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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/J019372/2
负责人：
Alan Hastie
金额：
$ 27.62万
依托单位：
University of Birmingham
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2013
资助国家：
英国
起止时间：
2013 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FJ019372%2F2
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.

第一个大陆是如何形成的？这是关于地球演化的一个基本问题，但科学家仍然无法得出结论性的答案。然而，解决这个问题对于地球科学家、化学家和生物学家来说至关重要，因为大陆的形成最终决定了整个地质时期地球内部、水圈和大气的化学演化。第一批大陆是由较古老的火成岩原岩部分熔融形成的。然而，原岩的组成及其构造亲和力都存在争议。实地和分析研究表明，早期大陆是由原始俯冲带洋壳部分熔融形成的。如果这是真的，早期的聚合边缘将通过将化学分馏的地壳物质回收回地球内部来改变地球的成分。此外，早期的火山弧会释放挥发性元素，并在化学上改变早期的大气和海洋，这将对生命的出现和进化产生影响。因此，了解大陆的形成对于多个科学学科都很重要，该项目旨在确定经历部分熔融形成第一批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可行性。早期大陆地壳由长闪长岩英云岩和花岗闪长岩/英安岩 (TTG/D) 火成岩组组成。最古老的TTG/Ds被认为源自变质的角闪石-斜长石-石榴石基性火成岩原岩。对于变基性岩，需要 ~1.0-1.6 GPa (30-50 km) 的压力来稳定角闪石、斜长石和石榴石的矿物学。如今，洋中脊 (MOR) 生成的基本洋壳厚度约为 7 公里，并俯冲到较年轻的洋壳之下，形成岛弧。在远离板块边界的地方，玄武岩海洋岛屿很常见，其中许多岛屿被认为是由从地球深处上升到地表喷发的热地幔柱产生的。过去尝试识别经过部分熔融形成 TTG/D 的基性火成岩原岩，涉及对来自 MOR、岛弧和板内海洋岛屿的变基性物质在 0.1-32 GPa 的压力范围内进行部分熔融实验。不幸的是，所得熔体与最早的 TTG/D 的成分不匹配，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 MOR 之下，地幔比今天的 MOR 之下的地幔更热，化学成分也更丰富，当它经历部分熔融时，它形成了更厚（> 20 km）和更丰富的 MOR 地壳。大洋高原源自地幔柱的大规模部分熔融，相对于现代 MOR 地壳，其地壳更厚（8-30 公里），成分也更丰富。因此，海洋高原可能是早期地球海洋板块的现代模拟。因此，本研究的目的是（1）分析俯冲所罗门群岛翁通爪哇海洋高原上方的岩石，以及（2）在1.0-1.6 GPa压力范围内对现代海洋高原岩石进行部分熔融实验，以确定海洋高原玄武岩原岩是否可以产生与地球早期大陆地壳成分相同的熔岩。如果成功，最古老大陆地壳的源区可以在约 30-50 公里的深度被识别和生成。此外，通过确定源区必须处于这个压力范围内，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替代模型（地壳内熔化和大规模表面重铺）涉及较高和较低的压力，导致石榴石和斜长石不能稳定在一起。