Ancient Diamonds Solve a Mystery

Scientists have unearthed diamonds more than 4 billion years old trapped inside crystals of zircon in the Jack Hills region in Western Australia, suggesting that the Earth might have cooled much more rapidly than previously thought, with the continental crust and oceans forming as early as 4.4 billion years ago.

About 4.5 billion years ago, Earth formed from a cloud of dust around a proto-Sun. During its youth, Earth smashed into a planet-size body and its surface temperatures likely soared above 10,830 degrees Fahrenheit (6,000 degrees Celsius).

When the molten Earth cooled off, the liquid lava condensed into rocks. Details about the rocks and when they began to form, a subject of intense debate, have been limited by a lack of substantive data.

One such debate centers on whether early Earth was covered by oceans of hot lava or if the planet's surface had cooled enough for rock formation and was covered instead by oceans of water.

Diamond formation requires exposure of carbon-bearing materials to high pressure but at a comparatively low temperature range between approximately 1652–2372 °F (900–1300 °C). These conditions are found in two places on Earth: in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.

Tough, heat-resistant diamond crystals retain their chemical nature and can provide vital clues about past events that occurred in the Earth's crust and mantle, supporting the quick-cooling theory, said study team member Thorsten Geisler of the Institute of Mineralogy at the University of Münster in Germany.

The scientists, led by Martina Menneken of the Institute, ran chemical analyses of the zircons, finding the ancient crystals and the enclosed diamonds were more than 4 billion years old. That's nearly a billion years older than the previous oldest-known terrestrial diamonds and suggests the diamonds were present in material that crystallized within 300 million years of the formation of Earth.

"Jack Hills is the only place on Earth that can give us this kind of information about the formation of the Earth," said study team member Alexander Nemchin, a geochemist at Curtin University of Technology,
Western Australia. "We're dealing with the oldest material on the planet."

After considering different diamond-formation scenarios, the scientists concluded the jewels most closely resembled those found in ultra-high pressure conditions. They interpret the findings to indicate that Earth had a relatively thick continental crust by 4.25 billion years ago.

Earth must be cooler than previously thought because otherwise the team could not have found diamonds, because to form diamonds, heavyweight pressures of a thick continental crust must be in place.

Posted by Casey Kazan