Carbon is present in the Earth’s mantle as a trace element; yet, the mantle is the largest reservoir of carbon, which modulates the composition of the Earth’s atmosphere. Unlike other trace elements, however, carbon, in oxidized form, affects mantle melting phase equilibria. Therefore, the compositions of the mantle-derived partial melts can, in theory, be used to decipher the presence and even concentrations of carbon in the mantle source regions for various volcanic centers. However, such an approach requires both a careful estimation of the primary melt compositions from natural samples and reliable experimental constraints on the partial melt compositions of mantle-equilibrated melts in the presence of carbon.

Here, using experimental phase equilibria and major element compositions of intraplate ocean island basalts, I will discuss how the mantle source regions of intraplate volcanism are generally more carbon-rich compared to the ambient mantle (Sun and Dasgupta, 2023 – EPSL). Future studies will need to assess whether such carbon-enriched deep mantle domains reflect primordial reservoirs or reservoirs modified by subducted carbon. I will also present recently published experimental results on mantle melting with low and variable bulk molar XCO₂ [CO₂/(CO₂+H₂O)] (0.0-0.17) at 2-4 GPa and 1200-1350 °C, aimed at constraining the effects of variable CO₂ in slab-derived H₂O-rich fluid fluxing the mantle wedge (Lara and Dasgupta, 2022 – EPSL; 2023 – JPet). The experimental partial melts show systematic evolution toward silica undersaturation with increasing bulk XCO₂ of the system. A comparison between our experimental partial melt compositions and a global dataset of the most primitive arc magmas suggests that the upper limit of XCO₂ in fluids inducing melting in mantle wedges is ∼0.10 at 2–4 GPa. This suggests that the sub-arc mantle domains are carbon-poor despite slab modification. Application of these new constraints to an H₂O and CO₂ mass balance model for subduction zones reveals that ∼35–85% of CO₂ entering subduction zones bypasses the sub-arc melt generation zone and is subducted to the convecting mantle, either carried by the slab or by the down-dragged limb of the mantle wedge directly above the slab.

How to cite: Dasgupta, R.: Carbon’s Role in Mantle Melting – Where it is Important and Where it is Not, and Implications for the Carbon Heterogeneity of the Mantle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4506, https://doi.org/10.5194/egusphere-egu26-4506, 2026.

Water in Earth’s interior exerts a profound influence on mineral and melt rheology, phase stability, and mass transport, with far-reaching implications for mantle dynamics, core evolution, and long-term planetary habitability. Constraining the budget, distribution, and accretion history of water in the deep Earth is therefore fundamental, yet remains challenging due to the lack of direct samples from the lower mantle and core. Here, we present constraints on Earth’s deep water budget, distribution, and accretion processes by integrating ab initio calculations with experimental and numerical studies. Recent advances highlight the unique role of hydrogen in governing the physical and chemical properties of Earth’s core, enabling improved constraints on hydrogen storage based on core–mantle partitioning and elasticity studies. Building on these constraints, the amount and distribution of water retained in the early mantle can be inferred from mineral–melt partitioning data. This framework allows estimation of the water inventory acquired prior to late veneer addition and, through comparison with present-day water budgets, reconstruction of Earth’s water accretion history. Our results indicate that substantial amounts of water were incorporated into the core and the basal magma ocean, with significant consequences for core–mantle interactions, mantle convection, and the thermal and chemical evolution of the planet.

How to cite: Li, Y., Wan, L., Brodholt, J., Vočadlo, L., and Ni, H.: Water in the Deep Earth: Budget, Distribution, and Accretion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4138, https://doi.org/10.5194/egusphere-egu26-4138, 2026.

Rocks are commonly treated as simple aggregates of minerals with well-defined compositions and crystal structures. However, they also contain nano- to micro-scale interstitial phases and grain boundaries with distinct geochemical properties that may represent an underappreciated reservoir for volatiles [1]. Resolving the distribution and speciation of these components requires analytical techniques with nanometer-scale spatial resolution. Photo-Induced Force Microscopy (PiFM) integrates atomic force microscopy (AFM) with infrared (IR) spectroscopy to enable phase identification at spatial resolutions of ~5 nm, well below the optical diffraction limit of conventional IR methods [2, 3]. In PiFM, a tunable IR laser is directed at a metal-coated AFM tip, inducing a photo-induced force (PiF) that corresponds to the sample’s IR absorption properties. Scanning the laser over a range of wavenumbers generates a PiF-IR spectrum, which aligns closely with conventional FTIR spectra, allowing for reliable phase identification through FTIR reference libraries [3].

      Here, we resolve volatile speciation and spatial distributions in mantle xenoliths and phases from high-pressure experiments. Spatially resolved volatile maps provide direct insight into their relationships with mineral phases and grain boundaries.  

References

[1] Alard et al., (2022) Nature Geoscience, 15, 856–857 [2] Nowak et al., (2016). Science Advances, 2(3), e1501571. [3] Otter, Förster et al., (2021). Geostandards and Geoanalytical Research. 45(1), 5-27.

