The world's largest diamonds, known as CLIPPIRs, are not just precious gems but also invaluable messengers from the Earth's deep interior. These extraordinary diamonds, which make up less than 1% of all diamonds on Earth, have captivated geologists and gem enthusiasts alike. What makes them truly fascinating is the journey they've undertaken, revealing secrets about our planet's hidden recycling system. As a geologist, I find myself drawn to the story these diamonds tell, and I'm excited to share my insights and analysis with you.
The Journey of CLIPPIRs
One of the most intriguing aspects of CLIPPIRs is their origin. Unlike ordinary diamonds, which form in the thick, rigid mantle roots beneath old continents, CLIPPIRs are part of a group known as "superdeep" diamonds. These diamonds are formed at depths far beyond our reach, in a region of the Earth's deep interior called the mantle transition zone, which lies more than 400km beneath our feet. This makes them incredibly rare and valuable, as they provide a unique window into the Earth's hidden processes.
What makes the journey of CLIPPIRs even more fascinating is the fact that they are linked to ancient seafloor rock, known as oceanic crust. This rock 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, a process called subduction. This process 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.
The Role of Kimberlites
Kimberlites, rare magmatic rocks that rise rapidly from deep within the Earth, play a crucial role in the journey of CLIPPIRs. These rocks act like natural elevators, carrying diamonds and other minerals from the mantle to the surface. 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.
However, 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.
The Journey Continues
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. This finding addresses one of the unresolved questions about CLIPPIR diamonds: what kind of rocks hosted them before kimberlite magmas carried them to the surface?
However, our findings also raise another question. These iron-rich rocks are so dense that they cannot rise back towards Earth's surface on their own. Earlier theories suggested that the rock hosting these diamonds could float upwards passively through the mantle. Our results suggest that the journey was probably more complicated. Instead of rising passively, this dense, iron-rich material would have needed a powerful lift. One possible mechanism is mantle plumes: columns of superheated rock that rise from deep within the Earth and can capture dense material, forcing it upward. These upwellings could have carried the diamond-bearing material upwards until it became stored at the base of the lithosphere, the thick, rigid root beneath old continents, where it likely remained for hundreds of millions of years.
The Value of CLIPPIRs
CLIPPIRs are not only rare and valuable gems. They record a long geological journey: from ancient seafloor, to deep mantle, to the roots of continents, and finally to the surface in rare volcanic eruptions. This journey reveals the deep processes that shape our planet, including the recycling of rocks and carbon in the Earth's interior. By studying CLIPPIRs, we can gain a better understanding of the Earth's hidden recycling system and the processes that have shaped our planet over hundreds of millions of years.
Finding More CLIPPIRs
Our findings may also help guide diamond exploration. In parts of Africa, including Sierra Leone and Angola, very large CLIPPIR diamonds have been found in river gravels. These diamonds must have come from primary source rocks, such as kimberlites, but in many cases, those sources remain unknown. Rivers can transport diamonds far from where they originally erupted, making it difficult to trace them back to the rocks that brought them to the surface. The famous Star of Sierra Leone, for example, was a 969-carat diamond recovered in 1972 from river deposits in the Kono district. Yet decades of mining in the region did not clearly identify a kimberlite source for diamonds of this type. More recently, CLIPPIR diamonds have been recovered from the Meya kimberlite in the same district, showing that primary sources for these exceptional diamonds do exist there.
Our approach gives exploration teams a new clue to look for. Kimberlites containing abundant iron-rich olivine and related minerals have higher potential to host CLIPPIR diamonds. This does not guarantee that giant diamonds will be found, but it helps geologists decide which kimberlites are worth investigating in more detail. Based on our findings, we would expect the most promising kimberlites in regions such as Sierra Leone and Angola to be those that show this iron-rich chemical signature. There are still many poorly characterized kimberlites in these areas, and some may be hiding more of the world's rarest diamonds.
In conclusion, the world's largest diamonds have travelled through Earth for hundreds of millions of years. By reading the chemical clues preserved in the rocks that brought them to the surface, we are beginning to understand not only where these diamonds may be found but also what they reveal about the deep processes that shape our planet. As a geologist, I find this journey of discovery incredibly fascinating, and I look forward to seeing what new insights and discoveries the future holds for us.
