Collaborative Research: Creating Earth’s earliest continents—an integrated investigation of the growth and modification of western Australia’s Pilbara Craton

地球上最早大陆的形成是地球科学中一个非常有趣的话题。一个主要问题是，从地球早期历史中保存下来的岩石非常罕见；地球历史上最初10亿年的岩石很少，而最初5亿年的岩石也没有保存完好。地质学家必须使用非常零碎的记录来确定第一个大陆地壳是什么时候形成的，它的成分是什么，以及它有多少。这笔拨款将支持地质学家研究地球上最古老的、未经修改的构造板块碎片——西澳大利亚的皮尔巴拉地块——来解决这些问题。方法是首先通过矿物（如锆石）的形成来确定岩石的年龄，这些矿物起到放射性时钟（地质年代学）的作用。一旦知道了年龄，岩石的化学成分就可以确定它们是来自早期的地壳还是直接从地幔中提取出来的（地球化学）。这些岩石的模式和结构（构造地质学），结合地球化学和地质年代学，将决定这些岩石是如何以及何时加入地壳形成这些早期大陆的。这项工作的主要成果将是更好地了解地球最早地壳的历史及其形成过程。这项工作包括学生培训、指导和支持一名初级科学家（博士后）以及国际合作。此外，与认知科学家一起工作将帮助学生想象大时间尺度上的地质过程。该提案将允许开发复杂的地球化学和地质年代学分析技术，并允许国内和国际访客在华盛顿州立大学同位素实验室使用这些技术。本提案旨在通过综合的年代学、地球化学和构造研究，通过检查花岗岩杂岩——埃德加山和科伦纳唐斯——以及西澳大利亚3.5 - 2.8 Ga东皮尔巴拉地体的相关壳上岩石，来解决地壳形成、改造和稳定的基本过程。东皮尔巴拉地体是地球上保存最完好的古-中太古代地壳之一，呈现出典型的穹窿-龙骨模式，其中花岗岩杂岩（穹窿）被陡峭倾斜的火山和火山碎屑壳上岩石地层（龙骨）所包围。两个相互关联的核心问题指导我们的研究：1)花岗岩复合体的就位机制和时间是什么；2)东皮尔巴拉地体的地幔和地壳源区是什么。为了研究这些问题，我们的工作将集中在：1)组成和变形非均质花岗岩穹窿杂岩的地球化学和结构；2)上地壳岩石地层变形区（“绿岩龙骨”）的时代、变质条件和构造；3)最小变形区上地壳岩石地层中的变质火山岩和花岗岩穹丘的地球化学特征，以确定地幔和地壳的源区。这种综合的地球化学和构造方法将提供有关一级构造过程的信息，这些构造过程导致了地球上最早的克拉通之一的形成、改造和稳定，因此，为地球上一些最早的大陆地壳的形成和保存提供了重要的见解。该项目得到了岩石学与地球化学和构造学两个项目的支持。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。

The formation of Earth’s earliest continents is topic of significant interest in the Earth Sciences. A major issue is that rocks preserved from Earth’s earliest history are rare; there are few rocks from the first billion years of Earth’s history, and no intact rocks at all remaining from the first 500 million years. Geologists must use a very fragmental record to determine when first continental crust formed, what was it composition, and how much of it existed. This grant will support geologists to study the Earth’s oldest, unmodified fragment of a tectonic plate – the Pilbara terrane of West Australia – to address these issues. The approach is to first determine the age of the rocks through the formation of minerals, such as zircons, that act a radioactive clocks (geochronology). Once the age is known, the chemistry of the rocks can determine if they were derived from an earlier crust or extracted directly from the Earth’s mantle (geochemistry). The patterns and fabrics of these rocks (structural geology), when integrated with geochemistry and geochronology, will determine how and when the rocks were added to the crust to create these early continents. The major outcome of this work will be a better understanding of the history of the Earth’s earliest crust and the processes that formed it. The work includes student training, mentorship and support for a junior scientist (post-doctoral fellow), and international collaboration. Additionally, work with a cognitive scientist will help students envision geological processes over large time scales. The proposal will allow technique development of sophisticated geochemical and geochronologic analyses and their use in the WSU isotope laboratory by national and international visitors.This proposal seeks to address the fundamental processes of crustal formation, modification, and stabilization by examining granitic complexes—the Mt Edgar and Corunna Downs—and associated surpracrustal rocks of the 3.5 – 2.8 Ga East Pilbara terrane of West Australia, through an integrated geochronological, geochemical, and structural study. The East Pilbara terrane is one of the best-preserved examples of Paleo- to Mesoarchean crust on Earth, displaying the classic dome-and-keel pattern in which granitic complexes (domes) are surrounded by steeply dipping volcanic and volcaniclastic supracrustal lithostratigraphy (keels). Two central, interlinking questions guide our research: 1) What were the mechanisms and timing of emplacement of the granitic complexes; and 2) What were the mantle and crustal source regions for the rocks of the East Pilbara terrane. To investigate these questions, our work will focus on: 1) The geochemistry and structure of the compositionally and deformationally heterogeneous granitic dome complexes; 2) The timing, metamorphic conditions, and structure in the deformed regions of the supracrustal lithostratigraphy (“greenstone keels”); and 3) The geochemistry of the metavolcanic rocks in the supracrustal lithostratigraphy and the grantic domes, in regions of minimal deformation, to address the mantle and crustal source regions. This integrated geochemical and structural approach will provide information on first-order tectonic processes that resulted in the formation, modification, and stabilization of one of Earth’s earliest cratons and, therefore, provide important insights into the formation and preservation on some of the earliest continental crust on Earth.This project is supported by both the Petrology & Geochemistry and Tectonics programs.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.