A macro- to nano-scale interrogation of 4 Gyr of tectonism, metamorphism and alteration in the Acasta Gneiss Complex

地球大陆地壳的起源和生长是地球科学中一个重要而持久的问题。在整个地球历史上，构造板块的形态和相互作用被认为在地球地壳演化中起着至关重要的作用。加拿大西北地区的阿卡斯塔片麻岩杂岩（AGC）展示了已知最古老的大陆地壳岩石。该项目将详细描述AGC中一种重要矿物锆石的地球化学特征，以评估板块构造风格（“停滞盖”与“活动盖”）在大陆形成中的作用。锆石中微量元素和同位素的比例将使pi能够追踪从40亿年前开始的大约10亿年地球历史中AGC地壳成分的变化。由于它们的极端年龄，AGC锆石颗粒在地质时期对晶体结构的破坏不断累积。因此，阿卡斯塔的许多（尽管不是全部）锆石颗粒的年龄和化学成分都发生了变化。这项工作结合了各种成像、质谱和锆石结构表征方法，使pi能够识别未改变的锆石化学成分，并评估蚀变过程的时间。锆石特征的结果还将提供有关大陆地壳生产变化的信息，以及构造风格与地球上最古老大陆形成之间的联系。这项研究还将支持与威斯康星大学地质博物馆合作的社区外展活动，以提高科学素养，扩大公众对地质时间的理解。加拿大4 ~ 3 Ga阿卡斯塔片麻岩杂岩（AGC）包含了超过10亿年的岩浆活动，包括已知最古老的长英质地壳暴露，因此是早期地球分异的重要档案。锆石矿物对AGC的年代学和岩浆活动来源研究具有重要意义；然而，年龄、微量元素和同位素化学信息的普遍变化（通常与辐射损伤引起的结晶度损失和再结晶有关）削弱了构建跨越AGC岩浆活动持续时间的可靠的U-Pb年代学、δ18O和微量元素记录的努力。本工作将采用一套新颖多样的表征方法（等离子体和次级离子质谱、拉曼光谱和原子探针层析成像）：1)结合全岩地球化学记录，扩展AGC锆石颗粒的原始U-Pb同位素、δ18O和微量元素记录，包括锆石中的构造物源代表Sc和Nb以及蚀变代表OH；2)利用原子探针层析成像技术研究锆石颗粒内重复热事件和流体相互作用的颗粒尺度到纳米尺度的表现。最终，该工作将阐明AGC的原生和次生锆石化学性质、多期热变质史中元素迁移的长度尺度，以及AGC的原生4-3 Ga岩浆和构造历史。这项研究还将支持与威斯康星大学地质博物馆合作的社区外展活动，以提高科学素养，扩大公众对地质时间的理解。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。

The origin and growth of continental crust on Earth is an important and enduring question in the Earth sciences. The configuration and interaction of tectonic plates throughout Earth’s history is recognized to play a critical role in Earth’s crustal evolution. The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, exposes the oldest known continental crustal rocks. This project will characterize the detailed geochemistry of an important mineral, zircon, in the AGC to evaluate the role of plate tectonic style (“stagnant-lid” vs. “mobile-lid”) in continent formation. Ratios of trace elements and isotopes in zircon will allow the PIs to track changes in AGC crustal compositions over the course of approximately 1 billion years of Earth’s history, starting 4 billion years ago. Because of their extreme age, AGC zircon grains accumulate damage to the crystal structure over geologic time. As a result, many (though not all), zircon grains in Acasta have had their age and chemistry altered. This work combines diverse methods of imaging, mass spectrometry and structural characterization of zircon, allowing the PIs to identify unaltered zircon chemistries and to evaluate the timing of alteration processes. Results of the characterization of zircon will also provide information about changes in continental crustal production and links between tectonic style and the formation of the oldest continents on Earth. This research will also support community outreach in collaboration with the University of Wisconsin Geology Museum to improve scientific literacy and expand the public’s understanding of geologic time.The 4 to 3 Ga Acasta Gneiss Complex (AGC), Canada, comprises over one billion years of magmatism, including the oldest known exposures of felsic terrestrial crust, and thereby is a critical archive of early terrestrial differentiation. The mineral zircon has been invaluable for tracking the chronology and sources of magmatism in the AGC; however, pervasive alteration of age, trace element, and isotope chemical information (often related to radiation-damage-induced loss of crystallinity and recrystallization) has impaired efforts to construct robust U-Pb geochronological, δ18O, and trace element records spanning the duration of AGC magmatism. This work will apply a novel and diverse set of characterization methods (plasma and secondary ion mass spectrometry, Raman spectroscopy, and atom probe tomography) to: 1) expand the primary U-Pb isotope, δ18O, and trace element record for AGC zircon grains in tandem with the whole-rock geochemical record, including tectonic provenance proxies Sc and Nb as well as alteration proxy OH in zircon; and 2) interrogate the grain-scale to nanoscale manifestations of repeated thermal events and fluid interactions within zircon grains by atom probe tomography. Ultimately, the work will clarify primary versus secondary zircon chemistry, the length scales of element mobility during a multistage thermal and metamorphic history, and the primary 4-3 Ga magmatic and tectonic history of the AGC. This research will also support community outreach in collaboration with the University of Wisconsin Geology Museum to improve scientific literacy and expand the public’s understanding of geologic time.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.