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: 2022
• 资助国家: 加拿大
• 起止时间: 2022-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=751210
• 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.

古代地形提供了对地球最早地壳的性质和进化的唯一约束。只有少数超过35亿年历史（GA）的地形限制了我们对构成第一大洲的过程的理解。此外，凭借其积极的构造，地球在重新加工和回收自己的外壳方面非常有效。因此，原始地壳是在地球形成后不久形成的，因此大多被破坏并重新融化以形成年轻的岩石。我的主要目的是限制形成第一个陆地外壳的化学性质和地质过程，以及对地球地幔早期化学演化的影响。大将克拉通是我们大洲最古老的核心。它们主要由在2.5至3 GA之间形成的岩石组成，而难以研究地球纪录的前十亿年。然而，这种大古壳可以包含其较旧前体的同位素特征。随着分析技术的越来越精确，我们可以使用新的同位素工具来更好地从古代地形中提取信息。这将使我们能够建立改进的图片，说明我们的第一大洲如何以及何时形成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，以了解最早的地壳历史，从而导致第一大洲的稳定。诸如147SM-143ND和176LU-176HF之类的同位素系统已成功地用于研究地壳的演变。但是，由于这些示踪剂也对早期大帝地壳形成后发生的地质过程也很敏感，因此通常会受到干扰，因此使最早的地壳历史难以破译。这种方法还经常要求研究人员对旧地壳的性质做出假设，这些外壳是建造大帝克拉通的前体来源。将常规的年代学和长期寿命的同位素系统耦合到新型短寿命的同位素系统（例如146SM-142ND）可以更好地了解早期的地壳历史。 146SM-142ND系统仅对在4 GA之前发生的地质过程敏感，因此是研究原始地壳参与Archean Cratons形成的理想示踪剂。该系统使我们能够清楚地确定Hadean Crust或Mantle是否参与了大陆的形成。将短暂的和长寿命的同位素系统结合在一起，将使我们对最古老的大陆的形成方式有最完整的了解。研究的主要领域将是1）上级克拉顿（魁北克北部，安大略省和曼尼托巴省），代表了大老地壳的最大范围之一，可能包括地球上最古老的岩石； 2）纳因省（拉布拉多），岩石古老的岩石年龄为3.9 ga； 3）来自巴西的太古壳，这仍然很大程度上没有研究； 4）Pilbara Craton（澳大利亚），年龄为3.5 GA。