Evolution of Earth's Earliest Crust

地球最早地壳的演化

• 批准号: RGPIN-2016-04479
• 负责人: Chacko, Thomas
• 金额: $ 3.06万
• 依托单位: University of Alberta
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=688464
• 关键词: Evolution; Earth; Earliest; Crust

Earth's continental and oceanic crust, the combination of which is likely unique in the solar system, plays a central role in the operation of our planet's lithosphere, atmosphere, and biosphere. The mechanisms by which this crust formed and stabilized early in Earth history remains one of the outstanding questions in the geological sciences. In this application, I propose to address this question by expanding my on-going research program on the Acasta Gneiss Complex (AGC) of the Northwest Territories. The AGC arguably contains the oldest rocks in the world (4.02 billion years old) and is certainly one of the few locations worldwide where evidence can be brought to bear on the first one billion years of Earth history. Despite its status as an old crust locality, much of the AGC remains unstudied. In particular, all previous studies of the AGC have focused exclusively on a ~30 km2 area, which makes up a small fraction of the complex as a whole (~1300 km2). The geology of the broader AGC is largely unknown but reconnaissance work by my group indicates that at least 700 km2 of the complex comprises rocks older than 3.3 Ga. I propose to increase our understanding of this important terrane through two complementary studies. The first study involves careful bedrock mapping of two heretofore unmapped 5 x 5 km areas in the broader AGC so as to document the field relationships between various rock units and identify the optimal samples for subsequent laboratory work. A sound field context is essential for the correct interpretation of state-of-the-art geochemical data that will be obtained on samples collected of the major rocks units in the complex. These include major- and trace-element and neodymium, lead and strontium isotope data on whole-rock samples, and uranium-lead, oxygen and hafnium isotope data on grains of the mineral zircon extracted from representative samples. The aim of these geochemical studies is to constrain the age and formation history of AGC rocks and in turn gain insights on crust-formation processes in the early Earth. The second study will take a novel approach of using the zircon age distributions and strontium-neodymium-lead isotope compositions of glacial sediment collected from across the AGC to derive an estimate of the relative proportions of different rocks suites present in the complex as a whole. These data will provide insight into when the majority of the AGC formed as well as the dominant rock compositions represented in the complex.**

地球的大陆壳和海洋壳的组合在太阳系中可能是独一无二的，在地球岩石圈、大气层和生物圈的运行中发挥着核心作用。 在地球历史早期，地壳形成和稳定的机制仍然是地质科学中的悬而未决的问题之一。 在本申请中，我建议通过扩展我正在进行的西北地区阿卡斯塔片麻岩杂岩体 (AGC) 的研究项目来解决这个问题。 AGC 可以说包含世界上最古老的岩石（40.2 亿年），而且无疑是世界上少数几个可以提供地球历史前 10 亿年证据的地点之一。 尽管它是一个古老的地壳区域，但 AGC 的大部分区域仍未被研究。 特别是，之前对 AGC 的所有研究都只集中在约 30 平方公里的区域，这仅占整个综合体（约 1300 平方公里）的一小部分。 更广泛的 AGC 的地质情况在很大程度上是未知的，但我的团队的勘察工作表明，该复合体中至少 700 平方公里的岩石包含年龄超过 3.3 Ga 的岩石。我建议通过两项补充研究来加深我们对这一重要地体的了解。 第一项研究涉及对更广泛的 AGC 中两个迄今为止未绘制的 5 x 5 公里区域进行仔细的基岩测绘，以便记录各种岩石单元之间的场关系并确定后续实验室工作的最佳样本。 声场环境对于正确解释最先进的地球化学数据至关重要，这些数据是通过从复杂的主要岩石单元收集的样本获得的。 其中包括全岩样品的主量元素、微量元素以及钕、铅和锶同位素数据，以及从代表性样品中提取的矿物锆石颗粒的铀-铅、氧和铪同位素数据。 这些地球化学研究的目的是限制 AGC 岩石的年龄和形成历史，进而深入了解早期地球的地壳形成过程。 第二项研究将采用一种新颖的方法，利用从 AGC 收集的冰川沉积物的锆石年龄分布和锶-钕-铅同位素组成，来估计整个复合体中不同岩石组的相对比例。 这些数据将提供有关大部分 AGC 形成时间以及复合体中主要岩石成分的见解。**