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

在太阳系的岩石行星中，地球有一个不寻常的双峰高度分布。今天，大约70%的地球表面位于海平面以下约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。这种双峰式的高度分布驱动了地球表面大部分的质量循环，并推动了地球可居住性的进化。要理解为什么地球是唯一一个具有如此明显双峰性的行星，就必须理解为什么地球有大陆--这是地质学最基本的问题之一。除了这些科学影响外，该项目还将支持一门关于科学插图和实地观察的课程。本科生将接受指导，该团队将与休斯顿地区高中的教师合作，向代表性不足的少数民族介绍地质学和自然历史，并在赖斯的继续教育计划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，这意味着与镁铁质洋壳相比，它富含二氧化硅，缺乏Fe和Mg。但是，当构成板块是镁铁质时，大陆地壳是如何获得长英质成分的，这一点还不清楚。在俯冲带中，地壳的组成与地壳厚度相关，而建立在厚地壳上的弧往往更像大陆地壳那样是长英质的。这一建议旨在了解地壳增厚和分异的基本机制，重点是石榴石在大陆地壳形成中的作用。检验这一假说的关键是石榴石分馏作用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三价铁，研究人员将研究来自北美科迪勒拉山脉和类似地区的样本，如新西兰和西藏。三价铁含量将使用电子微探针侧翼法测定，并根据穆斯堡尔光谱测量进行校准。将对这些地区的火山岩和深成岩进行微量元素和氧气压限制，以评估分异岩浆是否经历了氧化和石榴石分馏。岩浆和堆晶岩的联合研究将提供大陆成壳岩浆作用的综合观点。大陆地壳的形成是气候、构造、岩浆活动和地球深部之间反馈的结果。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。

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.