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The construction of Planet Earth

Final Report Summary - EARTHGROWTH (The construction of Planet Earth)

The first 150 M.yrs of Earth evolution were the most dramatic in the history of the planet, setting the scene for the development of plate tectonics, the Earth‘s magnetic field and the origin of life. By the end of this period Earth had separated into core, mantle, crust and atmosphere and the Moon had formed by, it is presumed, a giant impact near the end of accretion.

This project was aimed at understanding the processes by which, during Earth‘s earliest evolution, the chemical elements were distributed into different geological reservoirs, to determine the timings and conditions under which this partitioning occurred and to determine how Earth‘s interior reached its current composition. The project employed experiments at high temperatures (1100-2500 Celsius) and high pressures (up to 250000 atm, equivalent to 700 km depth) in which the silicate materials of Earth‘s mantle, crust and core are equilibrated with one another and with a gas phase under controlled conditions.

We find that the properties of the outer layers of the Earth are consistent with an early period of growth in which the materials added to Earth were very reduced, with high content of Fe metal, similar to the planet Mercury. Addition of Mercury-like bodies would show unusual fractionations of some elements and dissolve a small amount of uranium in the core. This uranium was important as a heat source to generate Earth’s magnetic field. As accretion progressed the Earth became more oxidised with addition of mixtures of oxidised iron and metallic iron and some volatile elements such as sulfur and carbon. Sulfur and carbon were strongly partitioned into the metallic core and their presence had strong effects on the partitioning of some elements, notably molybdenum and tungsten into the core. The current contents of molybdenum and tungsten in the outer silicate parts of the Earth demonstrate that volatile elements such as sulfur accreted in the later stages of growth of the Earth.

The Earth became extremely hot and largely molten at various stages in its history. We used this fact as a starting point in our investigation of how volatile elements were added to the growing planet. Using a novel furnace, in which liquid rock can be stirred at very high temperature (above 1100 Celsius) we measured how 13 important volatile elements (including zinc, lead and bismuth) partition into vapour above such molten rock. We find that the abundance patterns of these elements in the Earth was established by reaction between liquid and vapour on the small precursor bodies which were added to the growing Earth in the later stages of the accretion process.