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World's Oldest Diamonds Discovered in Australia

Anne Casselman
for National Geographic News
August 22, 2007
 
The world's oldest known diamonds have been found encased in a crystal in Western Australia, scientists say (see Australia map).

The minuscule gemstones are 4.25 billion years old and could provide a rare glimpse into Earth's distant geologic past.

"No one would have really predicted that diamonds were in there," said Simon Wilde, a geologist at Curtin University of Technology in Perth and a member of the team that made the find.

The discovery suggests that seas of molten lava that covered primordial Earth had cooled down faster than had previously been thought.

The find also suggests that plate tectonics, the process by which large shelves of Earth's crust move to create geologic activity, may have already been underway.

"A diamond would never form in a magma ocean," said Thorsten Geisler, a geologist at Westfalische Wilhelms-Universitaet in Munster, Germany, and another team member.

The discovery is a shocker to geologists, many of whom believed that the molten lava and volcanic activity persisted on Earth's surface for at least 500 million years after our planet formed some 4.5 billion years ago.

Diamonds Are Forever

The tiny diamonds were found trapped in zircon, a rare and exceptionally stable mineral that forms under temperatures between 1,112 and 1,652 Fahrenheit (600 and 900 degrees Celsius).

Once zircon has crystallized it may be moved around by geological processes, but its chemical makeup and structure don't change. This makes its age easy to pinpoint.

Zircon crystals represent the only record of the first 400 million to 500 million years of Earth's history, Wilde explained.

By analyzing a crystal's trace minerals and structure, geologists can deduce the conditions under which it formed.

John Valley, a geologist at University of Wisconsin in Madison who was not involved in this study, notes that there are four known "recipes" that create diamonds.

But the 4.25-billion-year-old diamonds "suggest the additional possibility that the diamonds have formed by some process that is not yet understood."

Study co-author Wilde said, "The bottom line is that we really honestly don't know why they're there."

The study, led by Martina Menneken, a master's student at the Westfaelische Wilhelms-Universitaet, appears in this week's issue of the journal Nature. Alexander Nemchin from Curtin University of Technology also contributed.

Clues to Earth's Earliest Life?

One exciting prospect is that if Earth cooled down earlier in its existence, then it's possible that life on Earth cropped up earlier too, Geisler said.

(Related: "Weird Australia Rocks Are Earliest Signs of Life, Study Says" [June 7, 2006].)

Geisler hopes that analyzing the various types of carbon in these diamonds could reveal whether this was the case.

"We don't know yet, but this is potential information contained in the carbon," he said.

Valley, the Wisconsin geologist, added, "Even though these diamonds are too small to be of commercial value as gems, scientists will find them even more valuable for the information they carry about the Earth."

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