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Carbon mobilized at shallow depths in subduction zones by carbonatitic liquids

Abstract

More than half a gigaton of CO2 is subducted into Earth’s interior each year1. At least 40% of this CO2 is returned to the atmosphere by arc volcanism2,3,4. Processes that are known to release carbon from subducting slabs—decarbonation or carbonate dissolution in fluids—can account for only a portion of the CO2 released at arc volcanoes5. Carbonatitic liquids may form from the subducting crust, but are thought to form only at very high temperatures. Melting of carbonated rocks could restrict the subduction of carbon into the deeper Earth. However, the behaviour of such rock types in subduction zones is unclear. Here I use laboratory experiments to show that calcium-rich hydrous carbonatitic liquids can form at temperatures as low as 870 to 900 °C, which corresponds to shallow depths of just 120 km beneath subduction zone arcs, in warm thermal regimes. I find that water strongly depresses the solidus for hydrous carbonate gabbro and limestone rocks, creating carbonatitic liquids that efficiently scavenge volatile elements, calcium and silicon, from the slab. These extremely mobile and reactive liquids are expected to percolate into the mantle wedge, and create a CO2 source for subduction zone magmatism. Carbonatitic liquids thus provide a potentially significant pathway for carbon recycling at shallow depths beneath arcs.

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Figure 1: Microstructures of quenched hydrous-carbonatitic liquids at 4.2 GPa, 900 °C.
Figure 2: Liquid compositions in the ternary carbonate system (molar proportions).
Figure 3: Reactions governing dehydration, vapour-absent melting, and generation of a hydrous carbonatitic liquid in an altered oceanic gabbro.

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Acknowledgements

I thank D. H. Green, M. W. Schmidt, N. Malaspina, P. Fumagalli, S. Tumiati, T. Hammouda and J. Hermann for fruitful discussions on the petrology of subduction zones. E. Erba introduced me to the world of pelagic sediments. A. Risplendente is acknowledged for assistance with the electron microprobe. Comments from T. John greatly improved the manuscript. Funding provided by the Italian program PRIN2012R33ECR.

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Correspondence to Stefano Poli.

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Poli, S. Carbon mobilized at shallow depths in subduction zones by carbonatitic liquids. Nature Geosci 8, 633–636 (2015). https://doi.org/10.1038/ngeo2464

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