Diamond formation in the very deep Earth may be a more common process than previously thought, according to a new study in the journal Nature Communications.
“Diamonds may not be as rare as once believed, but this finding won’t mean deep discounts at local jewelry stores,” said study authors Prof. Dimitri Sverjensky and Dr Fang Huang, both from the Johns Hopkins University.
“For one thing, the prevalence of diamonds near the Earth’s surface still depends on relatively rare volcanic magma eruptions that raise them from the depths where they form.”
“For another, the diamonds being considered in studies are not necessarily the stuff of engagement rings, unless the recipient is equipped with a microscope. Most are only a few microns across and are not visible to the unaided eye.”
Using a chemical model, the team found that diamonds could be born in a natural chemical reaction that is simpler than the two main processes that up to now have been understood to produce diamonds.
Specifically, the model shows that diamonds can form with an increase in acidity during interaction between water and rock.
“We show that diamonds could form due to a drop in pH during water–rock interactions,” the scientists wrote in the paper. “We use a recent theoretical model of deep fluids that includes ions, to show that fluid can react irreversibly with eclogite at 1,652 degrees Fahrenheit (900 degrees Celsius) and 5.0 GPa, generating diamond and secondary minerals due to a decrease in pH at almost constant oxygen fugacity.”
The common understanding up to now has been that diamonds are formed in the movement of fluid by the oxidation of methane or the chemical reduction of carbon dioxide.
“The new study showed that water could produce diamonds as its pH falls naturally – that is, as it becomes more acidic – while moving from one type of rock to another,” Prof. Sverjensky said.
“The more people look, the more they’re finding diamonds in different rock types now. I think everybody would agree there’s more and more environments of diamond formation being discovered,” he added.
“Overall, our results constitute a new quantitative theory of diamond formation as a consequence of the reaction of deep fluids with the rock types that they encounter during migration,” the scientists concluded.
Sverjensky, D.A. & Huang, F. 2015. Diamond formation due to a pH drop during fluid-rock interactions. Nature Communications 6: 8702; doi: 10.1038/ncomms9702
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