How were the first stable continents formed?

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.

The project will build upon previous work into determining how the Earth's first continental land masses were generated and how these processes could have affected the planet's primordial silicate interior, atmosphere and hydrosphere. The multidisciplinary nature of this novel research will partly benefit most areas of science (e.g. earth scientists, physicists, chemists and biologists) because it will aim to determine how the geochemical reservoirs on the Earth, and potentially other planetary bodies, evolved early in their life cycle. Consequently, the identification of these fundamental early geological processes will be made available to university users through several high impact peer reviewed publications, high profile conference presentations and the NERC data centre. The outcomes will benefit all levels of university research, from masters' students to internationally renowned professors. The results will also benefit and enhance the reputation of the quality of earth science research performed in the U.K. to the international scientific community.

Furthermore, Earth and environmental science is popular and appeals to many non-university users and, as this proposal aims to answer some of the most fundamental and stimulating questions about the formation of the planet, the results should be of interest to wider community users. Therefore, the rationale and results of the fellowship will be initially communicated to an independent government research centre - the British Geological Survey (BGS) (see support letter). The information will be circulated to members of the BGS by means of its outreach program, and subsequently, I will utilise the BGS outreach facilities and activities to disseminate the fellowship research to the general public, non-university educational users (teachers and students), geological societies, community groups and other interested parties (e.g. further dissemination to university users). The use of the BGS allows for a successful knowledge exchange plan with non-academic users in order to raise awareness of this new and exciting area of earth science research. The research outcomes will directly benefit the educational users and their students because it will bring the latest knowledge on the formation to the planet directly into classroom teaching.

Therefore, the dissemination of the research, via the BGS outreach program, fulfils a major goal of NERC's 2007-2012 strategy by continuing to strengthen NERC's science in society programs, which are intended to engage the public in the research that NERC is currently funding.