The Deep Lithosphere Filter and the Growth of Continental Arcs

了解大陆形成的过程不仅对了解整个地球系统，而且对人类社会的许多方面都是至关重要的。大陆地壳是在会聚边缘形成的，在那里海洋板块下沉到更浮力的大陆板块下方。在这两个板块之间的地幔楔形区融化产生岩浆，这些岩浆上升并最终成为新的大陆地壳的基石。鉴于大陆弧对现代社会的重要性，最近的许多科学重点都集中在大陆弧的浅部方面，如弧形火山的爆发和大型矿藏的形成。然而，所有这些浅层事件都是始于大陆弧下深部区域(岩石圈)的过程的最终结果，该区域受到的关注相对较少。例如，在地球表面取样的弧状岩浆是复杂的分异路径的最终产物，该路径始于地幔楔形部分熔化的地方。与火山喷发的爆炸性直接相关的因素，如岩浆中的水和二氧化碳含量，最终由地幔来源中存在的数量控制。研究深陆弧岩石圈是具有挑战性的，但了解大陆弧深处发生的过程及其与地表观测到的关系对地球科学至关重要。尽管岩石圈深处不能直接观测到，但采集该地区样本的捕虏体(被快速上升的岩浆夹带的外来岩石块)为了解下地壳和上地幔提供了一个独特的窗口。重要的是，地幔包体通常代表熔体残留物，因此它们是弧岩浆的补充。下地壳捕虏体代表在深部已经分馏的弧形岩浆(‘堆积物’)，并可能提供对岩浆早期分异路径的洞察。因此，这一建议的第一个目标是评估深部岩石圈分馏对岩浆分异的贡献。特别是，深层堆积物的化学成分是什么，这对大陆弧的物理演化有什么影响？由于这种堆积物富含石榴石等致密矿物，它们可能是导致大陆弧下深层岩石圈不稳定的原因。这一提议的第二个目标是将深部岩石圈捕虏体的地球化学特征置于大陆弧如何随时间演变的广泛背景下。特别是，弧形岩石圈如何以及何时增厚(或变薄)？弧体的增厚是否与岩浆流量异常高的时期有关？如果是这样，这是否限制了深部岩石圈在撞击板块之前在弧下生长的速度？回答这些问题需要使用多种工具，从地球化学仪器到物理模型。这项研究的所有方面都将有助于对弧和大陆地壳演化的变革性看法，因为到目前为止，深部岩石圈过程在很大程度上没有被认为是地壳分化的重要因素。这项研究将由本科生和研究生进行。

Understanding the processes involved in making continents is of fundamental importance not only to understanding the whole earth system, but also to many aspects of human society. Continental crust is formed at convergent margins where an oceanic plate sinks below a more buoyant continental plate. Melting in the mantle wedge region between these two plates generates magmas that rise and ultimately become the building blocks for new continental crust. Much of the recent scientific focus has been on the shallow aspects of continental arcs, such as explosive eruptions at arc volcanoes and the formation of large ore deposits, given their importance to modern society. However, all of these shallow events are the end-result of processes that begin in the deep region (the lithosphere) beneath continental arcs, an area that has received comparatively less attention. For instance, the arc magmas that are sampled at the Earth's surface are the end products of a complex differentiation path that started where the mantle wedge underwent partial melting. Factors such as water and carbon dioxide content of magmas, which bear directly on the explosiveness of volcanic eruptions, are ultimately controlled by the amounts present in the mantle source. Investigating the deep continental arc lithosphere is challenging, but knowledge of what processes go on deep beneath continental arcs and their relationships to what is observed at the surface is of paramount importance to earth science. Although the deep lithosphere cannot be observed directly, xenoliths (foreign pieces of rock entrained by rapidly ascending magmas) that sample this region offer a unique window into the lower crust and upper mantle. Importantly, mantle xenoliths often represent melt residues, thus they are the complement to arc magmas. Lower crustal xenoliths represent arc magmas that have fractionated at depth ('cumulates') and may provide insights into the early differentiation paths of magmas. Thus, the first objective of this proposal is to evaluate the contribution of deep lithospheric fractionation to magmatic differentiation. In particular, what is the chemical composition of deep cumulates, and what effect does this have on the physical evolution of continental arcs? Because such cumulates are rich in dense minerals like garnet, they may be responsible for de-stabilizing the deep lithosphere beneath continental arcs. The second objective of this proposal is to place the geochemical characteristics of deep lithosphere xenoliths into the broad context of how continental arcs have evolved over time. In particular, how and when does arc lithosphere thicken (or thin)? Is thickening in arcs related to periods of unusually high magmatic flux, and if so, does this place a limit on how rapidly the deep lithosphere can grow beneath the arc before hitting the slab? Answering these questions requires using a consortium of diverse tools, ranging from geochemical instruments to physical modeling. All aspects of the study will contribute to a transformative view of arc and continental crust evolution because deep lithospheric processes were hitherto largely unrecognized as important for crustal differentiation. This research will be conducted by undergraduates and graduate students.