Sound waves reveal diamond cache deep in Earth's interior

July 16, 2018 by Jennifer Chu, Massachusetts Institute of Technology
A composite image of the Western hemisphere of the Earth. Credit: NASA

There may be more than a quadrillion tons of diamond hidden in the Earth's interior, according to a new study from MIT and other universities. But the new results are unlikely to set off a diamond rush. The scientists estimate the precious minerals are buried more than 100 miles below the surface, far deeper than any drilling expedition has ever reached.

The ultradeep cache may be scattered within cratonic roots—the oldest and most immovable sections of rock that lie beneath the center of most continental tectonic plates. Shaped like inverted mountains, cratons can stretch as deep as 200 miles through the Earth's crust and into its mantle; geologists refer to their deepest sections as "roots."

In the new study, scientists estimate that cratonic roots may contain 1 to 2 percent diamond. Considering the total volume of cratonic roots in the Earth, the team figures that about a quadrillion (1016) tons of diamond are scattered within these ancient rocks, 90 to 150 miles below the surface.

"This shows that diamond is not perhaps this exotic mineral, but on the [geological] scale of things, it's relatively common," says Ulrich Faul, a research scientist in MIT's Department of Earth, Atmospheric, and Planetary Sciences. "We can't get at them, but still, there is much more diamond there than we have ever thought before."

Faul's co-authors include scientists from the University of California at Santa Barbara, the Institut de Physique du Globe de Paris, the University of California at Berkeley, Ecole Polytechnique, the Carnegie Institution of Washington, Harvard University, the University of Science and Technology of China, the University of Bayreuth, the University of Melbourne, and University College London.

A sound glitch

Faul and his colleagues came to their conclusion after puzzling over an anomaly in . For the past few decades, agencies such as the United States Geological Survey have kept global records of seismic activity—essentially, sound waves traveling through the Earth that are triggered by earthquakes, tsunamis, explosions, and other ground-shaking sources. Seismic receivers around the world pick up sound waves from such sources, at various speeds and intensities, which seismologists can use to determine where, for example, an earthquake originated.

Scientists can also use this seismic data to construct an image of what the Earth's interior might look like. Sound waves move at various speeds through the Earth, depending on the temperature, density, and composition of the rocks through which they travel. Scientists have used this relationship between seismic velocity and rock composition to estimate the types of rocks that make up the Earth's crust and parts of the , also known as the lithosphere.

However, in using seismic data to map the Earth's interior, scientists have been unable to explain a curious anomaly: Sound waves tend to speed up significantly when passing through the roots of ancient cratons. Cratons are known to be colder and less dense than the surrounding mantle, which would in turn yield slightly faster sound waves, but not quite as fast as what has been measured.

"The velocities that are measured are faster than what we think we can reproduce with reasonable assumptions about what is there," Faul says. "Then we have to say, 'There is a problem.' That's how this project started."

Diamonds in the deep

The team aimed to identify the composition of cratonic roots that might explain the spikes in seismic speeds. To do this, seismologists on the team first used seismic data from the USGS and other sources to generate a three-dimensional model of the velocities of seismic waves traveling through the Earth's major cratons.

Next, Faul and others, who in the past have measured sound speeds through many different types of minerals in the laboratory, used this knowledge to assemble virtual rocks, made from various combinations of minerals. Then the team calculated how fast would travel through each virtual rock, and found only one type of rock that produced the same velocities as what the seismologists measured: one that contains 1 to 2 percent diamond, in addition to peridotite (the predominant rock type of the Earth's upper mantle) and minor amounts of eclogite (representing subducted oceanic crust). This scenario represents at least 1,000 times more diamond than people had previously expected.

"Diamond in many ways is special," Faul says. "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."

The researchers found that a rock composition of 1 to 2 percent diamond would be just enough to produce the higher sound velocities that the seismologists measured. This small fraction of diamond would also not change the overall density of a craton, which is naturally less dense than the surrounding mantle.

"They are like pieces of wood, floating on water," Faul says. "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 percent diamond for cratons to be stable and not sink."

