Ian Sample

They shed light on the darkest age of earth’s evolution

Stones date back more than four billion years

Electron microscope used to examine rocks

Large spread of ages suggests two possibilities

A clutch of ancient diamonds plucked out recently from the hills of Western Australia (WA) has been identified as possibly the oldest remnants of the earth’s crust ever recovered.

The precious stones, which date back more than four billion years, were found inside rocks lodged in the sediments of a 50-mile-long coastal ridge generally known as the Jack Hills.

Evolution

The stones shed clear light on the darkest age of earth’s evolution — the period between the planet’s creation from a giant cosmic dust cloud 4.5 billion years ago and the formation of the oldest known rocks 500 million years later.

Hellish mass

The formal name given to the period — the Hadean — paints a picture of earth as a hellish mass of molten lava, but analysis of the diamonds suggests the blackened landscape may have cooled much more quickly than previously thought.

Natural diamonds form under intense pressure and temperatures up to 1,200 C more than 60 miles below the earth’s crust over extended periods of time.

The oldest of the diamonds is thought to be 4.25 billion years old, suggesting that by this time the planet had already cooled enough to form a crust of solid rock.

The scientists used a powerful electron microscope to examine ancient zircon rocks from an excavation site in the hills and identified microscopic inclusions of diamond inside them, measuring at most a twentieth of a millimetre across.

Over a billion years

Diamonds were found in 45 of the rocks studied, and they were believed to have formed over a period of more than a billion years.

The large spread of ages suggests two possibilities, according to the scientists’ report in the journal Nature.

Either the conditions for diamond formation occurred several times over in earth’s early history, or ancient diamonds were churned over in the earth’s molten interior for millennia before becoming locked into the crust at different times.

Another possibility

Another possibility, according to Ian Williams, a geologist at the Australian National University, is that carbon-rich fluids seeped deep into fissures in the zircon rocks several billions of years ago.

As the rocks were then drawn into the earth’s interior the carbon deposits were compressed into diamonds and preserved in rocks that erupted from the surface much later. — © Guardian Newspapers Limited, 2007