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How were the first stable continents formed?

第一个稳定的大陆是如何形成的？

基本信息

批准号：
NE/J019372/1
负责人：
Alan Hastie
金额：
$ 38.04万
依托单位：
University of Edinburgh
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2012
资助国家：
英国
起止时间：
2012 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FJ019372%2F1
关键词：
How were first stable continents

项目摘要

How were the first continents formed? This is a fundamental question regarding the evolution of the Earth, and yet, scientists can still not conclusively answer it. Nevertheless, resolving this question is essential for earth scientists, chemists and biologists as the generation of the continents are ultimately responsible for the chemical evolution of the planet's interior, hydrosphere and atmosphere throughout geological time. The first continents were formed by partial melting of an older igneous protolith; however, both the composition of the protolith and its tectonic affinity are controversial. Field and analytical studies suggest that the early continents were formed by partial melting of oceanic crust in primitive subduction zones. If true, early convergent margins would compositionally modify the Earth by recycling chemically fractionated crustal material back into the planet's interior. Also, early volcanic arcs would release volatile elements and chemically modify the early atmosphere and oceans, which would have implications for the emergence and evolution of life. Thus, understanding the generation of the continents is important to several scientific disciplines and this project aims to determine the affinity of the protolith that underwent partial melting to form the first continents and, if successful, may support the viability of subduction zones on the early Earth.The early continental crust is composed of the trondjhemite tonalite and granodiorite/dacite (TTG/D) suite of igneous rocks. The oldest TTG/Ds are thought to be derived from a metamorphosed amphibole-plagioclase-garnet-bearing basic igneous protolith. For metabasic rocks, pressures of ~1.0-1.6 GPa (30-50 km) are required to stabilise a mineralogy of amphibole, plagioclase and garnet. Today basic oceanic crust generated at mid-ocean ridges (MOR) is ~7 km thick and subducts beneath younger oceanic crust to form island arcs. Away from plate boundaries, basaltic oceanic islands are common, and many are thought to be generated from hot mantle plumes that ascend from deep within the Earth to erupt on the surface. Past attempts at identifying the basic igneous protolith that underwent partial melting to form the TTG/Ds has involved partial melt experiments on metabasic material from MORs, island arcs and intraplate oceanic islands at pressure ranges of 0.1-32 GPa. Unfortunately, the resultant melts do not match the compositions of the earliest TTG/Ds and few experiments have been performed within the essential 1.0-1.6 GPa pressure interval. Beneath the Earth's early MORs the mantle was hotter and chemically more enriched than mantle beneath today's MORs, and when it underwent partial melting, it formed thicker (>20 km) and more enriched MOR crust. Oceanic plateaus are derived from large scale partial melting of mantle plumes and, relative to modern MOR crust, have thicker crust (8-30 km) and are compositionally more enriched. Thus, oceanic plateaus may be a modern-day analogue for oceanic plates on the early Earth. Accordingly, this study aims to (1) analyse rocks above the subducting Ontong Java oceanic plateau, Solomon Islands and (2) perform experimental partial melt experiments on modern oceanic plateau rocks in the pressure range of 1.0-1.6 GPa to determine if lavas with identical compositions to the Earth's early continental crust can be generated form an oceanic plateau basaltic protolith. If successful, the source region of the oldest continental crust can be identified and generated at depths of ~30-50 km. Also, by identifying that the source region has to be in this pressure range, this research will suggest that primitive subduction zones could be viable processes for forming the first continental crust. This is because alternative models to explain the formation of the continents (intracrustal melting and large scale resurfacing) involve both higher and lower pressures that results in garnet and plagioclase not being stabilised together.

第一批大陆是如何形成的？这是一个关于地球进化的根本问题，然而，科学家们仍然无法得出确切的答案。然而，对于地球科学家、化学家和生物学家来说，解决这个问题是至关重要的，因为大陆的生成最终要对整个地质时期地球内部、水圈和大气的化学演化负责。第一个大陆是由较老的火成岩原岩部分熔融形成的；然而，原岩的成分及其构造亲和力都存在争议。野外和分析研究表明，早期大陆是由原始俯冲带中的洋壳部分熔融形成的。如果是真的，早期的会聚边缘将通过回收化学成分分离的地壳物质回到地球内部，从而在成分上改变地球。此外，早期的火山弧会释放挥发性元素，并对早期的大气和海洋进行化学修改，这将对生命的出现和进化产生影响。因此，了解大陆的形成对几个科学学科很重要，该项目旨在确定经历部分熔融形成第一个大陆的原岩的亲和力，如果成功，可能支持早期地球俯冲带的生存能力。早期大陆地壳由奥长花岗岩英安岩和花岗闪长岩/英安岩(TTG/D)系列火成岩组成。最古老的TTG/DS被认为来自变质的角闪斜长石榴石-含基性火成岩原岩。对于变质基性岩，稳定角闪石、斜长石和石榴石的矿物学需要大约1.0-1.6 Gpa(30-50公里)的压力。今天，在大洋中脊(MOR)形成的基本洋壳厚约7公里，并俯冲到较年轻的洋壳之下形成岛弧。除了板块边界，玄武岩海洋岛屿很常见，许多岛屿被认为是从地球深处上升到地表喷发的热地幔热柱产生的。过去试图确定经历了部分熔融形成TTG/DS的基性火成岩原岩的尝试涉及在0.1-32 Gpa的压力范围内对来自MORS、岛弧和板内洋岛的变质基性物质进行部分熔融实验。不幸的是，产生的熔体与最早的TTG/DS的组成不匹配，并且在基本的1.0-1.6 Gpa压力区间内进行的实验很少。在地球早期的MORS之下，地幔比今天的MORS下面的地幔更热，化学成分更丰富，当它经历部分熔融时，它形成了更厚(&gt；20公里)和更丰富的MoR地壳。大洋高原是由地幔热柱的大规模部分熔融形成的，相对于现代MOR地壳，具有更厚的地壳(8-30公里)和更丰富的成分。因此，海洋高原可能是地球早期海洋板块的现代类比。因此，这项研究的目的是(1)分析所罗门群岛俯冲的安通爪哇海洋高原上方的岩石；(2)在1.0-1.6 Gpa的压力范围内对现代海洋高原岩石进行部分熔融实验，以确定是否可以从海洋高原玄武岩原岩中产生与地球早期大陆地壳成分相同的熔岩。如果成功，可以在约30-50公里的深度识别和形成最古老的大陆地壳的源区。此外，通过确定源区必须在这个压力范围内，这项研究将表明原始俯冲带可能是形成第一个大陆地壳的可行过程。这是因为解释大陆形成的另一种模型(壳内熔融和大规模表面重新铺设)涉及更高和更低的压力，导致石榴石和斜长石不能稳定在一起。