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

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.