Discovering Earth's Hidden Diamonds Just Got Easier

Diamond prospectors know that the secret to finding diamonds is to locate rocks called kimberlites. A new study in Nature this week may help them focus their search a bit more closely, and also reveals a new understanding of the Earth's mantle.

Kimberlites – named after the South African town of Kimberley where they were first diamond was discovered – are generally only found in very old parts of the Earth's crust. They are the sites of small but violent volcanic eruptions that brought material – including diamonds – spewing to the surface. No one has ever seen a kimberlite erupt – the most recent took place about 40 million years ago, said study author Kevin Burke, a geologist at the University of Houston.

Scientists have known that kimberlites beneath Earth's surface erupt when shifting tectonic plates push them over plumes of heat rising from deep within the mantle. But these plumes are confined to certain regions of the mantle.

Burke's work reveals the best places to look for diamond-bearing kimberlites are the boundaries between the parts of the mantle that enclose plumes and those that don't.

Of course, the land that overlies these boundaries is in constant motion because of plate tectonics, so the search is complicated.

Where diamonds come from

About 2,000 miles (3,200 kilometers) below Earth's surface, at the boundary between the core and the mantle, where the temperatures reach 7,200 degrees Fahrenheit (4,000 degrees Celsius), plumes of heat begin their long, steady rise toward the planet's outer layers. As a heated plume creeps upward, it warms the solid layers of rock that lie over it.

"Most rock does not have a lot of volatile material," Burke said, so the heat from the plumes does not cause volcanic eruptions.

But if these solid layers contain kimberlites, they erupt violently when heated because of the volatile materials that kimberlites contain. The eruption carries the kimberlites – along with any diamonds they contain – to the surface.

Where to look

The trick to finding diamonds, Burke said, is to put together findings from plate tectonics, typically studied by seismologists and other geologists who study the Earth's surface, with studies of the Earth's deep geology. The two fields seldom combine their data, he said.

The new map, he said, reveals locations where diamonds are most likely to be found.

For example, many parts of Africa contain a high concentration of diamonds, because the continent contains kimberlites and was pushed over a plume in the last 540 million years. But parts of the continent also lie over a large section of the mantle with no plumes. By drawing a line along the boundary between the  two regions, Burke says he has highlighted the places where as-yet undiscovered diamonds are most likely to lie.

Burke's work also revealed that two large mantle regions with no plumes have been relatively stationary for much longer than previously thought. The regions are roughly elliptical, Burke said, and their centers are found along the Earth's equator – one lies beneath Africa, the other beneath the Pacific plate.

"Establishing the history of deep mantle structure has shown, unexpectedly, that two large volumes lying just above the core/mantle boundary have been stable in their present positions for the past 500 million years," he said.

"The reason this result was not expected," Burke explained, "is that those of us who study the Earth's deep interior have assumed that, although the deep mantle is solid, the material making it up would all be in motion all the time, because the deep mantle is so hot and under such high pressure from the weight of rock above it."

Not everyone agrees with this finding, Burke said. Other geologists would argue these mantle zones are not as stationary as his data show, but he believes that further investigations that also combine data from Earth's upper layers with findings from the lower layers will show that this work holds up.

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