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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Diamond is a high-pressure form of carbon. Most diamonds grow at depths of 150 to 200 kilometers before being ejected up to the surface through deep volcanic pipes called kimberlites. A few diamonds originate from much greater depths, but they were thought to consist only of small, heavily fractured specimens with many inclusions, and so were rarely of gem quality. But in 2016, Evan Smith and his co-workers discovered that some of the largest and most highly prized gems were also formed deep in the mantle, at depths of 600 to 650 kilometers. Since such diamonds are effectively messengers from the deep mantle, they have become some of the most valuable gemstones from a scientific standpoint as well. What can we learn from them about the deep mantle and the processes that led to their formation at such extreme depths? Evan Smith is a senior research scientist at the Gemological Institute of America in New York. Evan Smith, welcome to Geology Bites.

Evan Smith: Thank you. It's a pleasure to be here. I appreciate you having me.

Oliver Strimpel: We've been mining diamonds for centuries. How come this ultra-deep class of diamonds was only discovered so recently, and how did you and your colleagues identify them?

Evan Smith: We've been mining diamonds for a long time, but we've only really developed a geological understanding since the development of the electron microprobe, when we could really analyze inclusions in diamonds and develop an understanding of how they form. Now, there are some diamonds that we know more about than others, and the kind of diamonds that I started studying were ones that typically are not available to researchers. People have known for a long time that larger and higher-quality diamonds that seem to have few inclusions appear to stand out from other kinds of gem-quality diamonds. They look different in their physical characteristics. People had noticed that a long time ago, but no one had been able to take a detailed look at why that might be the case. So what I was able to do with my colleagues was parse out some of those individual samples and take a detailed look at what their inclusions are. And we recognized that this is actually another class of diamonds, another variety of diamonds that form in a distinct way.

Oliver Strimpel: So it was actually the inclusions, and you had to do some microscopic analysis in order to notice that this was a distinct class from the other gem-quality diamonds.

Evan Smith: Yeah. And when I say distinct class, we're talking in general about type II diamonds. Diamonds have nitrogen as their most abundant impurity, and some diamonds have enough that you can see it with infrared spectroscopy. Some diamonds have too little nitrogen to see with infrared spectroscopy, and we call those nitrogen-poor, or nitrogen-free, diamonds. Those are type II overall. Those are a very rare category; maybe one or two percent of mined diamonds are in that nitrogen-poor category. But when you look at, say, diamonds over 10 or 20 carats, which make up the best and biggest gemstones, a huge proportion of them are type II. So it stands out like a sore thumb. Why is there that strange relationship with the size and quality and color of diamonds? That's the reason why gemologists started to regard them as maybe a distinct and special category. And it took geologists a long time to understand why those larger, quite often type II, diamonds might be different. The difference comes down to studying their inclusions and getting a sense of the environment that those diamonds form in. And it turns out that they form in a completely distinct environment from all other kinds of diamonds.

Oliver Strimpel: So some of the distinctive features of these diamonds were actually used to come up with an acronym, which is the way these diamonds are commonly referred to. Could you explain that?

Evan Smith: Yeah, absolutely. I've loosely referred to this category as type II, but if you want to actually have a meaningful conversation about them, you have to recognize that there are some type II diamonds that don't fit in that category, and there are some diamonds that are clearly in that category that aren't quite type II. So a more meaningful encapsulation of their whole set of physical characteristics is put into this acronym, CLIPPIR. The diamonds that I'm referring to are Cullinan-like; that's the C. The Cullinan is the largest gem-quality diamond ever found. It was 3,106 carats in its rough state, and it's sort of the type diamond for this category. So diamonds that are like that diamond, Cullinan-like, overall as a group tend to be large. They tend to be inclusion-poor. They tend to be very pure, so they have very little nitrogen. They are irregular in their shape, so they very seldom have a nice crystal shape. And they're quite often very heavily resorbed; that's the R. They're sort of dissolved and etched in their outer surface characteristics.

