CAREER: Is Continental Crust Juvenile or Reworked? A Test of Growth Models Using the Extant Neoarchean Granitoid Record

目前，月球上的地质样本比太阳系形成的前5亿年里我们地球上的地质样本还要多。样品的缺乏导致地质学界就地球演化的三个非常重要的方面展开了激烈的辩论：1)何时，2)如何，以及3)大陆在早期地球上出现的程度。最近技术能力的进步使这些问题得以重新审视。地球是一个构造活跃的行星，这意味着古老的岩石不断被改变，留下的地球历史最早阶段的原始样本很少。通过在保存的岩石样本中寻找非常小但重要的同位素特征，人们可以推断出在地球历史的早期有多少大陆地壳。该项目将资助对26亿年前的岩石进行先进的同位素分析，以寻找超过40亿年前的大陆物质的证据，这些物质可能在后来的构造事件中被重新加工过。收集这些数据将使研究小组能够验证早期地球上存在大量古代大陆地壳的假设，并回答关于行星形成和演化的一个基本问题——大陆地壳是什么时候在地球上形成的？该项目还将支持本科生实地研究经验，这将有助于培养下一代地球科学家的技能，这些技能是雇主非常感兴趣的——实地地质学和空间推理技能。这一建议将结合岩石学和同位素地球化学来检验关于大陆地壳生长和改造的广泛假设。新太古代花岗岩类岩石是地球上保存最完好的最古老的岩石群。对样品的详细分析将解决大陆地壳何时在地球上形成的基本问题。该提案将分析来自北美三个不同克拉通的花岗岩类，以获得它们的火成岩化学特征。将从几个组成组中测量锆石年龄和同位素信息（U-Pb-Hf-O），以测试用于地壳熔化和套印的常用示踪剂的保真度。对异晶锆石和大块岩石nd同位素比率的分析将用于寻找新太古代（4.0 Ga）地壳的改造，这一时期被认为经历了大陆记录的广泛重叠。研究小组专注于142号分析，因为这些数据特别擅长识别冥古宙地壳遗迹。该提案将资助REU现场经验，旨在通过现场地质和空间推理培训培养多样化和具有战略重要性的劳动力。本科生研究人员绘制的地图和收集的样本将用于科学研究过程中进行的高级同位素分析，本科生参与者不仅将获得重要的、急需的空间批判性思维技能，还将接触到先进的地球化学技术和数据集。这些参与者将完成他们的本科学习，为从事地球科学领域的多种职业做好准备。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。

There currently exist more geological samples from the Moon than from our own planet during the first 500 million years of the Solar System. This dearth of samples has led to vigorous debate amongst the geological community regarding three very important aspects of Earth evolution: 1) when, 2) how, and 3) to what extent did continents emerge on the early Earth. Recent advances in technical capabilities now allow these questions to be revisited. Earth is a tectonically active planet, which means that old rocks are constantly being altered, leaving few pristine samples from the earliest phases of Earth history. By looking for very small, but important isotopic signatures in preserved rock samples, one may infer how much continental crust was around very early in Earth history. This project will fund advanced isotopic analyses of ~2.6-billion-year-old rocks in a search of evidence for great than 4.0-billion-year-old continental material, material that may have been reworked during later tectonic events. Collection of this data will allow the research team to test the hypothesis that very large volumes of ancient continental crust existed on the early Earth and to answer a fundamental question about planetary formation and evolution – when did continental crust form on Earth? This project will also support an undergraduate field research experience, which will serve to train the next generation of geoscientists in skills that employers are keenly interested in – field geology and spatial reasoning skills.This proposal will combine petrology and isotope geochemistry to test broad hypotheses regarding the growth and reworking of continental crust. Neoarchean granitoid rocks represent some of the oldest well-preserved suites of rocks on Earth. Detailed analysis of samples will address the fundamental question of when continental crust formed on Earth. This proposal will analyze suites of granitoids from three distinct North American cratons for their igneous chemical signatures. Zircon age and isotopic information (U-Pb-Hf-O) will be measured from several compositional groups to test the fidelity of commonly used tracers for crustal melting and overprinting. Analyses of xenocrystic zircons and bulk rock Nd-isotope ratios will be used to search for reworking of truly ancient (4.0 Ga) crust in the Neoarchean—a time that has been suggested to have experienced broad overprinting of the continental record. The research team focus on 142Nd analyses as these data are particularly adept at identifying Hadean crustal relics. This proposal will fund an REU field experience aimed at developing a diverse and strategically important workforce with training in field geology and spatial reasoning. The maps created and samples collected by undergraduate researchers will be used for advanced isotopic analyses conducted during the scientific study, and undergraduate participants will not only gain important—and in-demand—skills in spatially-focused critical thinking but will also be exposed to advanced geochemical techniques and datasets. These participants will finish their undergraduate experience well positioned to pursue many types of careers in the geosciences.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.