Carbonate dissolution during subduction revealed by diamond-bearing rocks from the Alps

Subduction zones modulate the global carbon cycle. Carbon is transported into the mantle by the subducting slab and returned to the surface by degassing at arc volcanoes above the subduction zone(1,2). However, the mechanisms for the transfer of carbon from the subducting slab and sediments into the overlying mantle wedge are poorly understood. Decarbonation-a metamorphic reaction between silicate and carbonate minerals that releases CO(2)-was thought to be the primary mechanism. Yet, thermodynamic models show that decarbonation occurs at much greater pressures and temperatures than those found in typical subduction zones(3-6). Carbon should therefore be retained in the slab and transported to great depths in the mantle, rather than supply the arc volcanoes. Here we identify diamonds in ultrahigh-pressure rocks from the Italian western Alps that have an oceanic origin. We assess the geochemistry of diamond-bearing fluid inclusions and find that they contain bicarbonate, carbonate and sulphate ions, silica monomers, and crystals of carbonate and silicate. This fluid geochemistry indicates that carbon was released from the slab at relatively shallow depths through dissolution, not decarbonation. We conclude that dissolution, driven by fluids released from the subducted slab, is an important mechanism for the transfer of carbon into the mantle and ultimately back into the atmosphere, helping to balance the carbon flux.