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

地球大陆被认为主要是在汇聚的构造边界形成的，在那里，由俯冲板块向深处输送的含水流体使上地幔熔化，并产生岩浆，最终促进上覆地壳的生长。然而，将弧区产生的原始岩浆成分推向以安山岩为特征的地球大陆的主体成分的过程，仍然存在激烈的争论。要理解这些“分化”过程，以及它们如何与汇聚大陆边缘的其他一阶特征相结合，一个主要的困难是，这些神奇活动区域的深层很少暴露在地球表面。然而，火山喷发有时能够将深来源的岩石碎片--也就是我们所知的“捕虏体”--输送到地球表面，为地球科学家提供了独特的一瞥，让他们得以一窥发生在大陆深处的过程，否则这些过程就很难研究。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口喷发，运送了大量来自弧根和其下地幔部分的捕虏体碎片。因此，这个地区提供了一个独特的机会来帮助解开在像安第斯山脉这样的火山活动地区的大陆地壳建造过程中运行的物理和化学过程。该奖项支持两名职业生涯早期的助理教授和一名博士生对这些独特的捕虏体以及将它们带到地表的罕见喷发进行详细的地球化学、岩石学和地质年代学研究。特别是，这项研究的重点是弧根的“重力不稳定”，这是一段地壳(即化学分异的)岩石，密度大于下地幔，以石榴石辉长岩、角闪岩和含石榴石的辉石岩等岩性为代表。将密度不稳定的根从弧底机械移除，这一过程被称为岩石圈下沉(又名分层)，不仅被认为是解释地球大陆化学的关键过程，而且被认为能够引起地表海拔的快速变化，例如推动安第斯高原中部新近纪的快速隆起。由于发生在下造山壳中的过程对于了解山脉的隆起史、大陆的地球化学和深部岩石圈的地球物理成像是至关重要的，因此这次调查的结果将对我们理解安第斯型板块边缘的地球化学、构造和构造史具有重要的意义。除了这次调查将产生的科研产品外，PIS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)向纽约州北部的公众解释弧形火山的起源和物理化学原理；2)继续组织RMSC“Next Level Science”夏令营的高中参与者参观校园，为当地学生提供直接体验大学和了解更多高级STEM教育的机会。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.