Diamonds, the Rare Treasures from the Depths

Once formed over 150 kilometers deep within the Earth’s mantle, diamonds now glitter on many a ring finger. A recent study sheds light on how these precious gems made their way to the surface. Through geological clues and model simulations, researchers revealed how the breakup of tectonic plates led to volcanic eruptions that brought these „treasure-rich“ materials within our reach.

Kimberlites are volatile-rich, occasionally diamond-bearing magmas that have erupted explosively at Earth’s surface in the geologic past1,2,3. These enigmatic magmas, originating from depths exceeding 150 km in Earth’s mantle1, occur in stable cratons and in pulses broadly synchronous with supercontinent cyclicity4. Whether their mobilization is driven by mantle plumes5 or by mechanical weakening of cratonic lithosphere4,6 remains unclear. Here we show that most kimberlites spanning the past billion years erupted about 30 million years (Myr) after continental breakup, suggesting an association with rifting processes. Our dynamical and analytical models show that physically steep lithosphere–asthenosphere boundaries (LABs) formed during rifting generate convective instabilities in the asthenosphere that slowly migrate many hundreds to thousands of kilometres inboard of rift zones. These instabilities endure many tens of millions of years after continental breakup and destabilize the basal tens of kilometres of the cratonic lithosphere, or keel. Displaced keel is replaced by a hot, upwelling mixture of asthenosphere and recycled volatile-rich keel in the return flow, causing decompressional partial melting. Our calculations show that this process can generate small-volume, low-degree, volatile-rich melts, closely matching the characteristics expected of kimberlites1,2,3. Together, these results provide a quantitative and mechanistic link between kimberlite episodicity and supercontinent cycles through progressive disruption of cratonic keels.

Diamonds are made of ordinary carbon, but what sets them apart from materials like coal is their purity and consistency. Naturally, their highly compact crystal structure can only form under immense pressure and heat in the Earth’s mantle, conditions found at depths over 150 kilometers. Over millions of years, diamonds were „baked“ in this extreme environment. The process of how they reached the Earth’s surface was roughly understood: diamond-bearing rocks melted due to geological processes, rose through fissures, and eventually surfaced during volcanic eruptions. The remains of these cooled volcanoes formed the deposits where raw diamonds are found today, typically embedded in a material known as kimberlite, named after the South African diamond-rich location, Kimberley. However, previous models couldn’t fully explain the origin of kimberlite melts deep within the Earth. It was evident that these processes were somehow linked to the restructuring of the Earth’s tectonic plates.

Unraveling the Mobilization Process

To investigate the geological processes leading to mobilization and kimberlite eruptions, the team led by Thomas Gernon from the University of Southampton delved into the matter. „The pattern of diamond eruptions is cyclic and follows the rhythm of supercontinents forming and breaking apart over hundreds of millions of years. However, until now, we didn’t know the process that suddenly brings diamonds to the Earth’s surface after resting 150 kilometers deep for millions or billions of years,“ says Gernon.

To gather new insights, the researchers analyzed the global correlation between the occurrence of kimberlites and the history of tectonic plate movements on Earth. They combined radiometric dating results with tectonic reconstructions, revealing that kimberlites formed over the past billion years typically erupted about 30 million years after the breakup of continental plates in the corresponding regions. This suggested an association with specific processes occurring at rift zones.

Natur Diamanten,natürliche Diamanten

Hot Processes at Plate Boundaries

To shed light on the exact mechanisms, the team developed geological model simulations that provided a plausible picture of the processes. According to their explanation, a continental plate thins considerably over many millions of years before it breaks apart. This process, known as „rifting,“ causes the Earth’s surface to sink, eventually forming a rift valley. This is currently happening in East Africa, where the Rift Valley is forming. The model suggests that something similar occurs deep within the Earth: pieces of the underside of the plate sink into the mantle while hotter rock flows in from below to fill the void—similar to seawater on the surface. This incoming magma destabilizes the surrounding rock containing diamonds, turning the previously ductile material into a liquid that then rises upward. Eventually, through volcanic eruptions, it reaches the surface and solidifies into diamond-rich kimberlite.

Furthermore, the researchers can explain why volcanic eruptions with diamond-rich kimberlite can occur relatively far from the continental edges. These eruptions are also ultimately caused by plate breakup. Dynamic processes that extend far and wide occur during this process. „These flows along the underside of tectonic plates remove a considerable amount of rock, dozens of kilometers thick. This chain reaction ultimately reaches regions of the continents that are far from rift zones,“ explains co-author Sascha Brune from the German Research Centre for Geosciences (GFZ) in Potsdam.

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