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Diamonds found in Earth's oldest cystals

August 22, 2007 By Ewen Callaway This article courtesy of Nature News.

Gems in 4.3-billion-year-old zircon baffle geologists.

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Diamonds have been found in some of Earth's oldest rocks, dating back to 4.3 billion years ago.

The precious gems are an exciting find for geologists, because they suggest that the planet's rocky crust was thicker in its infancy than suspected. But the news shouldn't get would-be bridegrooms excited about buying the world's most ancient engagement ring. At 70 micrometres, the largest of these diamonds is no thicker than a few strands of hair.

Announced on Tuesday at the Goldschmidt Conference for geochemistry and mineralogy in Cologne, Germany, and appearing today in Nature1, the diamonds have taken the geological world by surprise, including their discoverers.

"It came out of left field and we weren't expecting it," says Simon Wilde, a geochemist at Curtin University of Technology in Bentley, Australia, and a co-author of the paper. The diamonds may indicate that giant plates of Earth's crust were on the move very early in Earth's history, he says, although more careful study is needed to confirm this.

Under pressure

There aren't many rocks more than 4 billion years old, so little is known about the early geology of our approximately 4.5-billion-year-old planet.

Geologists once thought that unrelenting asteroid bombardment kept the planet molten for much of its first 500 million years. But in the early 1980s, Australian rock-hunters found 4.5-billion-year-old zircon crystals trapped within younger rock in the Jack Hills of Western Australia. This hinted at a cooler world that could form such crystals.

Most studies of these crystals have focused on the chemical makeup of the zircon itself. A team led by geologist Martina Menneken, from Westfalische Willhelms University in Munster, Germany, instead analysed the contents of tiny fissures in the zircon. There they found clear evidence of tiny diamonds in bits of crystal from 3.1 billion to 4.3 billion years old. If the diamonds are the same age as their surrounding material, this would make the oldest gem a billion years older than any other diamond found so far.

Diamonds are usually formed under high pressure. Given what geologists think the temperature of these zircons was when they formed, that means the diamonds were probably squeezed into being by the pressure of 100-150 kilometres of crustal rock lying above them. And that supports the idea that plate tectonics was in motion. "The easiest way to get a thickened crust is to bang two continental plates into one another," says Wilde.

Some have speculated that continental crusts skated on top of molten rock 4.4 billion years ago, creating a phenomenon similar to the plate tectonics that move the continents around today, creating mountains, ocean trenches and earthquakes. But most argue that this kind of crustal movement didn't get going until between 3 billion and 4 billion years ago (see "The start of the world as we know it").

Young or old?

The fact that the diamonds seem to span such a large age range is an enigma, the authors say. It is possible that the diamonds all formed 4.3 billion years ago, and later got shoved into younger crystals. Or perhaps diamonds were incorporated into zircons over and over again through some unknown process, in the same place, at different times in history.

Ian Williams, a geologist at the Australian National University in Canberra, isn't convinced of the gems' age. He bets that carbon was first inserted into zircons of different ages as pockets of graphite, which were converted to diamonds much later in history in one fell swoop.

"I think it's going to turn out to be young diamonds, but I'm quite happy to be proven wrong," says Williams.

One way to test these theories will be to look at the nitrogen in the gems, says Williams. If nitrogen is present as single atoms, it would hint that the diamonds were produced rapidly and at relatively low temperature - as might happen if the diamonds formed from pockets of graphite in the zircons in a single, rapid event. That would make the diamonds young. But if nitrogen is present in pairs or groups, this would indicate that the diamonds were formed over a long period of pressure. This would be consistent with the idea that the diamonds formed a very, very long time ago and are the oldest gems on Earth.

References

  1. Menneken et al. Nature 448, 917-921 (2007).

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