Scientists have found evidence to suggest that a thousand trillion tons of diamonds are buried deep in the Earth’s crust. The diamonds are located in an ancient area, known as cratonic roots, between 90 and 150 miles below the surface of the Earth.
Ulrich Faul is a research scientist in the Massachusetts Institute of Technology (MIT) Department of Earth, Atmospheric, and Planetary Sciences.
“This shows that diamond is not perhaps this exotic mineral, but on the geological scale of things, it’s relatively common,” said Faul. “We can’t get at them, but still, there is much more diamond there than we have ever thought before.”
Cratons are stationary sections of rock that are among the oldest on the planet. Shaped like inverted mountains, these rock walls extend as far as 200 miles into the Earth beneath the center of tectonic plates.
While studying the sound waves generated by earthquakes and other forms of seismic activity, the team noticed that the sound sped up significantly when passing through the roots of ancient cratons.
Cratons are known to accelerate sound waves slightly, but not as fast as what the scientists measured.
“The velocities that are measured are faster than what we think we can reproduce with reasonable assumptions about what is there,” said Faul. “Then we have to say, ‘There is a problem.’ That’s how this project started.”
The researchers set out to determine the composition of the cratonic roots to understand what may be causing the spikes. Using seismic data from the U.S. Geological Survey and other sources, the researchers developed a three-dimensional model of the velocities of seismic waves traveling through the planet’s cratons.
Faul and his team used measurements of sound speeds to assemble virtual rocks made from various types of minerals.
The experts calculated how fast sound waves would travel through each virtual rock, and found only one type of rock that would produce the speeds measured by seismologists.
The rock that matched their search was composed of one to two percent diamond, peridotite, and small amounts of eclogite.
“Diamond in many ways is special,” said Faul. “One of its special properties is, the sound velocity in diamond is more than twice as fast as in the dominant mineral in upper mantle rocks, olivine.”
This could easily explain the higher sound velocities that were detected. In addition, the small fraction of diamond would not affect the overall density of a craton, which has less density than the surrounding mantle.
“It’s circumstantial evidence, but we’ve pieced it all together,” said Faul. “We went through all the different possibilities, from every angle, and this is the only one that’s left as a reasonable explanation.”
The research was funded in part by the National Science Foundation.