The discovery of two diamonds originating from the depths of the Earth's mantle reveals a chemical coexistence that scientists had until now deemed unimaginable. These precious stones, formed several hundred kilometers beneath the surface, contain mineral inclusions that should normally neutralize each other.
These two exceptional specimens were extracted from a South African mine and have the particularity of simultaneously containing oxygen-rich carbonates and nickel alloys poor in this element. In chemistry, these states are respectively described as oxidized and reduced, two fundamentally opposing conditions.
The joint presence of these materials within the same diamond so surprised the research team led by Yaakov Weiss of the Hebrew University of Jerusalem that they set the samples aside for nearly a year before resuming their investigations.
Diamond from Earth's depths showing exceptional inclusions
Credit: Yael Kempe and Yakov Weiss The thorough analysis ultimately demonstrated that these inclusions were captured at the precise moment when the diamonds formed, between the oxygenated carbonates and the reduced metals of the mantle.
This observation constitutes the first direct evidence of such a formation mechanism in nature. The researchers explain that this reaction likely occurs when carbonate fluids are drawn deep down by the movement of tectonic plates, thus coming into contact with the metallic alloys of the Earth's mantle.
The original depth of these diamonds, estimated between 280 and 470 kilometers (174-292 miles), provides information about the chemical composition of the deep layers of our planet. Theoretical models predicted a gradual decrease in available oxygen with depth, but these samples reveal the presence of oxidized materials at much deeper levels than anticipated. This discovery challenges certain hypotheses concerning the origin of kimberlites, the volcanic rocks that transport diamonds to the surface.
The nickel-rich inclusions could also explain an enigmatic phenomenon observed in some diamonds: the occasional presence of nickel atoms substituting for carbon atoms in the crystal structure. Maya Kopylova, a professor at the University of British Columbia, emphasizes that this substitution seemed unlikely given the mass difference between these elements. The new study suggests that this peculiarity could be the signature of formation at great depth, thus opening new research perspectives.
Diamond formation in the Earth's mantle
Diamonds form under extreme conditions of temperature and pressure, typically between 150 and 200 kilometers (93-124 miles) deep in the Earth's mantle. Contrary to what one might imagine, they do not come from coal but from pure carbon subjected to colossal pressures exceeding 50,000 times normal atmospheric pressure.
The crystallization process can take millions of years and generally occurs in stable areas of the mantle where conditions remain constant over long periods. Diamonds form either by precipitation from carbon-rich fluids, or by direct transformation of graphite under the effect of pressure.
What makes diamonds so valuable to geologists is their ability to trap surrounding minerals during their formation. These inclusions preserve intact samples of the deep mantle, offering scientists direct witnesses to otherwise inaccessible conditions.
Diamonds then rise to the surface through rapid volcanic eruptions called kimberlitic eruptions, which can transport them from several hundred kilometers deep in just a few days.