The world’s most legendary diamonds, such as the Cullinan and the Hope Diamond, have long been shrouded in geological mystery. Unlike standard gem-quality diamonds that form at relatively shallow depths, these 'CLIPPIR' diamonds—an acronym for Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed—originate from the Earth’s deep mantle. New research led by the University of Cape Town has finally mapped the chemical journey of these stones, revealing that their brilliance is actually a product of ancient oceanic recycling.
Researchers have identified that CLIPPIR diamonds form within specific iron-rich zones of the lithospheric mantle. These zones possess a unique isotopic fingerprint, characterized by light oxygen and heavy iron isotopes. This signature is a tell-tale sign of oceanic crust that has undergone hydrothermal alteration on the seafloor before being dragged deep into the Earth’s interior through the process of subduction. This material does not simply vanish; it accumulates at the base of continents through mantle upwelling.
The study explains that as kimberlite magma—the volcanic vehicle that brings diamonds to the surface—rises, it interacts with these iron-rich subterranean pockets. This interaction triggers the growth of large crystals of olivine and garnet, which are hallmarks of the rocks hosting these rare diamonds. The crystallization process occurs under the immense pressures of the mantle transition zone, effectively turning recycled surface waste into the most coveted treasures on the planet.
This discovery does more than just solve a gemological puzzle; it provides a window into the 'conveyor belt' of plate tectonics. By tracing the isotopic path from the seafloor to the deep mantle and back to the surface, scientists are gaining a clearer picture of how carbon and other elements are cycled through the Earth over millions of years. It confirms that the deep mantle is a far more dynamic and heterogeneous environment than previously understood, acting as a massive repository for the Earth’s ancient history.