Decarbonation of subducting carbonate-bearing sediments and basalts of altered oceanic crust: Insights into recycling of CO2 through volcanic arcs

Abstract

The geological carbon cycle has played a key role in controlling climate throughout Earth's history. For the last ∼3 billion years plate tectonics has driven subduction. Subducted slabs have transported CO2 from the lithosphere, hydrosphere, and atmosphere into the Earth, from where it may be released back to the surface through processes such as arc volcanism or can be stored in the deep interior over geological time. Carbonate-bearing sediments and basalts of altered oceanic crust are the primary media by which carbon is subducted. Therefore, quantifying the depth and amount of CO2 released from different carbonate-bearing lithologies during subduction is fundamental to understanding whether CO2 is recycled through arc volcanism or buried in the mantle. The magnitude of CO2 released from subducting slabs at fore- and sub-arc depths is controlled by processes including ocean crust alteration (i.e., carbonation), metamorphic decarbonation, carbonate dissolution and slab-melting. However, the relative contribution of these processes to overall slab decarbonation is still debated, and will be complex given the variety of sedimentary lithologies and subduction geodynamics. Here, we present a global arc-by-arc lithology-specific analysis of the magnitude of slab CO2 released purely by metamorphic decarbonation of carbonate-bearing sediment and basalt during subduction of altered oceanic crust, using a thermodynamically rigorous model. We find that metamorphic decarbonation is highly efficient in low carbonate sediments, such as carbonated clay, and in carbonated basalts of altered oceanic crust, causing all of their CO2 to be removed. Sediments with medium and higher carbonate contents, such as chalk and limestone, are only partially decarbonated, but the combination of metamorphic decarbonation and carbonate dissolution promotes efficient carbon loss. Together they can explain observed magmatic CO2 emissions in carbonate-rich arcs. Warm slabs, such as Mexico and Cascadia, produce complete metamorphic decarbonation of carbonate minerals beneath fore-arcs. Under more common cold and intermediate thermal regimes metamorphic decarbonation of carbonate minerals occurs at depths between ∼80 and 170 km (∼2.3 to 5.5 GPa) promoting CO2 input into the mantle sources of volcanic arcs. Overall, our results demonstrate that sub-arc decarbonation is typically considered an important potential source of slab-derived CO2, which needs to be considered together with carbonate dissolution to explain observed volcanic CO2 emissions. In many arcs the modelled CO2 flux from sediment and basalts of altered oceanic crust into the wedge exceeds the observed CO2 output suggesting that the mantle wedge and arc lithosphere may sequester some CO2.