How were the first stable continents formed?

第一个大陆是如何形成的？这是一个关于地球演化的基本问题，科学家们至今仍无法给出结论性的答案。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是至关重要的，因为大陆的产生最终决定了整个地质时期地球内部、水圈和大气层的化学演化。第一个大陆是由一个古老的火成岩原岩部分熔融形成的;然而，原岩的成分和它的构造亲和力都是有争议的。实地和分析研究表明，早期大陆是由原始俯冲带洋壳的部分熔融形成的。如果这是真的，那么早期的会聚边缘将通过将化学分离的地壳物质再循环回地球内部，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改变，这将对生命的出现和进化产生影响。因此，了解大陆的产生对几个科学学科都很重要，这个项目的目的是确定经历部分熔融形成第一个大陆的原岩的亲和力，如果成功的话，可能支持早期地球上俯冲带的可行性。早期大陆地壳由奥长闪长英云闪长岩和花岗闪长岩/英安岩（TTG/D）套火成岩组成。最古老的TTG/Ds被认为是来自变质角闪石-斜长石-石榴石轴承基性火成岩原岩。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。今天，在大洋中脊（莫尔）生成的基性洋壳厚约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，许多岛屿被认为是从地球深处上升到地表的热地幔柱产生的。过去试图确定经历部分熔融形成TTG/Ds的基性火成岩原岩的尝试涉及在0.1 - 32 GPa压力范围内对来自MORs、岛弧和板内海洋岛屿的变基性材料进行的部分熔融实验。不幸的是，由此产生的熔体不匹配的组成最早的TTG/Ds和几个实验已经进行了必要的1.0 - 1.6 GPa的压力区间内。在地球早期MOR之下的地幔比今天MOR之下的地幔更热，化学成分更丰富，当它经历部分熔融时，它形成了更厚（> 20 km）和更丰富的莫尔地壳。大洋高原是地幔柱大规模部分熔融的产物，相对于现代莫尔地壳，其地壳厚度较厚（8 - 30 km），成分更为富集。因此，海洋高原可能是早期地球上海洋板块的现代模拟。因此，本研究的目的是（1）分析俯冲的Ontong Java海洋高原，所罗门群岛上方的岩石和（2）在1.0 - 1.6 GPa压力范围内对现代海洋高原岩石进行实验性部分熔融实验，以确定是否可以从海洋高原玄武岩原岩中生成与地球早期大陆地壳成分相同的熔岩。如果成功的话，最古老的大陆地壳的源区可以在30 - 50公里的深度确定和产生。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替代模型（壳内熔融和大规模表面重塑）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。

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.