的岩石圈板块中释放出来，因为它在穿过地幔下沉时变得加热。这些流体和熔体是从板块的不同部分释放出来的，即从代表洋壳的部分、从其沉积盖层(例如页岩、深海海洋软泥、粘土)和从下面的岩石圈地幔释放出来的，这些地幔因大洋中脊的热液活动而变得蛇纹状。这些化学浮力的流体和熔体与上覆的地幔橄榄岩柱相互作用，最终通过降低熔点来触发熔融。由此产生的含水玄武岩岩浆上升进入地壳，分异出更多的硅质成分，最终在覆盖的板块上引发爆发火山作用。虽然地质学家普遍知道这样的过程必须发生在俯冲带，但对其细节仍知之甚少。事实证明，火山岩的地球化学数据在总体上是板块构造过程的有用示踪物。在不同的板块构造环境中喷发的岩浆具有独特的地球化学“风味”。例如，轻稀土元素、铀和钍的浓缩，加上高场强元素的贫化，构成了俯冲带熔岩的特征，支持了俯冲板块的流体参与。尽管这一过程在概念上很简单，但细节仍然难以捉摸，最明显的是俯冲板块在深处所受的温度，提取的流体的性质，以及回收到地幔深处的残留物的化学成分。为了能够更详细地研究俯冲带过程，必须在实验室实验中再现俯冲带中产生流体和熔体的条件。传统上，这样的实验主要集中在板块中体积占主导地位的玄武岩和蛇纹岩部分，而对各种(和富含微量元素的)俯冲沉积物的实验数据很少。我们对含不同水量的红粘土高压熔融的初步研究强调了富含某些微量元素的辅助相在控制板坯释放的流体和熔体的化学方面所起的重要作用。这些矿物(特别是金红石、独居石、钛铁矿和磷灰石)的稳定性依赖于温度，这意味着喷发的弧状岩浆的化学成分具有作为俯冲带条件的精确地质温度计的未实现潜力。我们的目标是在红粘土和其他海洋沉积物上进行进一步的实验。将使用西印度群岛的化学数据作为实地例子，并将其与实验结果的地球化学特征进行比较。项目伙伴的参与将使我们的成果扩大到西南太平洋的汤加-克梅迪克弧区。

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.