CAREER: Testing models of early Earth crust formation and tectonics

板块构造是控制地球动力学的基本过程，包括地震和火山等自然灾害。板块构造是地幔对流的结果。然而，为什么地幔对流导致地球上的板块构造，而不是太阳系中其他也有对流地幔的岩石行星，还不是很清楚。同样，我们也不知道板块构造是什么时候在地球上开始的。这一提议的目标是使用地幔对流和地球早期地壳形成的数值模型来限制板块构造可能开始的时间。对3-4旋回前形成的岩石的化学成分的观察为当时的构造作用提供了关键的制约因素。研究人员的模型将根据这些观测结果测试板块构造和非板块构造的早期地壳生成情景，从而评估哪些构造过程与古代地质记录相一致。这项工作的结果将对整个地球科学产生广泛的意义。限制在早期地球上运行的构造过程有助于揭示板块构造是如何发展的，以及为什么在其他太阳系行星上没有板块构造。这项工作还有助于进一步限制地球大陆是如何形成的，并对早期地球的气候状态和可能的生命环境产生影响。该提案还为科学教育做出了重大贡献。它支持一名研究生，他将执行大部分拟议的工作，从而促进他们的研究生涯。该项目还通过开发远程授课的数据分析和可视化暑期短期课程，为本科教育作出贡献。这一短期课程将面向少数族裔服务机构的地球科学学生，目的是帮助促进地球科学的多样性。该项目还包括从短期课程参与者中挑选学生进行本科生研究实习。这些研究实习将为来自代表性不足群体的学生提供第一手的研究经验，这对他们作为地球科学家的职业生涯的进步至关重要。早期地球的特征是以俯冲和表面板块运动为特征的“活动盖子”模式，还是没有俯冲作用的“停滞盖子”模式，这一点一直备受争议。海甸和太古宙长英质地壳的地球化学观察为当时的构造作用提供了重要的制约因素。然而，仍然存在很大的模糊性，因为俯冲和非俯冲，即在厚厚的地壳高原底部熔融，已经提出了解释地球早期长英质地壳形成的模型。在这里，结合锆石中记录的Hf同位素提供的新数据的地球动力学模型将检验从Hadean到~3.5Ga的俯冲和高原熔融模型。许多太古代克拉通中的锆石中的Hf同位素表明，地球上最早的长英质岩石(3.5Ga)的镁铁质地壳来源在地表持续了100s Myr。更多的岩石学证据表明，今天保存下来的最早的长英质地壳是由镁铁质原壳的浅(30公里)熔融形成的。这些观测将被整合到二维地幔动力学模型中，以测试高原融化和俯冲的情景。建模工作还将探索俯冲在早期地球上是否可行的根本问题。将使用地幔对流的全球模型，包括地壳浮力和粒度演化，以评估在早期地球条件下，地壳浮力是否阻止俯冲，或极大地改变其动力学，以及在俯冲活跃时是否会发生板块融化。这项工作的结果将为早期地球上运行的构造过程提供至关重要的新约束，从而揭示板块构造可能何时开始。这对整个地球的演化有重大影响，包括发电机、地壳，甚至大气和海洋。该提案还涉及一个重要的教育部分，包括培训一名研究生，以及针对地球科学中代表性不足群体的学生开设的数值模拟暑期短期课程和REU方案。缺乏编程和定量技能以及研究机会，可能会成为不同学生在地球科学领域取得进步的障碍；这里开发的教育项目将寻求帮助解决这个问题。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。

Plate tectonics is the fundamental process governing Earth’s dynamics, including natural hazards such as earthquakes and volcanoes. Plate tectonics is a result of convection in Earth’s mantle. However, why mantle convection results in plate tectonics on Earth, but not the other rocky planets in the solar system that also have convecting mantles, is not well understood. Likewise, it is also not well known when plate tectonics started on Earth. The goal of this proposal is to use numerical models of mantle convection and the formation of Earth’s early crust to constrain when plate tectonics might have started. Observations of the chemical composition of rocks formed 3-4 Gyrs ago provide key constraints on the tectonic processes operating at this time. The investigator's models will test both plate-tectonic and non-plate-tectonic scenarios for the generation of early crust against these observations, and thus assess what tectonic processes are compatible with the ancient geologic record. The results of this work will have broad significance across the geosciences. Constraining the tectonic processes that operated on the early Earth sheds light on how plate tectonics developed, and potentially why it is absent on other solar system planets. This work also further helps constrain on how Earth’s continents formed, with implications for the climate state of the early Earth and possible environments for life. The proposal also makes significant contributions to science education. It supports a graduate student who will carry out much of the proposed work, therefore furthering their research career. The project also contributes to undergraduate education through the development of a remotely taught summer short course on data analysis and visualization. This short course will be targeted at geoscience students from minority serving institutions, with the goal of helping promote diversity in the geosciences. The project also includes undergraduate research internships for students to be selected from the short course participants. These research internships will give students from underrepresented groups firsthand research experience, which is critical for progressing in their careers as geoscientists. Whether the early Earth was characterized by a “mobile lid” mode of tectonics, featuring subduction and surface plate motion, or a “stagnant lid” mode where subduction is absent, is highly debated. Geochemical observations of Hadean and Archean felsic crust provide important constraints on the tectonic processes operating at this time. However, there is still significant ambiguity, as both subduction and non-subduction, i.e. melting at the base of a thick crustal plateau, models have been proposed to explain the formation of Earth’s early felsic crust. Here, geodynamical models integrating new data provided by Hf isotopes recorded in zircons will test the subduction and plateau melting models, from the Hadean until ~ 3.5 Ga. Hf isotopes in zircons from many Archean cratons suggest that the mafic crustal source of Earth’s earliest felsic rocks ( 3.5 Ga) persisted at the surface for 100s of Myrs. Additional petrological evidence indicates that the earliest felsic crust still preserved today formed from shallow (30 km) melting of mafic protocrust. These observations will be integrated into two-dimensional mantle dynamic models, to test plateau melting and subduction scenarios. The modeling work will also explore the fundamental issue of whether subduction was feasible on the early Earth. Global models of mantle convection including crustal buoyancy and grain size evolution will be used to assess whether crustal buoyancy prevents subduction, or drastically alters its dynamics, at early Earth conditions, and whether slab melting can occur when subduction is active. The results of the work will provide vital new constraints on the tectonic processes that operated on the early Earth, and thus shed light on when plate tectonics might have begun. This has major implications for the evolution of the Earth as a whole, including the dynamo, crust, and even the atmosphere and oceans. The proposal also involves a significant educational component, including training of a graduate student, and a numerical modeling summer short course and REU program, geared towards students from underrepresented groups in the geosciences. A lack of programming and quantitative skills, as well as research opportunities, can be a barrier for diverse students advancing in the geosciences; the educational program developed here will seek to help address this problem.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.