Deep Mantle Dynamics and Carbon Mineralogy

The Earth’s deep mantle, extending from the transition zone around 410 km depth to the core–mantle boundary, hosts a dynamic interplay of thermal convection, compositional layering and redox reactions that governs the storage and mobility of carbon. Subducted oceanic lithosphere transports oxidised carbonates into the mantle transition zone and lower mantle, where pressure, temperature and oxygen fugacity dictate whether carbon is sequestered as stable carbonate minerals, elemental carbon or diamond. Diamond formation captures mineral inclusions that record pressure–temperature–redox histories, offering direct insight into deep-Earth processes. Reactions between carbonates and silicates or metallic iron can yield bridgmanite, perovskite and iron carbides, while redox-driven freezing of carbon in subducted slabs creates resistant diamond phases. Mantle upwellings and plumes can return carbon to the surface via carbonatitic or kimberlitic magmas, influencing long-term climate and surface carbon reservoirs. Integrating experimental petrology, geochemistry and geophysics has revealed key mechanisms by which deep mantle dynamics regulate Earth’s carbon cycle and lithospheric evolution.

Research from Nature Portfolio

Recent studies have shown that sublithospheric diamonds record Neoproterozoic to Palaeozoic subduction events, with isotopic dating of CaSiO₃ and sulphide inclusions revealing crystallisation ages of 450–650 million years. These diamonds remained attached to continental keels for over 300 Myr, suggesting slab-derived buoyant diapirs may stabilise supercontinents. High-pressure experiments have demonstrated that under lower-mantle conditions (above 80 GPa and 3,100 K) magnesium carbonate transforms to a dense polymorph which then reacts with silica to produce diamond and bridgmanite, indicating that cold subducting slabs can generate super-deep diamonds. Complementary work on redox mechanisms has revealed that oxidation of Fe–Ni sulphides to magnetite within the upper mantle nucleates diamond at depths of approximately 320 km, emphasising the role of redox freezing in immobilising carbon. Further experiments on carbonate–silicate cation exchange show that magnesium carbonate can react with silicates to form calcium carbonate under deep transition zone conditions, altering predictions for the fate of subducted carbon in the lowermost mantle.

Deep Mantle Dynamics and Carbon Mineralogy publication trend

The graph below shows the total number of articles in deep mantle dynamics and carbon mineralogy across all publications each year (not limited to Nature Index journals).

Technical terms

Subduction: Descent of oceanic lithosphere into the mantle at convergent plate margins.

Perovskite: A high-pressure crystal structure typified by CaSiO₃ or MgSiO₃ in the lower mantle.

Bridgmanite: The most abundant lower-mantle mineral, the high-pressure form of MgSiO₃ perovskite.

Oxygen fugacity: A quantitative measure of a system’s redox state, controlling mineral stability and carbon speciation.

Redox freezing: Immobilisation of carbon via oxidation–reduction reactions, forming diamond or carbonate phases.

References

1. Sublithospheric diamond ages and the supercontinent cycle. Nature (2023).
2. Imperfections in natural diamond: the key to understanding diamond genesis and the mantle. La Rivista del Nuovo Cimento (2023).
3. Diamond formation in the deep lower mantle: a high-pressure reaction of MgCO3 and SiO2. Scientific Reports (2017).
4. Redox-freezing and nucleation of diamond via magnetite formation in the Earth’s mantle. Nature Communications (2016).
5. Reversal of carbonate-silicate cation exchange in cold slabs in Earth’s lower mantle. Nature Communications (2021).
6. The fate of carbonate in oceanic crust subducted into earth's lower mantle. Earth and Planetary Science Letters (2019).
7. The Mg-carbonate–Fe interaction: Implication for the fate of subducted carbonates and formation of diamond in the lower mantle. Geoscience Frontiers (2019).