Scientists Trace Origin of Rare CLIPPIR Diamonds to Iron-Rich Mantle Zones Deep Beneath Earth’s Surface
The formation of the world’s rarest and most valuable gem-quality diamonds known as CLIPPIR diamonds (Cullinan-like, large, inclusion-poor, pure, irregular and resorbed) has long puzzled scientists.
A new study led by University of Cape Town (UCT) now provides critical insights into their origin deep within the Earth’s mantle.
Conducted by the Kimberlite Research Group (KRG) in the Department of Geological Sciences and published in Nature Communications, the research uses the chemistry of the mineral olivine found in kimberlites to investigate processes occurring far below the Earth’s surface.
The findings reveal that kimberlites hosting CLIPPIR diamonds consistently originate from anomalous, iron-rich zones located at the base of the lithosphere more than 150 km underground.
These deep mantle regions exhibit distinct isotopic signatures, including light oxygen and heavy iron, which are characteristic of hydrothermally altered oceanic crust.
The study was carried out in collaboration with researchers from the Carnegie Institution for Science and the China University of Geosciences.
Together, the teams concluded that these iron-rich domains likely formed from ancient oceanic crust that was subducted into the Earth’s interior and later accumulated beneath continental plates through mantle upwelling.
According to lead author Geoffrey Howarth, interactions between ascending kimberlite magma and these iron-rich zones produced the large olivine and garnet megacrysts typically associated with CLIPPIR-bearing kimberlites.
The diamonds themselves crystallised under extreme pressures exceeding 11 gigapascals, within the mantle transition zone.
Beyond explaining the origin of these exceptional gemstones, the study highlights the broader significance of iron-rich, isotopically distinct mantle domains as a key source of geochemical diversity in volcanic rocks worldwide.
“These extraordinary diamonds among the largest and most valuable on Earth have remained a geological mystery,” Howarth explained. “Our research shows they formed in unique iron-rich environments deep beneath continents, derived from ancient oceanic crust that was subducted and later integrated at the base of the lithosphere.”
He added that analyzing the chemical signatures preserved in olivine carried to the surface by kimberlite eruptions now provides a powerful tool for tracing the origin of such diamonds and potentially guiding future exploration efforts.
