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Diamonds Tell How Old Continents May Have Formed

Cratons contain the oldest rocks on the planet and provide the nucleus around which younger continental material assembles. File Image by David Seal
 Washington - Sep 10, 2002
Diamonds are much more than just pretty gemstones. Scientists have found that these valuable minerals, and the smaller minerals sometimes included in them, can reveal the details of how and when the oldest parts of our planet formed.

The September 6, 2002, issue of Science, published a paper by Steven Shirey and David James, staff members of the Carnegie Institution of Washington's Department of Terrestrial Magnetism.

Collaborating on the study were Stephen Richardson, a visiting investigator from the University of Cape Town, Matthew Fouch, a former postdoctoral fellow at Carnegie who is now a professor at Arizona State University, and a team of international scientists including Jeff Harris from University of Glasgow, Pierre Cartigny from the University of Paris, Peter Deines from the University of Pennsylvania, and Fanus Viljoen from the GeoScience Centre in Johannesburg.

The scientists looked at evidence documented over the past two decades from 4,000 diamonds, and data from seismic P-waves from the Kaapvaal-Zimbabwe craton in southern Africa for their research.

Their goal was to determine if the composition and age of the diamonds correlate to the geologic structure of their deep-seated source region. They found that there was a correlation, and that diamonds can reveal a lot about the evolution of the cratonic roots in the area.

Cratons contain the oldest rocks on the planet and provide the nucleus around which younger continental material assembles.

They also hold much of the Earth's mineral wealth including diamonds, which typically form beneath the cratonic crust in root-like structures called mantle keels.

These keels extend to depths of more than 200 kilometers, where pressure is high enough for diamond formation. They date to the Archean period

(3. 9 to 2.5 billion years ago), are thought to be as old as the overlying crust, and have long been a focus of Carnegie scientists.

Diamonds in the Kaapvaal craton are found in much younger volcanic eruptions of kimberlite -- volatile-rich magma that carries diamonds to the surface from deep within the cratonic mantle keel. The scientists looked at the trace element and isotopic composition of the diamonds, and the age and composition of mineral inclusions.

Based on the seismic velocity of P-waves, they also constructed maps of the cratonic mantle keel. The resulting imagery of the deep structure showed the type of mantle above which diamond mines are located.

"When the picture emerged, we wondered if regional patterns of diamond age and composition would fit the seismic structure. What we found was the first general framework for diamond formation that is applicable on a continental scale," says Shirey.

Through detailed analysis, the researchers determined that the diamond formation in the cratonic mantle keel occurred episodically rather than continuously -- that there were multiple generations of diamonds.

Their examination of the diamonds suggested that the craton formed in at least two stages and was subsequently modified in a third stage.

The cratonic nuclei were created first by a process of mantle melting, which was followed by an accretion process involving old oceanic lithosphere -- the rigid layer of crust and mantle under the ocean basins. This latter stage helped stabilize the cratonic mantle keel.

Subsequent tectonic and magmatic events added new diamonds whose inclusion compositions closely corresponded with changes in the composition of the cratonic mantle keel.

Support for this work came from the National Science Foundation (NSF) Continental Dynamics Program, the South African National Research Foundation (NRF), and the Diamond Trading Company (De Beers).

Related Links
The multinational-multidisciplinary Kaapvaal Lithosphere Project
The Carnegie Institution of Washington
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