There might be over a quadrillion tons of diamonds hidden in the Earth’s interior, as reported by a new study conducted by MIT and other universities. However, the results are unlikely to set off a diamond rush. The researchers estimate that the precious minerals are buried over 100 miles below the surface, which means far deeper than any drilling expedition in the past has ever reached.
The ultradeep cache might be scattered within cratonic roots — these are the oldest and most immovable sections of rock which lie beneath the center of most continental tectonic plates. Cratons, shaped like inverted mountains, can stretch as deep as 200 miles through the planet's crust and into its mantle. Geologists actually refer to their deepest sections as “roots.”
In this new study, scientists estimate cratonic roots might contain 1 to 2% diamond. Considering the total volume of cratonic roots on the planet, the team claims that approximately a quadrillion (1016) tons of diamond are scattered within the ancient rocks, about 90 to 150 miles below the surface.
According to Ulrich Faul, a research scientist in the Department of Earth, Atmospheric, and Planetary Sciences of MIT, this shows that perhaps diamond isn't an exotic mineral, but instead, it's relatively common on the geological scale.
Faul’s co-authors include researchers from the University of California at Santa Barbara, the University of California at Berkeley, Ecole Polytechnique, the Carnegie Institution of Washington, the Institut de Physique du Globe de Paris, Harvard University, the University of Science and Technology of China, the University of Melbourne, the University of Bayreuth, and University College London.
Faul and his colleagues concluded after puzzling over an anomaly in seismic data. For the last few decades, agencies like the United States Geological Survey have maintained global records of seismic activity — basically, sound waves traveling through the Earth, triggered by earthquakes, explosions, tsunamis, and several other ground-shaking sources. Seismic receivers around the globe pick up sound waves from such sources, at ranging speeds and intensities, that seismologists can use to determine where, for instance, an earthquake originated.
Scientists can also use the seismic data to construct a picture of what the Earth’s interior may look like. Sound waves move at ranging speeds through the globe, depending on the density, temperature, and composition of the rocks through which these travel. Scientists have used the relationship between seismic velocity and rock composition in order to estimate the types of rocks that make up the crust of the Earth and parts of the upper mantle, which is also known as the lithosphere.
Nevertheless, in using seismic data to map the interior of the Earth, scientists have not managed to explain a curious anomaly: Sound waves speed up significantly when they pass through the roots of ancient cratons. These cratons are known to be colder as well as less dense than the surrounding mantle; this would in turn yield slightly faster sound waves, though not quite as fast as what has been estimated.
The team aimed to identify cratonic roots' composition which may explain the spikes in seismic speeds. To do that, seismologists first used seismic data from the USGS and several other sources to generate a 3D model regarding the velocities of seismic waves passing through the Earth’s major cratons.
Then, Faul and others, that in the past have measured sound speeds through several different types of minerals in the laboratory, used that knowledge to assemble virtual rocks, made from various combinations of minerals. Next, the team calculated how fast sound waves could travel through each virtual rock, and discovered only one type of rock which produced the same velocities as what the seismologists measured: one that contains one to two percent diamond, along with peridotite (the predominant rock type of the upper mantle of the Earth) and not to mention minor amounts of eclogite (representing subducted oceanic crust). That scenario represents at least 1,000 times more diamond than people had previously expected.
According to Faul, one of diamond's unique properties is that its sound velocity is "more than twice as fast as in the dominant mineral in upper mantle rocks, olivine."
The team of researchers discovered that a rock composition of 1 to 2% diamond would be enough to produce the higher sound velocities which the seismologists measured. The small fraction of diamond wouldn't change the overall density of a craton, that is by change less dense in fact than the surrounding mantle.
“They are like pieces of wood, floating on water,” Faul said. “Cratons are a tiny bit less dense than their surroundings, so they don’t get subducted back into the Earth but stay floating on the surface. This is how they preserve the oldest rocks. So we found that you just need 1 to 2% diamond for cratons to be stable and not sink.”
In a way, Faul says that cratonic roots made partly of diamond makes sense. Diamonds are forged in the high-pressure, high-temperature environment of the deep Earth, and merely make it close to the surface through volcanic eruptions which occur every few tens of millions of years. Those eruptions carve out geologic “pipes” made of a type of rock called kimberlite (named after the town of Kimberley, South Africa, where the first diamonds in this type of rock were found). Diamond, as well as magma from deep in the Earth, can spew out through kimberlite pipes, onto the Earth's surface.
For the most part, kimberlite pipes have been discovered at the edges of cratonic roots, such as in particular parts of Canada, Australia, Siberia, and South Africa. It would then make sense that cratonic roots contain some diamond in their makeup.
This research was supported, in part, by the National Science Foundation.
References: techgear.gr, news.mit
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