Faculty of Science and Technology

Scientists pondering how layers of orange and green glass beads came to be formed by volcanic eruptions on the Moon have come up with a radical new theory of how diamonds reach the Earth's surface.

Professor Lionel Wilson, a volcanologist at Lancaster University, and Professor James Head, leader of the Planetary Geology Group at Brown University in Providence Rhode Island, hit upon the theory while searching for a solution to the problem of how molten rocks from the deep interior of the Moon reached the surface to form the round glass droplets found by the Apollo astronauts. As they talked over one proposed mechanism they realized it could also explain how diamonds reach the Earth's surface.

Their hypothesis is published in the May 3 issue of Nature. If it proves true, their theory would suggest that diamonds have been found in South Africa or in the Canadian Shield not because they have any special surface geology but mainly because these parts of the Earth's crust are very old. In theory, diamonds could turn up anywhere at any time if you wait long enough.

The vast majority of diamonds are found on Earth are found in Kimberlites - very rare and characteristically fragmental volcanic rocks.

Diamonds can only be formed at very high temperatures and pressures and must come from the deep interior of the Earth - the mantle. Once they are cold at the low pressure of the Earth's surface they are stable. But in between the deep interior and the surface of the Earth diamonds are not stable and can quickly crumble into common graphite. For a diamond to survive it must travel from the hot high pressure of the mantle, at around 200 km deep, to the cool low pressure surface of the Earth very quickly - preferably in less than half an hour.

The model put forward by Professors Wilson and Head suggests that carbon dioxide has a key role to play in this process.

They suggest that, as a kimberlite ascends through a fracture in the Earth, pressurized carbon dioxide gas gets concentrated into the upper tip of this fracture. Behind the gas pocket is a region at least many hundreds of metres long, full of high-pressure CO2 gas bubbles. When the tip of the fracture breaches the surface the violent expansion of all the gas cools the magma very fast, and the "popping" of all the gas bubbles generates shock waves that shatter the chilling magma and the surrounding rocks.

Professor Wilson said: "This idea came about when we were investigating the explosive volcanic eruptions on the Moon that produced the layers of orange and green glass beads found by the Apollo astronauts on the Moon. We were thinking about the processes that could have caused this and we suddenly realised it could also explain a lot about how diamonds reach the surface of the earth.

"It is a complicated process and until now much research has focussed on the small details. By taking a broad view of the physics of the process we have come up with an entirely new model."

Wed 09 May 2007

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