CAREER: Testing models of early Earth crust formation and tectonics

职业：测试早期地壳形成和构造的模型

• 批准号: 2046598
• 负责人: Bradford Foley
• 金额: $ 67.17万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-06-15 至 2026-05-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2046598&HistoricalAwards=false
• 关键词: CAREER; Testing; models; early; Earth

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.

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岩石行星也有对流地幔，还没有得到很好的理解。同样，板块构造何时开始在地球上也不清楚。这项提议的目的是利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 Gyrs前形成的岩石的化学成分的观察提供了对此时运行的构造过程的关键约束。研究人员的模型将测试板块构造和非板块构造的情况下产生的早期地壳对这些观察，从而评估什么构造过程是兼容的古代地质记录。这项工作的结果将在整个地球科学领域具有广泛的意义。限制早期地球上的构造过程有助于了解板块构造是如何发展的，以及为什么它在其他太阳系行星上不存在。这项工作还有助于进一步限制地球大陆的形成方式，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。它支持一个研究生谁将进行大部分拟议的工作，从而促进他们的研究生涯。该项目还通过开发关于数据分析和可视化的远程教学夏季短期课程，为本科生教育做出贡献。这一短期课程的对象是来自少数民族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选出的学生的本科生研究实习。这些研究实习将为来自代表性不足的群体的学生提供第一手的研究经验，这对于他们作为地球科学家的职业发展至关重要。早期地球的特征是具有“移动的盖”构造模式，以俯冲和表面板块运动为特征，还是没有俯冲的“停滞盖”模式，这一点存在很大争议。地球化学观测的Hadean和太古代长英质地壳提供了重要的制约构造过程中运作的这个时候。然而，由于俯冲和非俯冲，即在厚地壳高原底部熔融，仍然存在重大的模糊性，已经提出了解释地球早期长英质地壳形成的模型。在这里，地球动力学模型集成了锆石中记录的Hf同位素提供的新数据，将测试俯冲和高原熔融模型，从Hadean直到~ 3.5 Ga。来自许多太古代玄武岩的锆石中的Hf同位素表明，地球最早的长英质岩石（3.5 Ga）的镁铁质地壳源在地表持续了数百万年。其他岩石学证据表明，最早的长英质地壳仍然保留今天形成浅（30公里）熔融的镁铁质原地壳。这些观测结果将被纳入二维地幔动力学模型，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基本问题。地幔对流的全球模型，包括地壳浮力和粒度演变将被用来评估是否地壳浮力阻止俯冲，或大大改变其动态，在地球早期条件下，以及是否板熔融时，俯冲是活跃的。这项工作的结果将为早期地球的构造过程提供重要的新约束，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组成部分，包括培训一名研究生，以及一个数值模拟夏季短期课程和REU计划，面向来自地球科学代表性不足群体的学生。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进步的障碍;这里开发的教育计划将寻求帮助解决这个问题。这个奖项反映了NSF的法定使命，并被认为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来支持。