In a way, Faul says cratonic roots made partly of diamond makes sense. Diamonds are forged in the high-pressure, high-temperature environment of the deep Earth and only make it close to the surface through volcanic eruptions that occur every few tens of millions of years. These eruptions carve out geologic "pipes" made of a type of rock called kimberlite (named after the town of Kimberley, South Africa, where the first in this type of were found). Diamond, along with magma from deep in the Earth, can spew out through kimberlite pipes, onto the surface of the Earth.

For the most part, kimberlite pipes have been found at the edges of cratonic roots, such as in certain parts of Canada, Siberia, Australia, and South Africa. It would make sense, then, that cratonic roots should contain some diamond in their makeup.

"It's circumstantial evidence, but we've pieced it all together," Faul says. "We went through all the different possibilities, from every angle, and this is the only one that's left as a reasonable explanation."

Explore further: Scientists find seismic imaging is blind to water

More information: Joshua M. Garber et al. Multidisciplinary Constraints on the Abundance of Diamond and Eclogite in the Cratonic Lithosphere, Geochemistry, Geophysics, Geosystems (2018). DOI: 10.1029/2018GC007534

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14 comments

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betterexists
1 / 5 (3) Jul 16, 2018
Can Antarctica Expedition find and retrieve such Diamonds ?
pntaylor
3.7 / 5 (3) Jul 16, 2018
"We can't get at them, but still, there is much more diamond there than we have ever thought before."

Still not reading the articles, huh?
rrwillsj
2 / 5 (4) Jul 16, 2018
The writer of this article & the researchers know how to get our attention. By using the word diamonds. Though it is technically true that these formations are of the carbon allotrope we call diamonds. I suspect it is in a form I wouldn't recognize.

be, that Thule Society gibberish you rely on for info about Antarctica? Only in the flying saucer comicbooks is there an entry descending into the Hollow Earth.

However since you wish to expedite an South Pole mining expedition? Be sure you wear your cuddly Dr. Denton jammies with the power flap & those adorable bunny feet! Cause Baby, it's cold outside!

You'll only have to dig through several miles of ice & then several hundred miles of rock. Succeed? You will annoy the Russians & DeBeers with more product for a glutted market.

Jmmm, wait a few decades & all that ice will be unnaturally melted away. Perhaps the landmass, relieved of that weight will rebound above the rising ocean water level? Perhaps not?
TheGhostofOtto1923
5 / 5 (1) Jul 16, 2018
The writer of this article & the researchers know how to get our attention. By using the word diamonds. Though it is technically true that these formations are of the carbon allotrope we call diamonds. I suspect it is in a form I wouldn't recognize... Jmmm, wait a few decades & all that ice will be unnaturally melted away. Perhaps the landmass, relieved of that weight will rebound above the rising ocean water level? Perhaps not?
Nope. Another post from willis with no content whatsoever. No willis, vacuous opinions and idiot catchphrases are not content.

Doesnt bother you in the slightest does it?
TheGhostofOtto1923
not rated yet Jul 16, 2018
Re the article, does this increase the possibility that diamonds can be found in asteroids that are fragments of protoplanets?
Cryptodiamond
5 / 5 (1) Jul 16, 2018
The fragments of these lithospheric roots, "Mantle Nodules" can contain up to 5000 carats of diamond per tonne or 1kg per tonne, are found whilst mining kimberlites. But these are rare even in the richest kimberlite, most nodules are not diamond bearing only one in one hundred have diamonds. It wold seem the rocks sourced from the base of the lithosphere are saying that 1-2% diamond everywhere is an over-estimate. Also not every kimberlite that samples at these depths has diamond. I would believe the rocks themselves before an interpretation of the sound waves. The mantle nodules say there is sporadic vein like diamond distribution less than 0.1% diamond.
Mark Thomas
5 / 5 (1) Jul 16, 2018
This scenario represents at least 1,000 times more diamond than people had previously expected.


