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

地球上陆壳的起源和生长是地球科学中一个重要而又经久不衰的问题。在整个地球历史上，构造板块的配置和相互作用被认为在地球地壳演化中起着关键作用。加拿大西北地区的Acasta片麻岩杂岩(AGC)暴露出已知的最古老的陆壳岩石。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，以评估板块构造样式(“停滞盖层”与“活动盖层”)在大陆形成中的作用。锆石中的微量元素和同位素比率将使PI能够跟踪AGC地壳成分在大约10亿年的地球历史过程中的变化，从40亿年前开始。由于年龄过大，AGC锆石颗粒会随着地质时间的推移而对晶体结构造成损害。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。这项工作结合了不同的锆石成像、质谱学和结构表征方法，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。锆石表征的结果还将提供有关大陆地壳生产变化以及构造样式与地球上最古老大陆形成之间的联系的信息。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推广，以提高科学素养和扩大公众对地质时代的理解。加拿大的4-3Ga Acasta片麻岩杂岩(AGC)由超过10亿年的岩浆作用组成，包括已知的最古老的长英质陆壳暴露，因此是早期陆地分异的关键档案。矿物锆石对于追踪AGC中岩浆活动的年代学和来源是非常宝贵的；然而，年龄、微量元素和同位素化学信息的普遍变化(通常与辐射损伤导致的结晶度损失和重结晶有关)损害了建立跨越AGC岩浆作用期间的可靠的U-Pb年代学、δ18O和微量元素记录的努力。这项工作将应用一套新颖和多样化的表征方法(等离子体和二次离子质谱仪、拉曼光谱和原子探针层析成像)来：1)扩展AGC锆石颗粒的原始U-Pb同位素、δ18O和微量元素记录，同时扩展全岩地球化学记录，包括构造物源Sc和Nb以及锆石中的蚀变代理OH；2)利用原子探针层析成像技术查询锆石颗粒内反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，这项工作将阐明原生和次生锆石化学、多阶段热和变质历史中元素活动性的长度尺度，以及AGC的原始4-3Ga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推广活动，以提高科学素养并扩大公众对地质时代的理解。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。

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.