Collaborative Research: Caught in the Act- The Petrology of Modern Lower-Crust Formation and Foundering in the North Andean Arc

地球上的大陆被认为主要形成于汇聚的构造边界，在那里，含水流体通过俯冲板块被输送到深处，使上地幔融化，并产生岩浆，最终促进上覆地壳的增长。然而，在弧区产生的原始岩浆组成（玄武岩）向地球大陆的主要组成（安山岩）转变的过程仍然存在激烈的争论。要理解这些“分化”过程，以及它们如何与收敛大陆边缘的其他一级特征相结合，一个主要的困难是，这些岩浆活跃区的深根很少暴露在地球表面。然而，火山爆发有时能够将深层岩石碎片（称为“捕虏体”）运送到地球表面，为地球科学家提供了独特的机会，了解大陆深处发生的过程，否则这些过程很难研究。在安第斯造山带的北段（哥伦比亚南部），最近一次罕见的火山喷发带来了大量的捕虏岩碎片，这些捕虏岩碎片来自于弧的深层根部和它下面的地幔部分。因此，这个地方提供了一个独特的机会，可以帮助解开像安第斯山脉这样岩浆活跃地区大陆地壳构造过程中的物理和化学过程。该奖项支持两位刚出道的助理教授和一位女博士生对这些独特的捕虏体进行详细的地球化学、岩石学和地质年代学研究，以及将它们带到地表的罕见喷发。特别地，本研究侧重于“重力不稳定”的弧根，这是一段密度大于下伏地幔的地壳（即化学分化）岩石，以石榴石辉长岩、角闪岩和含石榴石辉长岩等岩性为代表。机械移除弧形底部密度不稳定的根，这一过程被称为岩石圈沉降（又名分层），不仅被认为是解释地球大陆化学的关键过程，而且还被认为能够引起地表高度的快速变化，例如那些推动新近纪中部安第斯高原快速隆起的因素。由于发生在下造山带地壳的过程对于理解山脉的隆升历史、大陆的地球化学和深部岩石圈的地球物理成像至关重要，因此这项研究的结果将对我们理解安第斯型板块边缘的地球化学、构造和构造历史具有重要意义。除了此次调查将产生的科学研究成果外，pi还将与罗切斯特博物馆和科学中心（RMSC）合作开展以下活动：1)向纽约州北部公众解释弧火山的起源和物理化学原理；2)继续组织参加RMSC“Next Level Science”夏令营的高中生参观校园，为当地学生提供直接体验大学生活的机会，并更多地了解高级STEM教育。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Earth's continents are thought to have mainly formed in convergent tectonic boundaries, where hydrous fluids transported to depth by subducting plates enable melting of the upper mantle and produce magmas that ultimately contribute to the growth of the overlying crust. Nevertheless, the processes responsible for driving the composition of primitive magmas generated in arc regions, which are basaltic, towards the bulk composition that characterizes Earth's continents, which are andesitic, remain intensely debated. A major difficulty in understanding these 'differentiation' processes, and how they may couple with other first-order features of convergent continental margins, is the fact that the deep roots of these magmatically-active regions are rarely exposed in the Earth's surface. However, volcanic eruptions are sometimes capable of transporting deep-sourced rock fragments - known as 'xenoliths' - to the Earth's surface, providing Earth scientists with unique glimpses of processes occurring deep within the continents that are otherwise difficult to study. In the northern segment of the Andean orogen (southern Colombia), a rare eruption from a recent volcanic vent transported an extraordinary cargo of xenolith fragments sourced from the deep roots of the arc and mantle section beneath it. Therefore, this locality presents a unique opportunity to help unravel the physical and chemical processes that operate during construction of continental crust in magmatically-active regions like the Andes. This award supports two early-career Assistant Professors and a woman who is a PhD student to conduct a detailed geochemical, petrologic and geochronologic study of these unique xenoliths, and the rare eruptions that brought them to the surface. In particular, this research focuses on the 'gravitationally unstable' root of the arc, a section of crustal (i.e., chemically differentiated) rocks with densities greater than the underlying mantle, represented by lithologies such as garnet gabbros, hornblendites, and garnet-bearing pyroxenites. Mechanical removal of density-unstable roots form the base of arcs, a process known as lithospheric foundering (aka, delamination), has not only been identified as a crucial process to explain the chemistry of Earth's continents but is also thought capable of inducing rapid changes in surface elevation, such as those driving the fast Neogene uplift of the Central Andean Plateau. Because the processes taking place in the lower orogenic crust are critical for understanding the uplift history of mountains, the geochemistry of the continents, and the geophysical imaging of the deep lithosphere, results from this investigation will have important implications for our understanding of the geochemical, structural and tectonic history of Andean-type plate margins. In addition to the scientific research products that will result from this investigation, the PIs will collaborate with the Rochester Museum and Science Center (RMSC) on activities that will: 1) explain the origin and physicochemical principles of arc volcanoes to the public of Upstate NY; and 2) continue to organize campus visits for high-school participants of RMSC's 'Next Level Science' summer camps, to provide local students with a direct opportunity to experience college and learn more about upper-level STEM education.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.