EAR-PF: Geochemical Evidence for the onset of Plate Tectonics on the Early Earth from the Wyoming Province and Singhbhum Craton

板块构造是一种理论，它认为地球的最外层，即地壳，被分成不同的板块，这些板块穿过地幔（地壳正下方的一层）。这种板块运动导致了地球上大多数的火山和地震，并被认为是地球上生命形成的重要控制因素。然而，关于板块构造何时开始存在分歧。这项工作旨在回答地球科学中的一个关键问题：板块构造何时、为何以及如何开始？为了了解这些变化，将使用一种叫做锆石的矿物。锆石之所以重要，是因为它具有物理和化学的弹性，形成环境的多样性，以及它提供的丰富的化学信息。它也是40.1亿年前（Ga）之前唯一的岩石遗迹。先前的研究发现了锆石源岩在38亿Ga到大约3.6 Ga之间发生变化的证据（Bauer et al. 2020）。这种变化被认为表明了地球板块构造过程的变化。然而，要了解板块构造在地球历史的最初~ 10亿年间是否活跃，还需要更多的信息，包括关于地表或近地表水的范围和岩石类型变化的信息。通过对怀俄明州和兴兴克拉通两个研究地点冥古宙（4.01 Ga）和太古宙（2.5 Ga至4.01 Ga）锆石的痕量和稀土元素、年龄、氧同位素、硅同位素和矿物包裹体的高分辨率空间分析，将对地球表面和内部约10亿年的历史进行解释。这些研究将探索这些时期地球上液态水的时空范围、构造转变和地壳组成，并进一步限制这些早期时期的地壳过程。该项目将包括对哈佛大学本科生的指导，为高中生提供研究经验，并通过博物馆、哈佛大学和其他社区项目的活动向大波士顿社区推广。该项目将分析来自怀俄明州和兴兴克拉通的冥古宙和太古代锆石，以扩大人们对早期地球的认识。具体而言，将研究三个主题：1)支持和反对地球早期不同时期现代俯冲的证据；2)早期地球地表或近地表水的时空分布；3)早期地球岩石类型的分布。锆石的原位分析将包括Si-O耦合同位素、微量元素和包裹体研究的测量。通过这个项目，最初设计用于了解背景锆石或那些具有非常著名的地质起源的技术将应用于地球最早历史的样本。在此过程中，将面临与这些技术的可伸缩性相关的假设。例如，我们是否可以假设，在过去几亿年中支配地球的物理过程，如板块构造，在40多亿年前就开始起作用了？如果不是，那么在早期地球上是什么过程在起作用呢？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。

Plate tectonics is the theory that the Earth’s outermost layer, which is called the crust, is divided into different plates which move across the mantle (the layer directly below the crust). This plate movement causes most volcanos and earthquakes on Earth and is theorized to be an important control on life formation on Earth. However, there is disagreement on when plate tectonics began. This work seeks to answer a crucial question in the earth sciences: When, why, and how did plate tectonics start? To understand these changes, a mineral called zircon will be used. Zircon is important because of its physical and chemical resilience, the wide variety of environments where it forms, and the rich chemical information it provides. It is also the only remnant of rock from before 4.01 billion years ago (Ga). Previous studies found evidence of a change in the source rocks of zircon between 3.8 billion Ga and approximately 3.6 Ga (Bauer et al. 2020). This variation has been argued to indicate a change in plate tectonic processes on Earth. However, more information is needed to understand whether plate tectonics was active during the first ~1 billion years of Earth’s history, including information on the extent of surface or near-surface water and variations in rock types. Through high-resolution spatial analysis of trace and rare earth elements, ages, oxygen isotopes, silicon isotopes, and mineral inclusions of Hadean ( 4.01 Ga) and Archean (2.5 Ga to 4.01 Ga) zircon from two under-investigated sites, the Wyoming Province and the Singhbhum Craton, interpretations of ~1 billion years of history related to Earth’s surface and interior will be made. These investigations will explore the spatial and temporal extent of liquid water on Earth, tectonic transitions, and crustal composition during these periods and provide further constraints on crustal processes during these early eons. This project will include mentorship of undergraduate students at Harvard, research experiences for high school students, and outreach to the greater Boston community through events at museums, Harvard and other community programs. In this project, Hadean and Archean zircons from the Wyoming Province and the Singhbhum Craton will be analyzed in order to expand people’s understanding of early Earth. Specifically, three topics will be investigated: 1) evidence for and against modern-style subduction at different periods in early Earth’s history, 2) the spatial and temporal distribution of surface or near-surface water on early Earth, and 3) the distribution of rock types on early Earth. In-situ analyses of dated zircons will include measurements of Si-O coupled isotopes, trace elements, and inclusion studies. Through this project, techniques originally designed to understand in-context zircons, or those with a very well-known geologic origin, will be applied to samples from Earth’s earliest history. Within this, assumptions related to the scalability of these techniques will be confronted. For example, can it be assumed that the physical processes which have governed Earth over the past few hundred million years, such as plate tectonics, were in effect over 4 billion years ago? If they were not, what processes operated on early Earth?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.