While most diamonds are formed beneath continents, at depths between 150 and 300 kilometres, a paper published in Scientific Reports has shown that two rarer types of diamond – those found in oceanic rocks, and those that form more than 300 kilometres below the continental crust – have a common, and unexpected, origin.
The team of researchers from Curtin University found that oceanic and super-deep continental diamonds “actually share the same compositions,” according to Dr Luc Doucet, lead author on the paper.
“The composition of the carbon is organic, so that means it’s organic matter that formed the diamonds.”
Diamonds are made from carbon placed under high pressure, but that carbon can come from different sources: either organic carbon, from once-living matter, or inorganic carbon – like carbonate minerals, which are commonly found in rocks.
According to Doucet, it’s surprising that diamonds formed so deep beneath the Earth’s surface are mostly made from organic matter.
“Particularly in the ocean, organic carbon is very minor [in abundance] compared to non-organic carbon,” he says.
If the super-deep diamonds are forming mostly from organic carbon, Doucet says that means there’s some sort of carbon-fixing process going on – or possibly something even more complicated happening during their formation.
The researchers propose that once formed, these very deep diamonds are brought to the surface via ‘mantle plumes’ – material from the Earth’s mantle rising to the surface.
“Ballooning rocks from the Earth’s deeper mantle, called mantle plumes, then carry the diamonds back up to the Earth’s surface via volcanic eruptions,” explains Doucet.
The researchers examined a range of oceanic, deep-continental and the more common lithospheric diamonds, each of which had been collected and tested in previous studies. They concluded that the oceanic and deep-continental diamonds had organic origins by looking at their ‘isotopic signature’ – the different concentrations of slightly heavier and lighter carbon atoms within the diamonds.
Doucet says this research is useful for understanding how carbon on the surface – and in the atmosphere – can get into the earth. It has particular implications for carbon capture and storage.
“Diamonds are really good targets to understand what’s going on inside the earth,” he says.
“They are made of carbon, so they can help us to understand the carbon cycles.”