Evolution of Earth’s Earliest Crust

地球的大陆地壳和海洋地壳的结合在太阳系中可能是独一无二的，它们在地球岩石圈、大气和生物圈的运作中起着核心作用。这个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。在这个申请中，我建议通过扩展我正在进行的西北地区阿卡斯塔片麻岩复杂（AGC）的研究项目来解决这个问题。AGC可以说包含了世界上最古老的岩石（40.2亿年），当然也是世界上为数不多的可以证明地球前10亿年历史的地方之一。尽管它是一个古老的地壳地区，但AGC的大部分地区仍未被研究。值得一提的是，以往对AGC的所有研究都集中在约30平方公里的区域，这只占整个复合体（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，但我的小组的侦察工作表明，该建筑群中至少有700平方公里的岩石比3.3亿年更古老。我建议通过两个互补的研究来增加我们对这一重要地形的了解。第一项研究涉及在更广泛的AGC中对两个迄今未测绘的5 × 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现场关系，并确定后续实验室工作的最佳样品。声场背景对于正确解释最先进的地球化学数据至关重要，这些数据将从复杂岩体的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元素和微量元素、钕、铅和锶同位素数据，以及从代表性样品中提取的锆石颗粒的铀-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，从而深入了解地球早期的地壳形成过程。第二项研究将采用一种新颖的方法，利用从整个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套件的相对比例。这些数据将有助于了解大部分AGC形成的时间，以及复杂岩体中所代表的主要岩石成分。

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.