The seismic rumblings could provide key clues about where miners should look for diamonds, according to recent research. Matt Fouch, assistant professor of geological sciences at ASU, studies vibrations caused by earthquakes to visualize the earth at depths of hundreds of kilometers, where diamonds are formed. His maps of the earth below South Africa provide new information about Earth’s structure in regions where many diamonds are found.
In the July 1, 2001 issue of Geophysical Research Letters, Fouch and his coauthors, David James, John VanDecar (both of the Department of Terrestrial Magnetism, Carnegie Institution of Washington), and Suzan van der Lee (of the Institute of Geophysics, Zürich, Switzerland), show that some of southern Africa’s most profitable diamond mines are located near areas where the earth is exceptionally stable and cool up to 250 kilometers below the surface. The paper will be published in a special section of the journal, with seven other studies on geochemistry, composition, and rock dating of southern Africa.
Many diamonds come from regions, called cratons, that are some of the most geologically stable places in the world. Two cratons, the Kaapvaal and Zimbabwe cratons, covering an area roughly the size of the nation of South Africa, are the source of most of southern Africa's diamonds.
"The region we're studying in southern Africa is over 3 billion years old, and in some places it's even 3.6 billion years old," says Fouch. Geologists think diamonds develop up to several hundred kilometers deep within these ancient cratons and are then driven straight up to the surface.
Miners scout the best places to dig for gems by looking for diamonds that have made their way to the surface. Other techniques, such as drilling for samples deeper in the rock or studying anomalies in the gravitational or magnetic properties of the earth in the area, increase the chances of finding diamonds. But none of these approaches guarantee success. "If people knew exactly how it worked all the time, then we'd have a lot more diamond mines," Fouch jokes. "Nearly all diamonds come from cratons, but not all cratons contain diamonds. So the question is, why do some cratons produce diamonds and others don't? Another question is, why do some of those areas have diamonds that are commercially profitable, and others don't? Some regions have diamonds, but they're just too chewed up to be gem quality."
Fouch and his colleagues think they may have found part of the answer deep in the earth's mantle -- the layer of rock that extends several hundred kilometers beneath the crust. By imaging the earth at these depths, they looked at the very source of diamonds, rather than waiting for them to travel to the surface.
Fouch created three-dimensional images of deep layers of the earth by using an array of 82 seismometers, sensors that detect vibrations caused by earthquakes from all around the world. The seismometers, placed at roughly 100-kilometer intervals across South Africa, Zimbabwe and Botswana, recorded data from more than 200 earthquakes occurring over a two-year period, mainly from the Himalayan and Andean mountain ranges. They used seismic tomography, a technique very similar to CAT scans in medical imaging, to produce the images.
"As people, we never want earthquakes to happen, but as seismologists we know they are an inevitability. So our job is to use them in the most productive way possible," says Fouch. "Every time an earthquake happens, it's like shining a flashlight on a particular part of the earth. The seismic waves from each earthquake bounce off of different layers of the earth and illuminate different internal features."
The speed and angle of earthquake waves' motion depends on what kind of material they travel through. For example, the rippling caused by dropping a pebble in a bowl of water will move differently than in water containing ice cubes or in a bowl of jelly. By analyzing the timing and angle of the vibrations' spread past the seismometers, Fouch and coworkers mapped the physical properties of the earth below.
They found that the mantle directly below the most productive diamond mines looks distinctly different than in the surrounding areas. In diamond-producing areas, the mantle is "seismically fast," meaning that it propagates earthquake vibrations quickly because the mantle rock may be cooler or chemically different from the surrounding areas.
"There are a few distinct pockets of the faster seismic velocities," Fouch explains. "One of these regions is beneath the Kaapvaal craton in South Africa, and one -- a little more diffuse -- is beneath the Zimbabwe craton. ... Most of the gem-quality diamond mines in southern Africa lie very close to these regions." By looking for similarly cold, seismically fast parts of the mantle, diamond miners may be able to identify new promising areas for mining.
Industry collaborators in southern Africa are very interested in Fouch's research, and some even allowed the seismologists to install seismometers on their property.
"This is certainly a technique that could be used in conjunction with other methods to possibly determine whether a region might be more prone to having diamonds," Fouch says.
Journal
Geophysical Research Letters