Synmagmatic crustal thickening and the importance of garnet fractionation in making continental crust

在太阳系的岩石行星中，地球具有不寻常的双峰性。今天，大约70%的地球表面位于海平面以下~4公里，而地球表面的其余部分位于海平面以上，表现为大陆。这种双峰假设驱动了地球表面的大量循环，并推动了地球宜居性的进化。理解为什么地球是唯一一个具有如此明显的双峰性的行星，取决于理解地球为什么有大陆——这是地质学中最基本的问题之一。除了这些科学影响外，该项目还将支持一门关于科学说明和实地观察的课程。该团队将与休斯敦地区高中的教师合作，向未被充分代表的少数民族介绍地质学和自然历史，并在莱斯大学的继续教育项目和休斯顿自然科学博物馆提供短期课程和讲座。大陆地壳是长英质，这意味着与基性海洋地壳相比，它富含二氧化硅，缺乏铁和镁。但是，当大陆地壳的构造块是铁质的时候，大陆地壳是如何形成其硅质成分的还不清楚。在俯冲带，地壳成分与地壳厚度相关，而建立在厚地壳上的弧往往更像大陆地壳。这一建议试图了解连接地壳增厚和分化的基本机制，重点是石榴石在大陆地壳形成中的作用。验证这一假设的关键是石榴石分馏是否驱动残余岩浆的氧化或还原。因此，目标是量化石榴石中有多少铁是铁，研究人员将研究来自北美科迪勒拉和类似地区（如新西兰和西藏）的样品。铁含量将使用电子探针侧面法测定，根据穆斯堡尔光谱测量校准。将对这些地区的火山岩和深成岩进行微量元素和氧压约束，以评价分异岩浆是否经历了氧化和石榴石分馏。岩浆和堆积物的联合研究将对大陆地壳形成岩浆作用提供一个全面的认识。大陆地壳的形成是气候、构造、岩浆活动和地球深部相互作用的结果。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 has an unusual bimodal hypsometry among the rocky planets in the solar system. Today, about 70% of Earth's surface lies below ~4 km below sea level while the remaining parts of the Earth's surface ride above sea level, manifested as continents. This bimodal hypsometry drives much of the mass cycling on Earth's surface and fuels the evolution of Earth's habitability. Understanding why Earth is the only planet with such a pronounced bimodality rests on the understanding of why Earth has continents - one of the most fundamental questions of geology. In addition to these scientific impacts, this project will support a course taught on scientific illustration and field observation. Undergraduates students will be mentored, and the team will collaborate with instructors in the Houston area high schools to introduce geology and natural history to under-represented minorities, and provide short courses and lectures at Rice's continuing education program and the Houston Museum of Natural Science.The continental crust is felsic, meaning that it is enriched in silica and depleted in Fe and Mg compared with the mafic oceanic crust. But how the continental crust achieved its felsic composition when the building blocks are mafic is unclear. In subduction zones, crustal composition correlates with crustal thickness and arcs built on thick crust tend to be more felsic, like the continental crust. This proposal seeks to understand the fundamental mechanism linking crustal thickening and differentiation, with an emphasis on the role of garnet in the formation of the continental crust. The key to testing the hypothesis is whether garnet fractionation drives oxidation or reduction of the residual magma. Thus, the objective is to quantify how much of the iron in garnet is ferric, and the researchers will study samples from the North American Cordillera and analog localities, such as New Zealand and Tibet. Ferric iron content will be determined using the electron microprobe flank method, calibrated against Mossbauer spectroscopic measurements. Trace element and oxybarometric constraints on volcanic and plutonic rocks from these areas will be conducted to evaluate whether differentiated magmas have experienced oxidation and garnet fractionation. Combined investigation of magmas and cumulates will provide a comprehensive view of continental crust-forming magmatism. The formation of continental crust is the culmination of feedbacks between climate, tectonics, magmatism, and the deep Earth.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.