In a recent letter published in Nature, researchers propose that most carbon goes no deeper than about 300 to 500 kilometers, at which point a carbon barrier limits carbon recycling into the deeper mantle.

Carbon was cycled from Earth’s surface to its depths, emerging through the crust from volcanoes, and descending to the mantle in subducting ocean floor. But how far down was the carbon subducted? In a letter by Andrew Thomson, Michael Walter, Simon Kohn, and Richard Brooker (University of Bristol, UK) published in Nature, the authors propose that most carbon goes no deeper than about 300 to 500 kilometers, at which point a carbon barrier limits carbon recycling into the deeper mantle [1].

Downwelling slabs of mid-ocean ridge basalt (MORB) efficiently dehydrate at sub-arc depths, but may retain a considerable portion of their carbon cargo. Thomson et al made high pressure-temperature melting experiments on materials that replicated carbonated basalt from the IODP 1256D site on the East Pacific Rise. They show that upon reaching transition zone depths carbonatite melts are produced along a deep solidus depression. The melts infiltrate and react with the overlying mantle, causing diamond production, refertilization and associated metosomatism of the surrounding mantle. This melting of recycled crust in the transition zone was an effective barrier to carbon transport into the lower mantle.

The major difference between this work and other melting studies of carbonated MORB above 8GPa was the different phase assemblage resulting from lower and more realistic CO₂ and CaO contents of this study’s bulk composition. The resulting change in phase relations produces a deep solidus depression in carbonated oceanic crust at upper-most transition zone depths. The authors estimate that melting would occur to depths of at least 7 kilometers into the crustal section, and that only the coldest modern-day slabs would survive the solidus depression and carry carbonate beyond the transition zone.

The compositions of superdeep diamond-hosted inclusions provide strong evidence of carbonate melt-peridotite reaction. These diamonds form at transition zone depths, and have isotopic characteristics consistent with subducted carbon. The diamonds confirm that carbon must survive subduction beyond sub-arc dehydration reactions, and may record the process of slab melting in the transition zone.

Dr. Andrew Thomson said, “superdeep diamonds are a unique pristine snapshot of the deepest portions of the Earth’s carbon cycle. They contain a wealth of information that makes them invaluable and unparalleled tools for better understanding the interior of our planet”.

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.”