Pervasive subduction zone devolatilization recycles CO₂ into the forearc

The fate of subducted CO₂ remains the subject of widespread disagreement, with different models predicting either wholesale (up to 99%) decarbonation of the subducting slab or extremely limited carbon loss and, consequently, massive deep subduction of CO₂. The fluid history of subducted rocks lies at the heart of this debate: rocks that experience significant infiltration by a water-bearing fluid may release orders of magnitude more CO₂ than rocks that are metamorphosed in a closed chemical system. Numerical models make a wide range of predictions regarding water mobility, and further progress has been limited by a lack of direct observations. Here we present a comprehensive field-based study of decarbonation efficiency in a subducting slab (Cyclades, Greece), and show that ~40% to ~65% of the CO₂ in subducting crust is released via metamorphic decarbonation reactions at forearc depths. This result precludes extensive deep subduction of most CO₂ and suggests that the mantle has become more depleted in carbon over geologic time.