What sparkles on many a ring finger today originated more than 150 kilometers deep in the earth’s mantle. A study now sheds light on how the diamonds found their way to the surface. Using geological clues and model simulations, the researchers were able to show how the breaking apart of tectonic plates meant that volcanic eruptions could bring the “treasury” material within our reach.
Actually, they only consist of the common element carbon: what distinguishes diamonds from materials like coal is their purity and consistency. Naturally, the particularly compact crystal structure can only develop under high pressure and heat in the earth’s mantle. The corresponding conditions only occur at depths of more than 150 kilometers. There the diamonds were “baked” over millions of years. It was already clear, at least in broad terms, how they were then able to get to the surface of the earth: diamond-bearing rocks were melted by geological processes, pushed up through fissures and then came to light in volcanic eruptions. The deposits where rough diamonds can be found today were formed from the remains of the cooled volcanoes. They are typically embedded in a material which is called kimberlite after the South African diamond site Kimberley. However, previous models have not been able to clearly explain how the kimberlite melts at depth. It was only fundamentally apparent that these processes are related to the restructuring of the earth’s tectonic plates.
Mobilization on track
The scientists around Thomas Gernon from the University of Southampton have now got to the bottom of the geological processes that lead to the mobilization and the kimberlite eruptions. “The pattern of diamond flares is cyclical, following the rhythm of supercontinents merging and breaking up repeatedly over time—hundreds of millions of years. Until now, however, we didn’t know what process causes diamonds to suddenly be brought to the surface after lying dormant 150 kilometers below the surface for millions or billions of years,” says Gernon.
To get new clues, the researchers first analyzed the global connection between the occurrence of kimberlites and the history of plate motion on Earth. To do this, they linked radiometric dating results of the rocks with tectonic reconstructions. It is becoming apparent that kimberlites, which had formed over the past billion years, typically emerged about 30 million years after the rupture of continental plates in the corresponding areas. This suggested that mobilization is associated with specific processes that occur at rupture zones.
Hot processes at plate boundaries
In order to shed light on exactly what could happen, the team developed geological model simulations. The researchers report that a plausible picture of the processes then emerges from them. As they explain, a tectonic plate thins out significantly over the course of many millions of years before it breaks apart. In this process, known as “rifting”, the earth’s surface sinks and eventually forms a rift valley. This is happening in East Africa, where the so-called Rift Valley is emerging. At some point, seawater flows into the resulting structure, as is the case with the Red Sea. According to the model, something similar also happens at depth, the researchers explain: pieces of the underside of the plates sink into the earth’s mantle, while hotter rock flows from below into the free space – similar to seawater on the surface. This inflowing magma destabilizes the rock surrounding the diamond: the previously viscoplastic material becomes liquid and then makes its way upwards. It can eventually reach the surface through volcanic eruptions, where it solidifies into the diamond-bearing kimberlite.
The researchers can also explain why volcanic eruptions with diamond-bearing kimberlite can also occur relatively far away from the continental margins. Apparently these leaks are ultimately due to the breakup as well. Because this can lead to dynamic processes that are far-reaching: “These currents along the underside of tectonic plates remove a considerable amount of rock that is dozens of kilometers thick. Ultimately, the chain reaction also reaches regions of the continents that are far away from rift zones,” explains co-author Sascha Brune from the German Research Center for Geosciences (GFZ) in Potsdam.
In addition to the geoscientific importance, the results could also have a “lucrative” side, the GFZ concludes in its report on the study: The model simulations could provide indications of where on earth undiscovered diamond deposits are slumbering underground.
Source: German Research Center for Geosciences, specialist article: Nature, doi: 10.1038/s41586-023-06193-3