The Deep Lithosphere Filter and the Growth of Continental Arcs

了解形成大陆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具有根本性的重要性。大陆地壳是在海洋板块下沉到浮力更强的大陆板块之下的会聚边缘形成的。这两个板块之间的地幔楔区域的融化产生了岩浆，岩浆上升并最终成为新大陆地壳的基石。鉴于对现代社会的重要性，最近科学研究的焦点大多集中在大陆弧的浅层方面，如弧火山的爆炸性喷发和大型矿床的形成。然而，所有这些浅层事件都是大陆弧下深层区域（岩石圈）开始的过程的最终结果，而这一区域受到的关注相对较少。例如，在地球表面取样的弧岩浆是一个复杂分化路径的最终产物，这个路径始于地幔楔经历部分熔融。诸如岩浆中的水和二氧化碳含量等因素直接影响火山爆发的爆炸性，最终受地幔源中存在的量的控制。调查深大陆弧岩石圈是具有挑战性的，但了解大陆弧深处发生了什么过程，以及它们与地表观测的关系，对地球科学至关重要。虽然深层岩石圈无法直接观测，但从该地区取样的捕虏体（被快速上升的岩浆夹带的外来岩石）为研究下地壳和上地幔提供了一个独特的窗口。重要的是，地幔捕虏体通常代表熔体残留物，因此它们是弧岩浆的补充。下地壳捕虏体代表了在深处分异的弧岩浆（“堆积”），可能为岩浆的早期分异路径提供洞见。因此，本文的第一个目的是评价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？因为这些堆积物富含像石榴石这样的致密矿物，它们可能是破坏大陆弧下深层岩石圈稳定的原因。本提案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大背景下。特别是，弧岩石圈如何以及何时增厚（或变薄）？弧的增厚是否与异常高的岩浆通量有关？如果是这样，这是否会限制弧下深处岩石圈在撞击板块之前的增长速度？回答这些问题需要使用多种工具，从地球化学仪器到物理建模。这项研究的所有方面都将有助于对弧和大陆地壳演化形成一种变革性的看法，因为迄今为止，人们在很大程度上尚未认识到深岩石圈过程对地壳分化的重要性。本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.