Where do the world’s biggest diamonds come from? Geology now has answers
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Photomicrograph in cross-polarised light showing olivine in a piece of kimberlite rock from the Karowe kimberlites, famous for recovering some of the largest gem-quality diamonds ever found.<br>Author supplied
https://theconversation.com/where-do-the-worlds-biggest-diamonds-come-from-geology-now-has-answers-285003
https://theconversation.com/where-do-the-worlds-biggest-diamonds-come-from-geology-now-has-answers-285003
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Some of the most extraordinary gems ever found belong to a rare class of diamonds known as CLIPPIRs (Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, Resorbed). They make up less than 1% of all diamonds on Earth, and include three of the largest diamonds ever recovered, the Cullinan, at 3,106 carats, found at the Premier mine in South Africa; the 1,111 ct Lesedi La Rona, discovered at the Karowe mine in Botswana in 2015 and sold for US$53 million in 2017; and the 2,492 ct Motswedi recovered from the Karowe mine in 2024.
CLIPPIRs are not ordinary diamonds. They are part of a group known as “superdeep” diamonds that form more than 400km beneath our feet in a region of Earth’s deep interior called the mantle transition zone. This is much deeper than ordinary gem-quality diamonds, which usually form in the thick, rigid mantle roots beneath old continents, at depths of up to about 200km. The mantle is the thick layer of hot rock between Earth’s core and outer “shell” known as the crust.
Although their size and value attract attention, our interest in CLIPPIR diamonds lies in the journey they record. Because they originate at depths far beyond our reach, diamonds are valuable for understanding parts of Earth’s interior that we cannot directly observe. CLIPPIRs are especially rare messengers from the deep mantle. Studying them is one of the few ways we can learn how rocks and carbon are recycled in the Earth’s deep interior, and how our planet reshapes itself over hundreds of millions of years.
As a geologist at the University of Cape Town, I specialise in the magmas that carry diamonds to the surface, known as kimberlites. My colleagues and I set out to investigate what the world’s largest diamonds could tell us about Earth’s hidden recycling system – specifically, what rocks hosted these rare diamonds, and how they were eventually brought to the surface.
We explored these questions by focusing on olivine, a mineral found in kimberlite rocks. Kimberlites are rare magmatic rocks that rise rapidly from deep within the Earth, acting like natural elevators that carry diamonds and other minerals from the mantle to the surface.
Olivine is the most abundant mineral in the mantle. As kimberlite magmas rise, they pick up olivine from the mantle rocks they pass through. The chemistry of this olivine gives us a fingerprint of those deep rocks, including clues about their iron content and oxygen isotope signatures. This helps us understand the mantle regions that kimberlites sampled, including the areas where diamonds may have been stored before being brought to the surface.
But CLIPPIR diamonds have been found in only a small number of kimberlites globally, and we still do not fully understand why some kimberlites contain these exceptional diamonds while most do not.
Our findings add new pieces of the puzzle. We identified unusual iron-rich domains in the mantle associated with the kimberlites that contain CLIPPIR diamonds. This gives us new clues about the rocks that hosted these diamonds before they were carried to the surface. Our findings also offer a practical tool for diamond exploration because kimberlites containing abundant iron-rich olivine and related minerals have higher potential to host CLIPPIR diamonds.
Journey through the deep Earth
Previous studies had shown that CLIPPIR diamonds formed far deeper than ordinary diamonds. Their chemistry also suggested a link to ancient seafloor rock, known as oceanic crust, that was dragged deep into the Earth by plate tectonics – the slow movement and interaction of sections of Earth’s rigid outer shell. When two plates meet, one can be forced beneath the other. This process, called subduction, carries oceanic crust deep into the mantle, where it is changed by heat, pressure and interaction with the surrounding rocks. Under these extreme conditions, carbon contained in this material can be transformed into diamond.
New evidence from our analysis of olivine chemistry suggests that CLIPPIR-bearing kimberlites are linked to a particular kind of recycled material: ancient basaltic oceanic crust that had been altered by hot fluids circulating through the seafloor before it was dragged deep into the Earth. This hydrothermally altered oceanic crust appears to have formed dense, iron-rich rocks in the deep mantle.
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