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: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=647032
• 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.**

地球的陆壳和洋壳在太阳系中可能是独一无二的，在我们星球的岩石圈、大气层和生物圈的运行中发挥着核心作用。地壳在地球历史早期形成和稳定的机制仍然是地质科学中悬而未决的问题之一。在这份申请中，我建议通过扩大我正在进行的西北地区Acasta片麻岩杂岩(AGC)的研究计划来解决这个问题。AGC可以说包含了世界上最古老的岩石(40.2亿年前)，而且肯定是世界上少数几个可以证明地球历史前10亿年的证据的地点之一。尽管它是一个古老的地壳所在地，但AGC的大部分仍未得到研究。特别是，以前对AGC的所有研究都只集中在约30平方公里的区域上，这只占整个综合体的一小部分(约1300平方公里)。更广泛的AGC的地质情况在很大程度上是未知的，但我的小组的勘察工作表明，该综合体至少700平方公里由3.3Ga以上的岩石组成。我建议通过两项相辅相成的研究来增加我们对这一重要地体的了解。第一项研究涉及在更广泛的AGC中对两个迄今未绘制地图的5×5公里区域进行仔细的基岩测绘，以记录各种岩石单位之间的野外关系，并为随后的实验室工作确定最佳样本。声场背景对于正确解释最先进的地球化学数据是必不可少的，这些数据将从从杂岩中主要岩石单元收集的样本中获得。这些数据包括全岩样品的主量元素和微量元素以及钕、铅和锶的同位素数据，以及从有代表性的样品中提取的矿物锆石颗粒的铀-铅、氧和氦同位素数据。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，进而获得对地球早期地壳形成过程的洞察。第二项研究将采用一种新的方法，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杂岩中存在的不同岩石套的相对比例。这些数据将提供对AGC的大部分形成时间以及杂岩中代表的主要岩石成分的洞察。