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

大陆地壳约占地球表面积的40%，对维持可居住性至关重要。上大陆地壳主要是山脉侵蚀形成的沉积岩，而下大陆地壳的组成和起源还不清楚。普遍的范例是，下地壳是由致密的岩石组成，富含铁（Fe）和镁（Mg）元素，作为从下面的地幔上升的熔体而就位。然而，变质沉积物在现在暴露在地球表面的下地壳岩石中很常见。这项研究解决了一个问题：沉积物是如何融入下地壳的？这个问题是解决通过重建的压力-温度-时间的下地壳，在意大利的Ivrea-Verbano区的一个特征部分的历史，以评估竞争的假设下大陆地壳的形成。该研究项目结合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，支持来自代表性不足群体的两名研究生的培训，并让本科生参与学术研究。 描述下陆壳的形成是理解地壳演化、体力发展、地壳流动和地球化学分异的基础。使用一套互补的技术，该项目测试三个假设的形成Ivrea-Verbano区：埋藏，地壳底侵，和再层。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石榴石年龄用于确定石榴石在峰值温度下生长和停留的时间。变质锆石和独居石的U/Th-Pb年龄+微量元素丰度获得的激光消融分流岩石年代学，使评估的时间尺度上的伊夫雷亚区组装。岩石学约束来自拟剖面、最佳温压法和微量元素测温法.结合起来，这些技术有利于重建的下降部分的变化，在P-T-T轨迹和峰值变质条件。完整的数据集用于对三个假设进行严格的测试。该项目由地球科学部的构造学项目和岩石学地球化学项目共同资助。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。

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.