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

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。在整个地球历史中，构造板块的构造和相互作用被认为在地壳演化中起着关键作用。位于加拿大西北地区的奥陶洛斯塔片麻岩杂岩（AGC）是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，以评估板块构造样式（“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率将使PI能够跟踪从40亿年前开始的大约10亿年地球历史过程中AGC地壳成分的变化。由于其极端的年龄，AGC锆石颗粒积累了地质时期的晶体结构的损害。因此，许多（虽然不是全部），锆石颗粒的年龄和化学性质发生了变化。这项工作结合了锆石的成像，质谱和结构表征的各种方法，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。锆石的定性结果还将提供有关大陆地壳生产变化的信息，以及构造样式与地球上最古老大陆形成之间的联系。这项研究还将支持社区外展与威斯康星州地质博物馆大学合作，以提高科学素养和扩大公众对地质时代的理解。4至3 Ga的片麻岩复杂（AGC），加拿大，包括超过十亿年的岩浆活动，包括已知最古老的长英质地壳暴露，因此是早期地球分化的重要档案。锆石矿物对于追踪AGC岩浆活动的年代学和来源是非常宝贵的;然而，普遍存在的年龄、微量元素和同位素化学信息（通常与辐射损伤引起的结晶度和重结晶损失有关）的变化削弱了构建跨越AGC岩浆活动持续时间的U-Pb年代学、δ 18 O和微量元素记录的努力。这项工作将采用一套新颖多样的表征方法（等离子体和二次离子质谱、拉曼光谱和原子探针层析成像）：1）扩展了AGC锆石颗粒的原生U-Pb同位素、δ 18 O和微量元素记录，并与全岩地球化学记录相结合，包括构造源区代用指标Sc和Nb以及锆石中的蚀变代用指标OH;以及2）通过原子探针层析成像术来询问锆石颗粒内的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，这项工作将澄清原生与次生锆石化学，元素活动性的长度尺度在多阶段的热历史和变质历史，以及主要的4-3 Ga岩浆和构造历史的AGC。这项研究还将支持社区外展与威斯康星州地质博物馆的大学合作，以提高科学素养和扩大公众对地质时代的理解。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.