Friday, 9 September 2011

On The Origin Of Earth's Precious Metals

When the Earth formed, it was a ball of molten rock. Slowly the denser and therefore hotter elements such as iron, nickel and osmium sank to the centre of the planet, forming a radioactive ball of metal kept solid by extreme pressure, the power source for the Earth. This process, known as diffusion, drew the heat away from the surface of the planet, allowing it to cool and stabilise to the point where solid rock formed. Yet a problem arises when we look at the metallic composition of the Earth's crust.

We know that it contains vast amounts of iron and nickel, now used in industry, and smaller, yet still vast, deposits of gold, silver and platinum, all heavy metals that should have sunk to the Earth's core over 4 billion years ago. The question is why do we find such metals on the surface today when science dictates that they should be over 8000 miles beneath our feet and totally inaccessible? In fact the silicate mantle contains over 1000 times the levels of gold previously anticipated.
An artist's impression of the Late Heavy Bombardment

A study by a team of geologists led by Dr Matthias Willbold from the University of Bristol has shown that the vast majority of our metal resources and ores could have been delivered to Earth in a vast 300 million year long meteorite shower called the Late Heavy Bombardment. Of course it is impossible to analyse every grain of dirt on the Earth for traces of gold and because such metals have an annoying habit of forming nuggets, so gathering any amount of meaningful data is a challenge.

Luckily, the team were able to use a more common metal that has similar properties to 'iron loving' elements such as gold, but with a few physical quirks that gave them a unique edge over the problem of the scale of the Earth and the conglomerate nature of iron loving metals. Tungsten is a metal that can exist in a number of unstable, radioactive forms called isotopes as well as in the more common stable form. Two interesting radioactive forms are tungsten - 182 and tungsten - 184.

Both have properties which allowed the team to identify the origin of the numerous deposits of crustal gold on Earth. All of the tungsten - 184 that existed on the new Earth would have sunk to the centre of the Earth with all the other iron loving, properly called siderophile, elements such as gold and osmium. However tungsten - 182 is formed by the radioactive decay of hafnium. Hafnium is not a siderophilic element and therefore would not have sunk to the centre of the Earth. Instead it decayed in the crust, depositing large amounts of tungsten - 182.

The team took rock samples from the Isua Greenstone Belts in Greenland, some of the oldest rocks in the world which were layed down 3.9 billion years ago, when the Late Heavy Bombardment was receding. They studied the tungsten isotope ratios in the samples and found a sharp rise in the amount of tungsten - 182 after the Late Heavy Bombardment had ended. They believed that the LHB was connected with the levels of tungsten in the crust.

A rock formation from the Isua Greenstone
Belts, dating to 3.9  billion years old
Dr Willbold concluded that the amounts of the tungsten isotope or its radioactive parent hafnium were deposited on the Earth by meteorites after the Earth had cooled. Therefore any heavy elements contained within would have been buried in the crust. As tungsten is a siderophilic element, they reapplied this theory to other iron loving, heavy elements such as gold and silver. Such a hypothesis is plausible and may have solved the mystery of the presence of heavy metals in the Earth's crust as the Earth would have been struck by over 20  billion billion tonnes of material.