Two diamonds from a South African mine have provided a detailed look into Earth’s mantle, at depths between 280 and 470 kilometers (173 and 292 miles).
The team from Hebrew University of Jerusalem found the first direct evidence of nickel-iron metallic alloys and nickel-rich carbonates deep within Earth’s mantle.
This discovery was made by studying tiny inclusions inside diamonds from the Voorspoed mine in South Africa.
Interestingly, it showcases the value of diamonds as more than just jewels.
“Their [diamonds] inclusions—whether nanometer-scale alloys or high-pressure minerals—offer one of the only natural records of conditions hundreds of kilometers beneath our feet,” the researchers noted in the press release on September 22.
Diamonds preserving chemical reactions
The Earth’s mantle is a massive layer that drives volcanoes, recycles the planet’s crust, and regulates its long-term development. But even at this scale, there are some reactions we can’t see.
Geologists have tried for a long time to understand it, using models and experiments.
These models predicted something specific: that at depths of roughly 250 to 300 kilometers, a very special kind of metal should exist.
They believed that nickel-rich metallic alloys would form there. But these predictions were hard to prove due to a lack of natural samples.
The breakthrough finally came from examining nano- and micro-inclusions—tiny imperfections trapped within South Africa’s mine diamonds.
These diamonds acted as “tiny time capsules,” preserving a rare snapshot of deep Earth processes.
“This is a rare snapshot of mantle chemistry in action,” said Yaakov Weiss, the study’s lead author.
“The diamonds act as tiny time capsules, preserving a reaction that would otherwise vanish as minerals re-equilibrate with their surroundings,” Weiss added.
The inclusions also contained other minerals that confirm the diamonds’ origin in the deep upper mantle and shallow transition zone.
Diamond formation
The study found the presence of both nickel-iron alloy and nickel-rich carbonate. Normally, these two types of substances would react immediately and not coexist.
This surprising coexistence is a critical piece of the puzzle, pointing to a metasomatic redox-freezing reaction. The process occurs when an oxidized, carbon-rich melt infiltrates a reduced, metal-bearing mantle rock.
The team explained that this interaction created nickel-rich carbonates and oxidized the surrounding mantle rock.
Interestingly, this also shows that diamonds can form from the reaction between carbonate minerals and reduced metals in the mantle.
“It joins earlier evidence from shallower depths that this is the main mode of formation of natural diamonds,” the researchers noted.
The study supports the theory that diamonds can form when carbonate fluids are carried deep into the mantle by subducting tectonic plates and react with metal alloys.
Plus, it offers a possible explanation for why some diamonds contain nickel atoms in their crystal structure, a long-standing mystery.
Moreover, the findings provide insights into how volcanoes form.
The enrichment of the mantle with elements like carbon and potassium during these reactions could be a key step in forming specific volcanic magmas. These magmas, such as kimberlites, are responsible for bringing diamonds to the surface.
The research was a collaborative effort with colleagues from the University of Nevada and the University of Cambridge.
The findings were reported in the journal Nature Geoscience on September 22.