Generation of Subduction-Zone Magmas from Melange Diapirs

在地球上，海洋板块不断地沿着破坏性的板块边缘被拖入地幔。这一过程被称为“俯冲”，是板块构造的基本表现之一。俯冲带是物质在地球表面和内部之间的主要运输机制。对俯冲带化学循环的了解为我们提供了更好地了解造山和岩浆过程的工具，这些造山和岩浆过程有助于全球关键元素的循环，形成宝贵的自然资源，如贱金属矿石，以及俯冲带的日常灾害，如地震、海啸和火山。传统上，从俯冲的大洋板块排出的流体迁移到炎热的上地幔，在那里它们引发熔融和岩浆的产生，导致火山链的形成，这些火山链在地球周围的俯冲带无处不在。这些已建立的模型可以解释俯冲带火山中发现的许多岩浆成分。然而，在俯冲带火山中观察到的一些岩浆类型不能用传统的模型来解释，需要一个不同于正常地幔的源区。通过计算机模拟、实地考察和地球化学研究，出现了关于物质运输的新模型。这些新的模型表明，来自俯冲洋壳的岩石和上覆地幔之间的混合导致了直接位于俯冲板块上方的厚层混合岩层的形成。据预测，这些混合岩层，即所谓的“混杂岩”，将漂浮上升进入上覆的热地幔楔形，在那里它们开始产生大量的熔融。在这个项目中，研究人员Marschall、Gaetani和Cruz-Uribe将通过在实验室进行高压实验，首次研究在地幔普遍存在的压力-温度条件下，天然混杂岩产生的熔体的组成。在这些实验中产生的熔体和矿物将通过现代显微分析方法进行表征，这些方法能够非常灵敏地、在非常小的范围内确定材料的化学成分。这些成分将与俯冲带火山喷发的熔岩进行比较。这项研究的结果可能有助于解释比传统模型更大范围的火山岩成分的形成，并有助于我们理解在地球深处运行的化学和机械过程，这些过程导致了地球上最活跃的火山活动的形成。这项关于楔形底辟中混杂熔融形成弧状岩浆的研究将对整个固体地球科学界产生重要影响，因此，这项工作的结果将引起广泛的科学家的兴趣。这包括对俯冲带岩浆生成感兴趣的岩石学家和地球化学家，以及对俯冲带的机械过程和热状态感兴趣的地球物理学家和数值模型师。

On planet Earth, oceanic plates are constantly dragged down into the mantle along destructive plate margins. This process is called "subduction" and it is one of the fundamental manifestations of plate tectonics. Subduction zones function as the primary transport mechanism for materials between the surface and interior of the Earth. An understanding of chemical cycling in subduction zones provides us with the tools to better understand the mountain-building and magmatic processes that contribute to the global cycles of critical elements, the formation of valuable natural resources, such as base-metal ores, and daily hazards of subduction zones, such as earthquakes, tsunamis and volcanoes.Traditionally, it is envisaged that fluids expelled from the subducting oceanic plate migrate into the hot overlying mantle, where they trigger melting and the generation of magmas that lead to the formation of the volcanic chains that are ubiquitous along subduction zones around the planet. These established models can explain a number of magma compositions found in subduction-zone volcanoes. However, some magma types observed in subduction-zone volcanoes cannot be explained by that traditional model and require a source region that is different from the normal mantle. New models on material transport have emerged from computer modeling, fieldwork and geochemical studies. These new models suggest that mixing between rocks derived from subducting oceanic crust and the overlying mantle leads to the formation of thick layers of mixed rocks directly above the subducting plate. These mixed-rock layers, so-called "mélanges", are predicted to buoyantly rise into the overlying hot mantle wedge, where they start to produce significant amounts of melt. In this project, researchers Marschall, Gaetani and Cruz-Uribe will investigate for the first time the composition of melts produced from natural mélange rocks under the pressure-temperature conditions prevailing in the mantle by performing high pressure experiments in the laboratory. The melts and minerals produced in these experiments will be characterized by modern micro-analytical methods that are capable of determining the chemical composition of the material very sensitively and at a very small scale. The compositions will be compared to those of lavas erupted from subduction-zone volcanoes. Results from this study may help to explain the formation of a larger range of composition of volcanic rocks compared to the traditional model, and it help us to understand the chemical and mechanical processes operating at depth that lead to the formation of the most vigorous volcanism on the planet.This study of the formation of arc magmas by mélange melting in wedge diapirs will have important consequences for the entire solid-Earth science community and thus, the results of this work will be of interest to a broad range of scientists. This includes petrologists and geochemists who are interested in the generation of magmas along subduction zones, as well as geophysicists and numerical modelers with an interest in the mechanical processes and the thermal state of subduction zones.