Gold rained down on early Earth

Posted by Kate Taylor

All the gold in the Earth's crust was delivered there by meteorites, accoring to researchers at the University of Bristol.

An analysis of some of the oldest rock samples on Earth provides clear evidence that all accessible reserves of precious metals are the remains of a bombardment of meteorites around 200 million years after the Earth was formed.

"Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tonnes of asteroidal material," says
Dr Matthias Willbold.

Heavy metals present at the formation of the earth would be expected to have sunk to the planet's core - and, indeed, this is what seems to have happened. Scientists reckon there's enough precious metals there to cover the entire surface of the Earth with a four-meter layer.

So it's been something a mystery as to where the stuff hanging round our necks all came from.
 
In an attempt to resolve the mystery, a team from Bristol's School of Earth Sciences analysed four-billion-year-old rocks and determined their tungsten isotopic composition. 

Tungsten's a very rare element, and comprised of several isotopes, atoms with the same chemical characteristics but slightly different masses. Tungsten arriving on board meteorites would have a different isotopic composition.

And the team found there was a 15 parts per million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks - a differencethat could explain the excess of accessible gold on Earth.

The meteorites were stirred into the Earth’s mantle by gigantic convection processes. Subsequently, geological processes formed the continents and concentrated the metals in ore deposits which are mined today.