A new process of diamond formation has been theorized that could indicate they are more plentiful than thought. It involves the acidification of water at great depths, producing minuscule diamonds in subsurface fluids that can be brought to the surface.
Diamonds are thought to form through redox reactions – reduction or oxidation, referring to the gain or loss of oxygen – at extremely high pressures and temperatures in Earth's mantle, or through a reduction of carbon dioxide. The diamonds are later brought to the surface through volcanic activity, often within an igneous rock called kimberlite. The redox reaction process, though, is poorly understood.
Dimitri Sverjensky from Johns Hopkins University in Baltimore, Maryland, has suggested a new theory in Nature Communications that would occur alongside – and separately – to the existing theory. He says that a drop in pH to more acidic levels in water-rock interactions deep in the Earth can lead to the formation of extremely small diamonds just thousandths of a centimeter across in size.
“I’ve been developing a new way to model ions in high pressures, and we can now talk about pH as a real variable,” he told IFLScience. “It turns out diamonds can be formed by a drop in pH, with the redox state being kept constant.”
Although Sverjensky is yet to find direct evidence for his theory, he says the reasoning is sound. Essentially, at depths of 100 to 200 kilometers (60 to 120 miles), the pH levels of water can be altered by the removal of hydrogen ions when it moves between rocks. As this occurs, diamonds can form within the fluid at extremely high pressures and temperatures due to the acidification, rather than redox reactions. These diamonds would still be brought to the surface in kimberlite, but it does hint at a new method of formation via a change in water chemistry alone.
Proving the method is true is difficult, though. It's not really possible to look at existing diamonds to determine which particular mechanism caused them to form, but studying tiny packets of fluid within diamonds trapped from the time they formed could provide a clue.
Does this mean there could be a greater number of diamonds hiding within the Earth? “I think this is consistent with that suggestion,” said Sverjensky. “There are indications of diamonds being found in increasingly different rocky types over the last 30 years, so it seems like they’re not as rare as we might have thought before.”