Oliver Strimpel: OK, so CLIPPIR, C-L-I-P-P-I-R, CLIPPIR diamonds.

Evan Smith: That's right, kind of alluding to clipper ships, but spelled a little bit differently to distinguish them.

Oliver Strimpel: How do we actually know that they originated 600 to 650 kilometers below the surface, which is about three times deeper than the vast majority of diamonds that we find?

Evan Smith: The depth of origin for CLIPPIR diamonds comes from recognizing the mineral inclusions that they sometimes contain, and there are a couple of key phases. One is majoritic garnet. This is a special high-pressure form of garnet that contains an excess of silicon. Normally, you have three atoms per formula unit of silicon in a garnet structure, and majoritic garnet has a little bit more than that. So there's a coupled substitution of silicon, aluminum, sodium, titanium and chromium into the garnet structure that makes it majoritic in its characteristics. And we know this is something that can't form in the lithospheric mantle; it requires higher pressures. So when you have majoritic garnet in a diamond, it indicates that it comes from greater depths. We have some examples of majoritic garnet in CLIPPIR diamonds. We also have examples of calcium silicate minerals, namely breyite, and we think that this is a retrogression or inversion product of the higher-pressure phase called calcium silicate perovskite, or davemaoite, as its mineral name. So, again, this is a mineral trapped in diamonds that we think could only have been trapped at very great depths. In the 2016 paper, I gave a looser bracket of somewhere between 360 and 750 kilometers deep in the Earth. But I think putting it somewhere within the mantle transition zone or the top of the lower mantle is probably about right, in terms of the processes and the environment where these diamonds might grow. We also have some information from geobarometry that indicates that they come from higher pressures. But the biggest constraint on the depth for CLIPPIR diamonds really comes down to the observation of these high-pressure minerals that could only have formed at great depths.

Oliver Strimpel: Do you think it's possible that some diamonds might have been able to form at even greater depths, and it's just that they don't make it to the surface, or we haven't found them yet?

Evan Smith: There are some people who believe that there are diamonds formed at the core–mantle boundary, and I wouldn't be so quick to rule that out. But I think at present we don't have any really firm evidence that shows diamonds must come from that depth. And based on our understanding of how these diamonds are forming, by and large, they involve subduction and ingredients derived from subducted slabs, and the processes going into them seem to occur where things are happening to subducting slabs, where they're losing volatiles. We think this makes a lot of sense: as a subducting slab reaches the mantle transition zone and lower mantle, it's losing volatiles, and diamond formation is part of that process.

Oliver Strimpel: So you mentioned some telltale minerals, such as the majoritic garnet, that tell us these diamonds actually originated at those extreme depths. But how can they survive at the pressures and temperatures up here on the surface?

Evan Smith: Diamond itself is a very robust crystal structure. It has incredible strength and an almost chemical inertness, so it can survive across a wide range of geological conditions after it's formed. We know, based on the morphology of CLIPPIR diamonds, that a lot of them appear to have been broken in the mantle, and they've certainly been very strongly resorbed. So a lot of the diamond might actually be missing; it's been dissolved or oxidized away. But the great thing about CLIPPIR diamonds is that because their average size is so large, it appears as though we still have a lot of diamond to deal with, even after a lot of it has been destroyed. So we end up with diamonds making their way all the way to the surface by a couple of transport mechanisms. There's some initial stage of upwelling, or vertical ascent, for the diamond, but we know the final stages of diamond ascent involve kimberlites, or related mantle-derived magmas, that sweep diamonds from a depth of, say, 200 kilometers all the way up to Earth's surface. That journey is not necessarily friendly to the diamonds. It can etch or dissolve or resorb away some of the diamond. But when we see them, we know that they've retained at least some of their diamond material, because we have enough to cut and polish.

Oliver Strimpel: And you mentioned that even though they're inclusion-poor, which is the IP in CLIPPIR, they still have some inclusions. So how do those inclusions survive the enormous changes of pressure? You'd think they would just kind of explode and blow the diamond to pieces.

