Coupled deep-mantle carbon-water cycle: Evidence from lower-mantle diamonds

Diamonds form in a variety of environments between subducted crust, lithospheric and deep mantle. Recently, deep source diamonds with inclusions of the high-pressure H2O-phase ice-VII were discovered. By correlating the pressures of ice-VII inclusions with those of other high-pressure inclusions, we assess quantitatively the pressures and temperatures of their entrapment. We show that the ice-VII-bearing diamonds formed at depths down to 800 ± 60 km but at temperatures 200–500 K below average mantle temperature that match the pressure-temperature conditions of decomposing dense hydrous mantle silicates. Our work presents strong evidence for coupled recycling of water and carbon in the deep mantle based on natural samples. [Display omitted] •A novel approach was developed to assess the pressure-temperature conditions of entrapment of inclusions in diamonds•The viscoelastic relaxation of diamond has a significant effect on the evolution of pressure and temperature•Ice-VII-bearing diamonds have formed in wet, cool environments at depths down to 800 km•The coupled recycling of water and carbon is present in the deep mantle