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, offer a unique window into the planet's hidden recycling system. As a geologist specializing in kimberlites, the magmas that carry diamonds to the surface, I find these rare diamonds particularly fascinating. They provide a rare opportunity to study the deep mantle and understand how rocks and carbon are recycled over hundreds of millions of years.
What makes CLIPPIRs truly remarkable is their origin at depths far beyond our reach. They form in the mantle transition zone, a region of the Earth's deep interior, at depths of over 400 kilometers. This is in stark contrast to ordinary gem-quality diamonds, which typically form in the mantle roots beneath old continents at depths of up to 200 kilometers. The journey of these diamonds from the deep mantle to the surface is a testament to the Earth's dynamic processes.
One of the key findings of our research is the identification of unusual iron-rich domains in the mantle associated with CLIPPIR-bearing kimberlites. This discovery provides new clues about the rocks that hosted these diamonds before they were brought to the surface. By studying the chemistry of olivine, a mineral found in kimberlite rocks, we can understand the mantle regions that kimberlites sampled and the areas where diamonds may have been stored.
Our analysis of olivine chemistry also suggests a link between CLIPPIR diamonds and ancient seafloor rock, known as oceanic crust. This oceanic crust, altered by hot fluids circulating through the seafloor, was dragged deep into the Earth by plate tectonics. Under extreme conditions, carbon contained in this material can be transformed into diamond. 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 the Earth's surface on their own. Instead, they would have needed a powerful lift, such as 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, where it likely remained for hundreds of millions of years.
The journey of CLIPPIR diamonds is a long geological one, from ancient seafloor to deep mantle, to the roots of continents, and finally to the surface in rare volcanic eruptions. This journey highlights the dynamic nature of the Earth and the interconnectedness of its various systems.
Our research also has practical implications for diamond exploration. In parts of Africa, such as 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. By identifying kimberlites containing abundant iron-rich olivine and related minerals, we can better understand the potential for hosting CLIPPIR diamonds. This approach provides exploration teams with a new clue to look for, helping geologists decide which kimberlites are worth investigating in more detail.
In conclusion, the study of CLIPPIR diamonds offers a unique opportunity to understand the Earth's deep processes and the journey of these rare gems. By reading the chemical clues preserved in the rocks that brought them to the surface, we are beginning to unravel the mysteries of the deep mantle and the dynamic nature of our planet.