Evan Smith: Yeah. So the diamond doesn't change very much in its volume as you change the pressure. It's not very compressible, but the minerals trapped inside it are, relatively speaking, much more compressible, and they want to expand as you bring that diamond up to the surface. So they exert a tremendous amount of pressure on the surrounding diamond, and the diamond can crack locally around an inclusion. It can deform locally around an inclusion in response to that tremendous pressure. But to some degree, that tremendous pressure can actually be preserved, in some cases. We do see inclusions sometimes with 1, 2, 3 gigapascals of pressure; I think the highest is even over 10 gigapascals of pressure, actually preserved at surface conditions inside an inclusion in diamond. So some of those tremendous pressures are another strong piece of evidence that requires the diamonds in those cases to have a sublithospheric, or super-deep, origin.

Oliver Strimpel: So these diamonds actually come from depths that correspond to the lower mantle transition zone, where we see a change in seismic wave velocity, which is thought to be caused by changes in the mineral phases that are present down there. Up till now, our only sources of knowledge about this region have come from seismic data and from lab experiments that try to mimic the temperatures and pressures of the mantle, such as those described by David Kohlstedt in his podcast episode. What new information have we learned about this zone from the CLIPPIR diamonds?

Evan Smith: First, just to be perfectly clear, we do have other kinds of sublithospheric diamonds. We've known since the late 1980s that some diamonds appear to come from the sublithospheric mantle. There's a body of literature already in existence before 2016 describing diamonds from western Africa, and the big one is the Juína region in Brazil, which contain a large proportion of diamonds that look like they come from the asthenosphere, or the transition zone, or the uppermost lower mantle. CLIPPIR diamonds add another layer to that story, because they tell us that there's actually another variety of diamonds, another suite, within that super-deep package. And what we're learning from all of those diamonds is a sense of what happens to subducted slabs. Sublithospheric diamonds seem to have some kind of genetic connection with subducted materials. They appear to form from volatiles released from the subducting slab. So the slab could be releasing carbon, it could be releasing water, it could be releasing boron, in the case of type IIb diamonds, which are often blue.

So that's the general picture we see from sublithospheric diamonds. And specifically for CLIPPIR diamonds, we're seeing another angle on this subduction story, because even though CLIPPIR diamonds usually don't contain inclusions, when they do, about 70% of the time those inclusions are this unusual metallic phase that we think was actually trapped as a molten metal. It's a mixture of iron, nickel, sulfur and carbon, and it's likely that it actually served as the diamond growth medium and has now been trapped as little blebs inside some of these CLIPPIR diamonds. That metallic melt is something totally bizarre that we really didn't expect to find. Something we learned more recently, in 2021, is that that metallic melt has a direct connection to serpentinization in the seafloor. It has a heavy iron isotopic signature that we can't explain in any other way than to say that the iron-rich precursor was a product of serpentinization, that is, ocean water interacting with the oceanic lithosphere before it's subducted down to the mantle transition zone or the top of the lower mantle. So the CLIPPIR diamonds are telling us that there's this unusual iron-rich metallic melt that is produced during serpentinization or that evolves during subduction. And that's an additional part of the subduction story that you can't really get easily from seismic observations, or even from experiments. So diamonds give you another window into deep subduction processes, to tell you what's happening to the slabs down there.

And a big question on everyone's mind is how much material is actually recycled and carried from Earth's surface deep down into the mantle. The diamonds seem to be a product of that process, the conveyor belt that's bringing things down into the mantle, and diamonds and their inclusions are capturing the release of these materials. They're capturing the sort of hidden secondary arc, if you will: as the slab enters the lower mantle, there's another wave of geological activity that is unseen to us, and it's that story that's captured really well by sublithospheric diamonds.

Oliver Strimpel: That's fascinating. Is it possible to explain how we can connect that metallic iron–nickel material to serpentinization?

