FAQ: Your diamond questions answered!

Michael Walter: Generally, yes. They typically form at depths greater than about 150 km and are brought to the surface in kimberlite magmas.

Michael Walter: Some are grown in large presses at pressures of about 60000 bars and maybe 1000 C, typically from carbon dissolved in a metal solvent. Other diamonds are grown through carbon vapor deposition (CVD), which is condensation from a carbon vapor onto a diamond substrate.

These diamonds tend to be more "perfect" than natural diamonds in the sense that they can have fewer defects, and the defects that control color can be controlled. The relative lack of defects is also how you can tell synthetics apart from natural diamonds.

Michael Walter:‘Space diamonds’ or interstellar diamonds likely form directly by condensation from gas—kind of like the CVD diamond I mentioned in the above question. They can also form when planets collide through shocking carbon to high pressures.

Michael Walter: Not easily. They can break apart by mechanical erosion or be destroyed by oxidation at high temperatures in magma.

Michael Walter: Not that we know of. They seem to be exclusively associated with deep, cold lithospheric mantle, which can only be found under continental crust, not oceanic crust. 

Richard Carlson: There is a mineable diamond deposit in the ocean off the coast of Namibia and South Africa, where the Orange River enters the Atlantic.  These diamonds likely eroded out of the kimberlites in central Africa and were then carried by the river to the coast. There are lamproites, which are sort of like kimberlites, in the Ontong Java Plateau—a large underwater plateau. 

Michael Walter: Very, very low.

Michael Walter: Yes. We don’t actually date the diamond itself, but minerals that are inclusions inside the diamond. We have to break or cut the diamond to get them out.

Michael Walter: Sometimes, the big diamonds are broken during crushing. But diamonds are tough, so most of the smaller ones probably make it through the crushers just fine and are then separated by density.

Michael Walter: Carnegie scientists receive funding from various sources, including federal grants, the Carnegie Science endowment, individual donors, and sometimes from the diamond industry.

Michael Walter: Many abrasive diamonds are synthetic, but naturals are also used. They're pretty cheap—like $1 per carat!

Michael Walter: We chase down diamonds because they trap bits of the deep mantle as inclusions, and they provide us with invaluable information from parts of the Earth that are difficult if not impossible to sample directly.

Diamonds are very durable and so can trap and store the inclusions for billions of years! They reveal the ages of rocks and the chemical processes that formed them, and that helps us piece together the history of how our planet (and even life) formed.

Michael Walter: Kimberlites are CO₂- and H₂O-rich magmas that form deep in the mantle—greater than 300 km in depth. As they move up, the solubility of the volatile elements—the CO2 and H2O—in the magma decreases. So the magmas become super-charged with low-density gas, which provides a huge driving force that shoots the magma up to the surface of the planet. Estimates are that they can travel from the deep mantle in a matter of days, which is almost no time at all on a geologic timescale! As they scream upward through the mantle, they pick up mantle rocks that contain diamonds and other minerals and bring them to the surface.

Michael Walter: Hexagonal diamonds—also called Lonsdaleite—form through the high shock pressures of meteorite impacts and not in the mantle, to my knowledge.

Michael Walter: Absolutely. Carbon is a common element and probably even more common in some other planets—based on their star compositions—than Earth.

Graham Pearson:  The remarkable thing about those cold deep mantle roots is that they translate with the continents as they move across Earth's surface.

The really amazing thing is that the processes that led to the formation of those roots early in Earth's history made the roots so strong that they survived billions of years of being pushed around on Earth's surface. Where the diamonds are old, they've dragged those diamonds around as well. So, not only are the diamonds deep, but they've had an interesting ride around Earth's surface on a bit of a bumper car ride before they then get sampled by the kimberlite. We can use what we find in the diamond's inclusions to piece together that history.

Graham Pearson:  Yes, they can. The tricky bit is that there is no unique signature of kimberlite; it depends on the host rock that the kimberlite is in. So, you've got a kimberlite that might be a kilometer across. If you're not so lucky, it might be tens of meters across. So you have to see that kimberlite signature within noise in whatever the host rock is—it might be limestone, it might be granite, it might be basalt.

So yes, you can use those techniques, but normally they are used after the kimberlite has already been found and the geologists have been able to establish what the host rock is. Then the geophysicists can do the modeling, and you can work out a signature.

Graham Pearson:   So, no, we don't share in the proceeds. There's a joke that you should never make a geologist the CEO of your company. As academics, we do it for the love of science.

We are sometimes given small pieces of very large stones to study. And, if you work with GIA, they have unbelievably valuable specimens that pass through their offices, so their scientists get to use nondestructive techniques to study them.

Graham Pearson:   That's a picture that we're still painting. If you look at the 300-400 km, there is abundant evidence that the carbon is coming from the subducting slabs bringing surface carbon down. When you get down to 700 km, that surficial relationship isn't really obvious. There is evidence that the carbon is just normal mantle carbon, but there is also some evidence that carbon might be making it all the way down there from subducting slabs.

Ask me again in 2 years, and I'll probably have a better answer.

Michael Walter:   Any mineral can have inclusions of other minerals or even frozen melts, and we study those as well. Diamonds are great because they are so durable, and virtually nothing can diffuse through them at any appreciable rate, so they preserve inclusions much better than other minerals can.

Michael Walter:   Cloudy diamonds generally form as a consequence of trapping lots of small fluid inclusions.

Michael Walter:   We use many techniques starting with an optical microscope and then including Raman spectroscopy, X-ray spectroscopies (diffraction, XANES, etc.), FTIR, electron microprobe, SIMS—basically whatever tools we can find that will give us more information!

Michael Walter:   Well, we definitely find that most superdeep diamonds seem to carry a subduction signal in their chemistry. Diamonds from the cratonic lithosphere may also be linked to subduction processes in some cases, especially those of the 'eclogitic' variety.

Michael Walter:   Good question! The answer is not clear. In part, the reason may be due to sampling bias. We have better access to the diamonds in Brazil because, overall, they tend to be lower quality stones. We don't have access to most diamonds mined in South Africa and Canada, for example, so we are missing out on a lot of samples!

Michael Walter:   We definitely study both, and they tell us different things. They are not typically of the same age and form by different processes.

Michael Walter:   The carbon isotopic composition of diamonds is quite variable. Diamonds with peridotitic inclusions tend to have values that are close to the mantle value. In contrast, those with eclogitic inclusions often have much lighter carbon, consistent with a source of organic carbon, possibly biogenic.

Michael Walter:   The iron isotopes of the metallic inclusions are unusually heavy, around 1 per mille heavier than normal mantle rocks. So, it's not likely the inclusion originates from the mantle. However, the iron looks a lot like the magnetite produced as a product of serpentinization—a process that occurs when seawater moves through fractures to alter underlying rocks in the seafloor as it subducts. What this says to us is iron, carbon, water, and other minerals from the Earth's surface area making it down to a depth of 700 km and are potentially responsible for the diamonds created there and the source of our planet's mysterious deep focus Earthquakes.