The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth’s mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth’s surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO3 reacts with silicates to form CaCO3. In combination with previous work indicating that CaCO3 is more stable than MgCO3 under reducing conditions of Earth’s lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth’s lowermost mantle may include CaCO3.

Lv, M., Dorfman, S.M., Badro, J. et al. Reversal of carbonate-silicate cation exchange in cold slabs in Earth’s lower mantle. Nat Commun 12, 1712 (2021). abstract

Images of selected recovered sample cross-sections obtained using backscattered scanning electron microscopy (a, d, g), scanning transmitted electron microscopy (b, e, h), and energy-dispersive X-ray mapping (c, f, i) of the cross-section show the silicate layer sandwiched by two carbonate layers, with the reaction region along the contacting interface. ac Ex situ analysis of sample quenched from 33 GPa and 1650 K heated for 15 min (run #1) demonstrates reaction CaC-to-MgC: CaSiO3 is not present in starting materials but is indicated in EDX map by colocation of Ca and Si, shown in magenta. df Ex situ analysis of sample quenched from 88 GPa and 1800 K heated for 150 min (run #9) demonstrates reaction MgC-to-CaC: MgSiO3 is not present in starting materials but is indicated in EDX map by colocation of Mg and Si, shown in blue-green. CaCO3 also appears as a red (Ca, but no Si) ribbon within CaSiO3 starting material. gi Ex situ analysis of sample quenched from 133 GPa and 2000 K heated for 400 min (run #10) demonstrates reaction MgC-to-CaC: MgSiO3 appears as Ca-depleted, Si-rich region (blue or blue-green) adjacent to CaSiO3 starting material (magenta).