Building Earth's oldest crust: geochemistry and geochronology of Archean cratons and early mantle

构建地球最古老的地壳：太古代克拉通和早期地幔的地球化学和地质年代学

• 批准号: RGPIN-2020-06323
• 负责人: ONeil, Jonathan
• 金额: $ 1.82万
• 依托单位: University of Ottawa
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=717329
• 关键词: Building; Earth; oldest; crust; geochemistry

Ancient terrains provide the only constraints on the nature and evolution of Earth's earliest crust. Only few terrains older than 3.5 billion years old (Ga) are preserved, limiting our understanding of the processes that formed our first continents. Moreover, with its active tectonics, the Earth is very efficient at reworking and recycling its own crust. The primitive crust, formed shortly after Earth's formation, has therefore mostly been destroyed and re-melted to form younger rocks. My main objective is to constrain the chemical nature and geological processes that formed the first terrestrial crust as well as the impact on the early chemical evolution of Earth's mantle. Archean cratons are the most ancient cores of our continents. They are mostly composed of rocks formed between 2.5 to 3 Ga, leaving more than the first billion years of Earth's crustal history difficult to study. This Archean crust can, however, contain the isotopic signature of its older precursor. With the increasing precision of analytical techniques, we can use new isotopic tools to better extract information from ancient terranes. This will allow us to build an improved picture of how and when our first continents formed and investigate the nature of Earth's primitive crust. This project combines multiple geochemical and isotopic tools to understand the earliest crustal history, leading to the stabilization of the first continents. Isotopic systems such as 147Sm-143Nd and 176Lu-176Hf have successfully been used to study the evolution of the crust. However, because these tracers are also sensitive to geological processes occurring well after the formation of the early Archean crust, they can often be disturbed and thus make the earliest crustal history difficult to decipher. This approach also often requires that researchers make assumptions about the nature of the older crust that served as a precursor source to build Archean cratons. Coupling conventional geochronology and long-lived isotopic systems to novel short-lived isotopic systems such as 146Sm-142Nd can provide a much better understanding of the early crustal history. The 146Sm-142Nd system is only sensitive to geological processes occurring before 4 Ga and is thus the ideal tracer to study the involvement of primitive crust in the formation of Archean cratons. This system allows us to clearly establish whether Hadean crust or mantle were involved in the formation of the Archean continents. Combining short-lived and long-lived isotopic systems will give us the most complete understanding of how our oldest continents formed. The main areas of research will be 1) the Superior craton (Northern Quebec, Ontario and Manitoba) representing one of the largest extents of Archean crust and possibly including the oldest rocks on Earth; 2) the Nain Province (Labrador) with rocks as old as 3.9 Ga; 3) the Archean crust from Brazil, which is still largely unstudied; and 4) the Pilbara craton (Australia), as old as 3.5 Ga.

古代地形是对地球最早地壳的性质和演化的唯一限制。只有少数超过 35 亿年 (Ga) 的地形被保存下来，这限制了我们对第一块大陆形成过程的理解。此外，由于其活跃的构造，地球在改造和回收其地壳方面非常有效。因此，地球形成后不久形成的原始地壳大部分被破坏并重新熔化，形成更年轻的岩石。我的主要目标是限制形成第一个地壳的化学性质和地质过程以及对地幔早期化学演化的影响。太古代克拉通是我们大陆最古老的核心。它们大多由 2.5 至 3 Ga 之间形成的岩石组成，使得地球地壳历史的前十亿年以上难以研究。然而，这个太古宙地壳可能含有其较古老前体的同位素特征。随着分析技术精度的不断提高，我们可以使用新的同位素工具更好地从古代地体中提取信息。这将使我们能够更好地了解第一批大陆如何以及何时形成，并研究地球原始地壳的性质。该项目结合了多种地球化学和同位素工具来了解最早的地壳历史，从而实现第一批大陆的稳定。 147Sm-143Nd和176Lu-176Hf等同位素系统已成功用于研究地壳演化。然而，由于这些示踪剂对早期太古代地壳形成后发生的地质过程也很敏感，因此它们经常会受到干扰，从而使最早的地壳历史难以破译。这种方法通常还要求研究人员对作为构建太古宙克拉通前体来源的旧地壳的性质做出假设。将传统地质年代学和长寿命同位素系统与新颖的短寿命同位素系统（例如 146Sm-142Nd）耦合可以更好地了解早期地壳历史。 146Sm-142Nd系统仅对4 Ga之前发生的地质过程敏感，因此是研究太古代克拉通形成过程中原始地壳参与情况的理想示踪剂。这个系统使我们能够清楚地确定太古代大陆的形成是否涉及冥古宙地壳或地幔。结合短寿命和长寿命的同位素系统将使我们对最古老的大陆如何形成有最全面的了解。主要研究领域是：1）苏必利尔克拉通（魁北克省北部、安大略省和马尼托巴省）代表了太古代地壳最大范围之一，可能包括地球上最古老的岩石； 2) 纳因省（拉布拉多省）的岩石年龄可达 3.9 Ga； 3）来自巴西的太古宙地壳，其很大程度上仍未被研究； 4) 皮尔巴拉克拉通（澳大利亚），年龄为 3.5 Ga。