In a study published in Nature, a team of scientists describes an unexpected mechanism for diamond formation relying on ancient, subducted seawater.
Diamonds are crystals of carbon, formed deep in Earth. As diamond crystals grow, they sometimes trap fluids or other mineral crystals, micro-samples of their surrounding environment. In a study published in Nature, a team of scientists, including DCO’s Graham Pearson (University of Alberta, Canada), describes an unexpected mechanism for diamond formation relying on ancient, subducted seawater [1].
The team, lead by Yaakov Weiss (Columbia University, USA), analyzed 11 diamonds from the Ekati mine in the Northwest Territories of Canada. These diamonds, so called fibrous diamonds, are less than a millimeter in diameter. The center of many of the stones was familiar, a gem-like diamond. But surrounding this core the diamond was studded with millions of minute inclusions, giving it a “fuzzy” or fibrous appearance under a microscope.
The inclusions in the 11 diamonds studied provided the authors with new information about how, when, and where in Earth this carbon crystalized. For diamonds to have inclusions like these, they must have formed quickly, trapping surrounding fluids and minerals. Through a series of measurements, some involving a unique laser ablation method developed by Pearson’s research group, Weiss and colleagues showed that many of the inclusions contained fluids rich in chlorine and sodium. The source of such high levels of these two elements, combined with their isotopic fingerprint, are strongly indicative of ancient seawater that reacted with oceanic crust, that was subducted to depth.
During subduction, water, in the form of salty fluids or “brines” was transferred into the deep mantle beneath the Northwest Territories, as oceanic lithosphere descended beneath the overlying tectonic plate. The reaction of these brines with particular rock types in the mantle root appears to be a critical part of the diamond forming process.
“These results are particularly interesting to the Deep Carbon Observatory because they point to a new mechanism whereby carbon was cycled into, and stored in, deep Earth,” said Pearson. “Before now, it was unclear what the starting compositions were for the unusual fluids that form these diamonds. Diamonds with “salty” inclusions appear to be common beneath the Northwest Territories. Similar fluid compositions in diamonds from other parts of the world indicate that this diamond forming reaction was widespread beneath the deepest continents around the world.”