Evan Smith: Yeah, it's sort of a convoluted pathway, and to be fair, we don't understand it fully. But we know serpentinization loosely looks like this. You've got water circulating through the crust and mantle portions of the oceanic lithosphere. It reacts with olivine, and it makes, as products, serpentine, brucite and magnetite, and you've got a fluid that's evolving during that process. Because you're oxidizing iron from iron two-plus to iron three-plus to make magnetite, you end up, in some cases, with a kind of reducing fluid that has hydrogen in it. And that reducing fluid can actually start to precipitate iron–nickel alloy phases. We know that both the magnetite, which is an iron-rich phase, and the iron–nickel alloy, which is another iron-rich phase, both of those iron-rich phases produced during serpentinization, are isotopically heavy. We think that one or both of those iron-rich phases are contributing directly to the evolution of this metallic melt that we now see trapped in CLIPPIR diamonds. Again, we don't fully understand how that story of serpentinization in the seafloor ends up making this diamond-forming liquid. There's some kind of evolution process that happens during subduction. You've got a journey of a couple of hundred kilometers over a few million years between the time of serpentinization and diamond growth. And right now, that's what we're trying to figure out: how this metallic phase evolves, how widespread it might actually be, and what it might actually mean beyond just diamond formation.

Oliver Strimpel: So the isotopic signature is also further evidence that this material came from the seafloor. But it's interesting that you said it was actually an isotopically heavier material, because I thought that some of the materials that accumulate on the seafloor, because they're generated either by weathering processes or by biogenic processes, tend to produce isotopically lighter signatures.

Evan Smith: Yeah, that's exactly correct. During the deposition of carbon in the seafloor, when we see lighter signatures, we attribute that to biogenic processes. We think that that carbon has come about because of the activity of living things, and we do think that some of that isotopically light carbon may actually be subducted and go towards making diamonds with isotopically light signatures, light in terms of their carbon signature.

Oliver Strimpel: And is that what we actually measure in the diamonds?

Evan Smith: Yeah, yeah. We can measure the carbon isotopic signature of diamonds fairly easily, and that's one of the nice pieces of information that seems to connect diamonds with subduction. So we see a story of subducting slabs contributing to diamond formation, and that story is echoed by the mineral inclusions, by the carbon isotopes of the diamonds themselves, and by things like the iron isotope chemistry and the fluid chemistry in general that we see. Another observation is the boron isotopes in type IIb diamonds. These are blue diamonds, like the Hope diamond. Their boron isotopic signature is a little bit heavy; it's heavier than what you would expect to see for simply mantle-derived boron. It suggests very strongly that the boron we see in those diamonds must have come from Earth's surface and was deposited into the oceanic lithosphere by serpentinization or alteration.

Oliver Strimpel: One of the reasons that CLIPPIR diamonds were not identified as belonging to a different class is that we actually find them in the same locations as all the other diamonds. Is that just a bias, because that's where we're looking for diamonds, or are these in fact thought to be the only places at which CLIPPIR diamonds make it to the surface?

Evan Smith: That's a really good question. The fact that we recover CLIPPIR diamonds at places where we mine lithospheric diamonds is maybe a by-product of the fact that when we're looking for new diamond deposits, we're essentially looking for lithospheric diamonds. They seem to make up the bulk of mined gem diamonds. And the tools that we have for finding new deposits, like indicator minerals, which are mantle-derived minerals that accompany diamonds but are more abundant than the diamonds themselves, are exclusively lithospheric. And when we're looking at a deposit and trying to decide whether or not it's economic to mine, we take a bulk sample of that kimberlite to see how many diamonds are in it. Usually, we're looking exclusively, again, at the lithospheric diamonds, because those are a more abundant population. When we do find CLIPPIR diamonds in a deposit, they tend to be much less frequent, and it's really difficult to assess how many are there, because they're present in such low abundances. But that leaves open this interesting possibility that maybe there are some kimberlites that appear to be barren but actually contain small amounts of CLIPPIR diamonds, and we just don't have a way of detecting the presence of diamond in those deposits. Even if you dug up a big bulk sample, you might not find many diamonds at all. It's not until you mine the entire deposit that you realize, oh, actually, we've got some of these larger, higher-quality CLIPPIR diamonds that do make this deposit economic. But it would be a very risky venture to try that, so nobody does.

