The Virgin Islands: Petrogenesis of early Earth-like Rocks (VIPER)

我们的地球有45.6亿年的历史，在已知的行星中是独一无二的，它有维持生命的大气层、液态水海洋，以及由富含二氧化硅的花岗岩地壳组成的大陆，覆盖了地球表面的40%。剩下的60%的地壳被洋壳覆盖，洋壳更薄，由贫硅的玄武岩组成，并不断更新。板块构造过程不断地重塑今天的地球表面，在大洋中脊形成新的洋壳，并在不到2亿年的时间尺度上通过俯冲到相邻板块之下来摧毁它。洋壳通过与海水的相互作用而水化，然后当地壳被拖入地球内部时，这些水被释放出来，以触发与俯冲相关的岩浆活动，例如，山体。富士和Mt.圣海伦斯，太平洋火环的一部分。由此产生的富含二氧化硅的岩浆凝固为火成岩，然后通过侵蚀、沉积和造山过程进行循环，形成新的大陆地壳。然而，尽管板块构造解释了现今地球上的地壳形成，但人们对40亿年前早期地球上的构造过程知之甚少。对早期地壳形成过程缺乏了解，以及这些过程可能对早期环境产生的影响，意味着我们不知道(1)地球板块构造何时开始，(2)地球表面是如何分化的，(3)早期表面环境是如何被化学改变的，(4)为什么生命，特别是早期陆地表面的生物，能够进化。了解早期地球是如何发展的，以及它是如何演变成现代世界的关键在于大陆大陆的形成。这是因为，在过去的40亿年里，地球大陆地壳的生长、保存和侵蚀导致了地球内部、地壳表面和海洋-大气环境的化学变化。然而，对于最古老的大陆地壳是如何形成的，仍然没有达成共识。在大陆存在之前，来自地球内部的岩浆凝固形成了一个覆盖全球的厚厚的玄武岩地壳。大约40亿年前，大约在海洋中出现最早的生命的同时，这个玄武岩地壳不知何故开始重新熔化，形成了最古老的保存下来的大陆岩石。这些大陆岩石主要由独特的花岗岩类岩石组成，称为英云闪长岩、花岗闪长岩和花岗闪长岩(TTG)，这些岩石在今天的地球上很少形成。产生早期地球TTG的构造环境和源区的组成仍然存在争议和未知。需要研究的是与多个源区和构造环境相关的TTG建造区域，以确定TTG可能是如何形成的。在这里，我们将在加勒比海东北部的维尔京群岛上研究相对现代的早期类地TTG，因为这些岛屿由来自深处多个源区形成的火成岩组成，并与多种构造环境有关。值得注意的是，与保存在早期地球地形中的地壳形成过程的零碎记录不同，TTG岩石及其来源在维尔京群岛是完全可以获得的。因此，维尔京群岛提供了一个独特的天然实验室，用于测试40亿年前最早的TTG是如何形成的，因此也是最古老的大陆地壳。

Our Earth is 4.56 billion years old and is unique among known planets in having a life-supporting atmosphere, liquid-water oceans, and continents made of silica-rich granitic crust that cover about forty percent of its surface. The remaining sixty percent of the crustal surface is covered by oceanic crust, which is thinner, composed of silica-poor basaltic rock, and is continually renewed. Plate-tectonic processes constantly re-shape the Earth's surface today by forming new oceanic crust at mid-ocean ridges and destroying it by subduction beneath adjacent plates on time scales of less than 200 million-years. Oceanic crust is hydrated by interaction with sea water and this water is then released as the crust is dragged down into the Earth's interior, to trigger subduction-related magmatism as, for examples, Mt. Fuji and Mt. St. Helens, part of the Pacific Ring of Fire. The resulting silica-rich magmas solidify as igneous rocks and are then recycled through erosion, deposition and mountain-building processes to create new continental crust. However, although plate tectonics explains crust formation on the present-day Earth, the tectonic processes operating on the early Earth 4 billion years ago are very poorly understood. The lack of knowledge about early crust-forming processes, and the influences that these processes may have had on early environments, means that we do not know (1) when plate tectonics started on Earth, (2) how the Earth's surface differentiated, (3) how early surface environments were chemically modified and (4) why life, especially the organisms that colonised the early land surface, was able to evolve.The key for understanding how the early Earth developed and how it evolved into the modern world lies in the formation of the continental landmasses. This is because, over the last 4 billion years the growth, preservation, and erosion of the Earth's continental crust has been responsible for chemically modifying the planet's interior, crustal surface, and ocean-atmosphere environments. However, there is still no consensus on how the oldest continental crust formed. Before the existence of the continents, magma from the Earth's interior solidified to form a thick world-wide basaltic crust. Around 4.0 billion years ago, at about the same time as the earliest life appeared in the oceans, this basaltic crust somehow started to re-melt to form the oldest preserved continental rocks. These continental rocks are predominantly composed of distinctive granite-like rocks called tonalites, trondjhemites and granodiorites (TTG) that are rarely formed on the Earth today. The tectonic settings and the composition of the source regions responsible for generating early Earth TTG remain controversial and unknown. What is required is to study an area of TTG formation that is associated with multiple source regions and tectonic environments to determine how the TTG could have formed. Here, we will investigate relatively modern early Earth-like TTG on the Virgin Islands of the northeastern Caribbean because the islands are composed of igneous rocks formed from multiple source regions at depth and are associated with several tectonic environments. Significantly, unlike the fragmentary record of crust-forming processes preserved in early Earth terrains, TTG rocks and their sources are fully accessible in the Virgin Islands. Therefore, the Virgin Islands provide a unique natural laboratory for testing how the earliest TTG, and therefore the oldest continental crust, formed 4 billion years ago.