Carbon is 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 is 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 .
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 is 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 is 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”.
Image: Superdeep diamonds from Brazil. Credit: Michael Walter