Oliver Strimpel: Do we know anything about how old these CLIPPIR diamonds are and how long they took to reach the surface?

Evan Smith: The age of CLIPPIR diamonds is still very much an outstanding question. The fact that we find some CLIPPIR diamonds in older deposits, like the Premier kimberlite, tells us that there must be some CLIPPIR diamonds that are at least 1.15 billion years old. So whatever the process is that makes CLIPPIR diamonds, it has been operating for over a billion years. We also find CLIPPIR diamonds in younger deposits, like at Letšeng in Lesotho. This is a 90-million-year-old deposit, but we don't know what the age of those diamonds is; we just know that they're not any younger than 90 million years. We can take some clues to the ages by looking at other categories of sublithospheric diamonds, such as those from Juína, which are not necessarily great gemstones but are also part of this super-deep diamond-forming story. We have some ages for those diamonds. A recent study by Suzette Timmerman found that there are sublithospheric diamonds that appear to have formed a long time ago, in the Precambrian, maybe 600 million years ago, something like that, and those diamonds sat around in the mantle before eventually being brought to the surface. So we know that, at least in some cases, there is a period of mantle residence for sublithospheric diamonds. They have to be sitting somewhere in the mantle for a few hundred million years before they're brought to the surface.

And in the case of that study, what was really nice is that they found diamonds of similar ages in Brazil, on the South American continent, and some on the African continent, and those are obviously divided by the Atlantic Ocean. So they think that those packages of diamonds were actually adhered to the base of the continental lithosphere, and that's where they've been residing. They've sort of been sitting at the bottom, stuck, plastered onto the bottom of the continent, for millions of years. And as those continents have drifted apart, the diamonds have gone with them, because they're stuck to the bottom like chewing gum on your shoe. It's not until a kimberlite comes along and samples some of that mantle package and brings the diamonds to the surface that you know that they're present at all. So you need this interesting turn of events, coincidences that line up, to actually bring the diamonds to the surface.

Oliver Strimpel: That's fascinating. So it's really possibly a two-step process: they form at these great depths, down in the mantle transition zone region, and then some mechanism, maybe it's just convection, brings them up to the base of the lithosphere, where they get stuck and then wait for whatever process is associated with kimberlites, I don't know if it's considered to be mantle plumes or what, until one just happens to be above them. So it really conjures up a vision that maybe the entire lithosphere is peppered with these bits of valuable chewing gum, if you like.

Evan Smith: Yeah, it's possible. And I should make a comment on the ascent mechanism for sublithospheric diamonds. Right now, our best idea of the ascent mechanism might involve just the upwelling of subducted materials. The subducted slab as a whole is denser than the surrounding mantle; that's why it's sinking. But within that slab, there are perhaps more depleted, more magnesium-rich regions, with less iron, so they're less dense. Some of these packages of rock might actually be less dense than the surrounding mantle and form buoyant diapirs. They might not be all that big, but they just have to be big enough to capture packages of diamond-bearing rock, bring them up and plaster them onto the bottom of continents. So we think that it makes the most sense if that ascent mechanism is tied to the process of diamond formation and also involves subducted ingredients, because that's your best chance of having diamond formation and then the appearance of buoyant materials that can bring the diamonds up and park them at the base of the lithosphere.

Oliver Strimpel: As I mentioned in the introduction, these diamonds are extremely valuable, and I'd be surprised if your research budget enabled you to outbid celebrities and royalty. So how do you get access to them? And are you confined to nondestructive methods, such as spectroscopy?