So there may be a heck of a lot more diamond below us than we had believed, but most of it is out of reach 90-150 miles down. We know about "certain parts of Canada, Siberia, Australia, and South Africa," where diamonds have risen to the surface. So what are we missing? Are there surface areas that have been overlooked, or more likely, places where diamond rich minerals protrude from deeper layers to within a few miles of the surface, but nobody ever had a reason to drill that deep into before? Who knows, there could be a massive hoard of diamonds a couple miles down under Los Angeles for all we know. So let the race begin to produce the highest resolution maps possible to see where the nearest concentrations are.

In the rush to map the Earth's interior better to find diamonds, maybe we will learn a few things about what's really down there along the way.
rrwillsj
2.3 / 5 (3) Jul 16, 2018
good question otto, There is the possibility that during the early planetesimal phase that the rings of debris that would become planets and moons and asteroids sorted out elemental quantities by distance from the Sun. If Carbon concentrated in wanna-be-an-Earth's orbit? That could have resulted in less Carbon for Mars the Belt and the moons of the outer giants. Or maybe not.

Oh yah, uhhmm otto my boy. Before you IPO your "Lucy in the Sky with Diamonds? What makes the gemstone quality valuable is the Public perception of scarcity. In other words, check with the Cartel before your marketing campaign.

If there are reliable methods to specifically distinguish Space Diamonds? That'd make a great marketing gimmick.

However, I'd place my bets on the release of a reasonably priced 3-D printer pumping out pre-cut diamonds, gemstones. geodes. fancy crystal art before the first asteroid prospector (with a wisecracking robo-mule, natch) ever leaves the ground.
TheGhostofOtto1923
5 / 5 (1) Jul 17, 2018
good question otto, There is the possibility that during the early planetesimal phase that the rings of debris that would become planets and moons and asteroids sorted out elemental quantities by distance from the Sun
-And how could you possibly know willis? You pretending to be something you're not again hmmmm?
Anonym334113
3.7 / 5 (3) Jul 17, 2018
The world is already awash in diamonds. They would have very little worth were it not for the effective monopoly that controls their availability, thus keeping their market price unrealistically high. I never liked diamonds. Can't see the point of them, or any other 'bling'. Why should we try to impress one another with the money or goods we have? Useless, pointless, shallow. So let the highly compressed carbon stay wherever it is.
TheGhostofOtto1923
1 / 5 (1) Jul 17, 2018
Imagine diamonds the size of basketballs. Imagine having enough diamonds to make smartphone screens out of them.

Yeah I know we may soon be fabricating thin films for those purposes but what if we found enough in space to mine them for that?
antialias_physorg
3.7 / 5 (3) Jul 17, 2018
Can't see the point of them

Diamonds aren't just 'bling'. Most of them are used in the industry for various purposes. Only about a fifth of the world's diamond production is fed into the gem industry.
granville583762
5 / 5 (4) Jul 18, 2018
Diamonds in Berkshire lab
De Beers has gone mass market selling diamonds 80% cheaper than the real diamond to make those precious gifts cheaper, apparently that diamond ring for ever has been slipping through the cracks in the pavement on its journey back home to the depths
De Beers is in no hurry to tunnel 100s of miles to greater depths when the Berkshire labs supply his needs and then there is Element Six diamond factory
Diamonds are losing that lustre they once had; the research is interesting all the same.
Cryptodiamond
5 / 5 (3) Jul 18, 2018
The world is already awash in diamonds. .


good question otto, There is the possibility that during the early planetesimal phase that the rings of debris that would become planets and moons and asteroids sorted out elemental quantities by distance from the Sun
-And how could you possibly know willis? You pretending to be something you're not again hmmmm?


To Granville and others, I have spent 35 years looking for and sometimes discovering diamond deposits, this is difficult and risky concern, they are rare, . Due to discoveries in the 70 -80's by others, De Beers no longer have a cartel. There have been no major new discoveries since the 90's . Demand grows with Global GDP.

To Otto, true diamonds have been found in Asteroids,no gem, none of any size, all microns at most.
The true value of this article is on how the Earth works and what this means for the carbon cycle.

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