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If only this diamond truly lasted forever.

If only this diamond truly lasted forever

Most of the diamonds sparkling on wedding rings were born in the Earth's mantle, 150 kilometers underground, and shot up to the surface mixed into columns of hot, fast-moving lava called kimberlite. However, many diamonds may be rising by a slower process.

Peter H. Nixon of the University of Leeds, England, and his colleagues have studied a 50-kilometer-long slab of earth called Beni Bousera that was once in the mantle and surfaced in Morocco about 15 million years ago. In a letter to the March 2 NATURE, they cite evidence showing that before the deposit surfaced, it contained about 10,000 times the diamond-richness of the best kimberlites.

Actual diamonds no longer exist in the Beni Bousera, only bits of graphite. But Nixon says the shape of the graphite suggests a more lustrous past. All of the Beni Bousera graphite is shaped distinctively as diamond crystals, not as graphite crystals -- most of it in strikingly diamond-like octahedra. "You wouldn't expect to see graphite in an octahedral shape," says diamond expert Tony Erlank of the University of Cape Town in South Africa. He agrees this graphite probably was once diamond.

Graphite and diamond are both pure carbon, and at surface pressures, diamonds tend to revert to graphite. Kimberlite diamonds persist because they rise to the surface quickly. But the Beni Bousera probably came up slowly, its diamonds apparently retaining their shape but losing their value. Nixon suggests Beni Bousera was once a carbon-rich seafloor that sank to the mantle, where the diamonds formed. It then slowly surfaced through the shifting crust.

Had the Beni Bousera made it up faster, some of its layers might have been a staggering 15 percent diamond, or 10,000 times as rich as kimberlite. Nixon suggests that such deposits -- carbon-rich seafloor that goes to the mantle and back -- might be the original source of kimberlites. The kimberlites may pick up their few gems as they speed through these diamond wellsprings, he says. Could such a million-dollar mantle ever make it to the surface before its diamonds became worthless graphite? Nixon thinks it possible. Similar deposits in Tibet and the eastern Soviet Union have retained some of their diamonds, he notes.

The place to look for surfaced, diamond-rich mantle would be around the restless plate margins where deep material moves up. Diamond hunters normally range over the calm continental interiors where kimberlites are found, but Nixon suggests the plate margins deserve a closer look.
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Title Annotation:Beni Bousera geology
Author:Flam, Faye
Publication:Science News
Date:Mar 4, 1989
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