Collaborative Research: How Does Lower Continental Crust Form? A Petrochronological Investigation of the Ivrea Zone

陆壳约占地球表面积的40%，对人类生存至关重要。众所周知，上大陆地壳主要是由山脉侵蚀形成的沉积岩，而下大陆地壳的组成和成因还不是很清楚。流行的模式是，下地壳由致密的岩石组成，富含铁(Fe)和镁(Mg)元素，作为地幔下部上升的熔体就位。然而，变质沉积物在下地壳岩石中很常见，这些岩石现在暴露在地球表面。这项研究解决的问题是：沉积物是如何并入下地壳的？这个问题是通过重建下地壳的一个特征部分--意大利的伊夫雷亚-韦尔巴诺带--的压力-温度-时间历史来解决的，以评估关于下大陆地壳形成的相互矛盾的假说。该研究项目纳入了宾夕法尼亚州立大学、加州大学圣巴巴拉分校和洛桑大学之间的新合作，支持培训来自代表性不足群体的两名研究生，并让本科生从事学术研究。研究大陆下部地壳如何形成，是了解地壳演化、体力发展、地壳流动和地球化学分异的基础。利用一套互补的技术，该项目检验了伊夫雷亚-韦尔巴诺带形成的三个假说：埋藏、地壳底侵和重新分层。碎屑锆石U-Pb年龄被用来确定沉积物沉积年龄随深度的变化。Lu-HF石榴石枣用于确定石榴石生长和停留在最高温度的时间。变质锆石和独居石U/Th-Pb测年+激光消融分流岩石年代学获得的微量元素丰度，使我们能够评估伊夫雷亚带的时间尺度。岩石学约束来自于假选区、最佳温度压力法和微量元素测温法。这些技术结合在一起，有助于重建P-T-t进动轨迹和峰期变质条件下的剖面变化。完整的数据集用于对这三个假设进行严格的测试。该项目由地球科学部的构造学计划和岩石学和地球化学计划共同资助。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。

Continental crust occupies about 40 percent of the surface area of the Earth and is critical for the sustenance of habitability. Whereas the upper continental crust is known to be dominated by sedimentary rocks formed by the erosion of mountains, the composition and origin of lower continental crust is not well understood. The prevailing paradigm is that the lower crust is made of dense rock, rich in the elements iron (Fe) and magnesium (Mg), emplaced as rising melts from the mantle below. However, metamorphosed sediments are common in sections of lower crustal rock that are now exposed at the surface of the Earth. This research addresses the question: How are sediments incorporated into the lower crust? This question is addressed via a reconstruction of the pressure-temperature-time history of a characteristic section of the lower crustal, the Ivrea-Verbano Zone in Italy, to evaluate competing hypotheses for the formation of lower continental crust. The research project incorporates a new collaboration between Penn State, UC Santa Barbara, and the University of Lausanne, supports the training of two graduate students from under-represented groups, and engages undergraduates in academic research. Characterizing how lower continental crust forms is fundamental to understanding crustal evolution, development of body forces, flow of crust, and chemical differentiation of the Earth. Using a suite of complementary techniques, this project tests three hypotheses for the formation of the Ivrea-Verbano Zone: Burial, Crustal Underplating, and Relamination. Detrital zircon U-Pb dates are used to determine how sediment deposition ages vary with depth. Lu-Hf garnet dates are used to bracket the timing of prograde garnet growth and residence at peak temperatures. Metamorphic zircon and monazite U/Th-Pb dates + trace-element abundances obtained by laser-ablation split-stream petrochronology enable assessment of the timescale over which the Ivrea Zone was assembled. Petrologic constraints are derived from pseudosections, optimal thermobarometry, and trace- element thermometry. In conjunction, these techniques facilitate reconstruction of the downsection variation in prograde P-T-t paths and peak metamorphic conditions. The complete dataset is used for rigorous testing of the three hypotheses. This project is jointly funded by the Tectonics program and the Petrology & Geochemistry program in the division of Earth Sciences.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.