Evan Smith: When we know for sure they're part of the CLIPPIR category, we're talking about diamonds that are quite large, a few carats or more, you know, upwards of 10, 20, 100 carats. And sometimes these diamonds are completely flawless and D color, so they're the best of the best gem diamonds, and absolutely, they command the highest prices. You can't buy that on a research budget. You can't borrow it.

Oliver Strimpel: What sort of price are we talking about in today's market?

Evan Smith: Oh, many of the diamonds that I've studied are easily in the millions-of-dollars range. That brings me to my position at the Gemological Institute of America. This is the right place to be if you need to look at rare and expensive diamonds; this is a place where they come through with relative frequency. The infrastructure of laboratories grading diamonds sees millions of diamonds come through every year, and this is the way I was able to build up observations. We were screening all of these diamonds, trying to find the ones that are nice, gem-quality type IIa diamonds but that also have inclusions in them. So that's very few diamonds; it's something like one in 10,000 of all the diamonds that come through for grading that might be of interest to look at for a study like this. I was able to borrow these diamonds for a period of hours, or maybe a day or two, depending on the individual diamond. But these are polished, faceted diamonds coming through the lab for grading, and I could only observe them nondestructively. So this means looking at them under the microscope and taking lots of pictures. The biggest tool I was relying on was Raman spectroscopy. This is an instrument where you put the diamond under a microscope, essentially, and line up the inclusion under the crosshairs. Then you can shine a laser at it and collect the light that bounces off the inclusion, and get an idea of what mineral it is, or, in some cases, what volatiles, like methane or hydrogen, are actually trapped within that inclusion system as well.

Additionally, there were a few samples that I was able to do more invasive studies on, and actually polish them open and expose some of the inclusions to do things like electron microprobe work, where you need an exposed inclusion. Those samples were offcuts, things that had been trimmed off larger diamonds during the manufacturing process. These might be trimmed off because they're too heavily fractured, or because they have big black inclusions in them, and the manufacturer is looking to exclude that, quote-unquote, ugly material from the end product, the polished diamond. And that offcut is something that's extremely valuable for scientific study. An interesting side note on the Cullinan diamond: I mentioned this was a 3,106-carat rough diamond. Its largest faceted piece was just over 500 carats, and that means that they cut away a lot of material. If you look at original pictures of the Cullinan diamond, you can actually see a couple of these black specks in it. So even the type example did have inclusions. And according to the notes from the people actually cleaving this diamond, their very first cleave through the Cullinan diamond was to intersect what they called a big black spot, an inclusion inside the diamond. So even the very early people trying to polish diamonds were avoiding these inclusions. That's part of the reason why they seem to have so few inclusions: people are actively excluding them from the polished diamonds. And when we see these fantastic pieces come up for auction, they're completely inclusion-free.

Oliver Strimpel: Wow, that's great. So the diamond trading companies, like De Beers, are actually quite collaborative with you, in the sense that the bits that are of the greatest scientific interest are actually the blemishes, from their point of view.

Evan Smith: Yeah, that's right. In some cases, if they're actually trimming a diamond down using a laser, laser-cutting off some of the more included regions of the diamond, they're open to the idea that there might be something scientifically valuable that comes from that piece of diamond.

Oliver Strimpel: What are you working on at the moment?

Evan Smith: Right now, I think the most interesting thing I've got is still the metallic inclusions in CLIPPIR diamonds. We have an idea of their composition, and we know this very exciting tidbit of information, that they're actually connected to serpentinization in the seafloor. But, like I said, we don't understand the full story of how this metallic liquid evolves, how it behaves, what its life cycle is, and what happens after diamond formation. Does this metallic liquid just sink down into the core, or is it reacted away? And what does it mean for the whole recycling process and the mantle geodynamics of carbon, beyond just diamond formation? I think this is something that we didn't expect, and that's always the thing that you should pay most attention to. You've got to pay attention to those things that don't make sense. And right now, the metallic inclusions just don't make a lot of sense.

Oliver Strimpel: Evan Smith, thank you very much.

Evan Smith: Thank you.

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