News in Science

Earth's gold fell from the sky

Valuable impact A massive meteor bombardment 3.9 billion years ago provided most of the gold and other precious metals found near the Earth's surface today, according to a new study.

The research by scientists led Dr Matthias Willbold from the University of Bristol, explains why these minerals are surprisingly abundant in accessible parts of the planet, rather than sinking to the centre of the Earth during its early years.

As the Earth formed 4.5 billion years ago it was mostly a giant global magma ocean, which differentiated as it cooled. Precious 'iron-loving' metals, such as gold moved into the planet's iron core, leaving lighter silicates on the surface.

Reporting in the journal Nature, Willbold and colleagues claim the relative abundance of precious metals in the Earth's mantle today probably came from a huge meteor storm which ended about 3.9 billion years ago.

This cataclysmic event, called the late heavy bombardment, added between a half and one per cent of additional material to the Earth's mass, forming a veneer on the surface. It was also responsible for many of the Moon's craters.

Planet's earliest rocks

Willbold and colleagues examined ancient rock samples from Isua in Greenland, which have been dated to 3.8 billion years and are some of the oldest rocks on Earth.

They compared them with volcanic plume samples from places like Hawaii, which date from after the bombardment.

High precision measurements were carried out to determine the ratios of tungsten isotopes. Before the bombardment, most of the tungsten-184 migrated to the iron core, along with gold and other precious 'iron-loving' metals. Another isotope, tungsten-182, is formed by the radioactive decay of hafnium and remained in the silicates that formed in the Earth's crust.

Willbold and colleagues found a distinct change in the ratios of tungsten-182 and tungsten-184 immediately after the bombardment. This change in tungsten isotope ratio was most likely caused by meteorite impacts, which increased the amount of tungsten-182 and other precious metals in the Earth's crust.

Understanding the Earth's early history

Dr Bill Griffin, a professor of geochemistry at Sydney's Macquarie University, who was not invovled in the study, says lets us know a lot more about how the Earth was put together and what it went through in its first billion years.

"The plume samples show that the stirring process, which mixed the veneer material, probably went on from about 3.9 to 3 billion years ago," says Griffin.

"It's a lovely example of going from the absolute miniscule of tungsten isotope ratios, up to the global scale of Earth mantle convection."