How to cite: Förster, M. W., Dujardin, A., Demouchy, S., and Alard, O.: Distribution of volatiles in mantle xenoliths at nano-lengthscales visualized with Photo-induced Force Microscopy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5257, https://doi.org/10.5194/egusphere-egu26-5257, 2026.

The chemical evolution of Earth’s mantle is governed by the interplay between primordial reservoirs formed during accretion and recycled components introduced by subduction. Although geochemical evidence indicates the persistence of deep primordial water¹, viable mineralogical hosts within the iron-rich^2-4, high-temperature residues of a crystallized basal magma ocean (BMO) remain elusive. Here we report the synthesis and crystal structures of two hexagonal iron oxyhydroxides, Fe₅O₁₂H_x (x≥9) and Fe₇O₁₂H_x (x≥3) at 78–198 GPa and 2,400–2,800 K, using in situ single-crystal X-ray diffraction in a laser heated diamond anvil cell.  We identify the enigmatic "H-phase"⁵—a controversial feature in deep-mantle mineralogy—as our Fe₅O₁₂H_x oxyhydroxide. We show that its formation is triggered by trace adsorbed moisture even in nominally anhydrous systems, resolving long-standing debates regarding the stability of iron-bearing bridgmanite. Unlike previous candidates, these dense oxyhydroxides coexist with major lower-mantle minerals under conditions representative of BMO crystallization and the margins of large low shear velocity provinces. This finding identifies these oxyhydroxides as solid compounds that chemically anchor primordial water, reconciling early Earth solidification with the genesis of ultralow velocity zones and potentially serving as deep sources for volatile-rich mantle plumes.

Reference

1. Hallis, L. J. et al. Evidence for primordial water in Earth’s deep mantle. Science (2015) 350, 795–797.

2. Labrosse, S., Hernlund, J. W. & Coltice, N. A crystallizing dense magma ocean at the base of the Earth’s mantle. Nature (2007) 450, 866–869.

3. Boukaré, C. É., Badro, J. & Samuel, H. Solidification of Earth’s mantle led inevitably to a basal magma ocean. Nature (2025) 640, 114–119.

4. Wu, Z., Song, J., Zhao, G. & Pan, Z. Water-Induced Mantle Overturns Leading to the Origins of Archean Continents and Subcontinental Lithospheric Mantle. Geophys. Res. Lett. (2023) 50, 1–10.

5. Zhang, L., Meng Y., Yang W., Wang L., Mao W. L., Zeng Q. S., Jeong  J. S., Wagner A. J., Mkhoyan K. A., Liu W., Xu  R., Mao H. K.,  Disproportionation of (Mg,Fe)SiO₃ perovskite in Earth’s deep lower mantle. Science (2014) 344, 877–882 .

How to cite: Yuan, H., Man, L., Hu, Q., Frost, D., and Dubrovinsky, L.: Deep-mantle iron oxyhydroxides as reservoirs of primordial and recycled water, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15371, https://doi.org/10.5194/egusphere-egu26-15371, 2026.

Impurities and mineral or fluid inclusions in natural diamonds can provide valuable insights into the evolution of mantle conditions through geologic time (Stachel et al., 2015). In particular, information on the isotopic composition of the early mantle can be inferred by studying helium and hydrogen impurities in diamond, which are usually trapped either as fluid inclusions or as interstitial defects in the diamond lattice. However, a critical aspect to consider is whether or not the isotopic information carried by diamonds is still representative of the original diamond-forming fluid. Processes such as diffusion might occur during prolonged residence time in the Earth’s mantle at high temperature conditions, leading to the re-equilibration of He and/or H with the surrounding mantle, either by loss or gain of He and H themselves.  

Here we compute the diffusion of He and H in diamond using ab initio molecular dynamics and machine learning molecular dynamics simulations, as implemented in the Vienna Ab Initio Simulation Package (VASP); postprocessing was realized using the UMD package (Caracas et al.,2021).  All simulations were performed on a broad range of high pressure and high temperature conditions, compatible with those expected in the Earth’s mantle where diamonds are formed. We determine the diffusion coefficients as a function of both pressure (0, 5 and 10 GPa) and temperature (300 – 3000 K) for He and H. We show that diamonds at greater depths may act as closed systems throughout geological time.

MLF, MCD, FI, and MGP acknowledge funding from the European Union (ERC, INHERIT, Starting Grant No. 101041620)

Caracas, R., Kobsch, A., Solomatova, N. V., Li, Z., Soubiran, F., & Hernandez, J. A. (2021). Analyzing melts and fluids from ab initio molecular dynamics simulations with the UMD package. JoVE, e61534. doi:10.3791/61534

Cherniak, D. J., Watson, E. B., Meunier, V., & Kharche, N. (2018). Diffusion of helium, hydrogen and deuterium in diamond: Experiment, theory and geochemical applications. Geochimica et Cosmochimica Acta, 232, 206-224. https://doi.org/10.1016/j.gca.2018.04.029

Stachel, T., & Luth, R. W. (2015). Diamond formation—Where, when and how? Lithos, 220, 200-220. https://doi.org/10.1016/j.lithos.2015.01.028

How to cite: La Fortezza, M., Caracas, R., Day, M. C., Innocenzi, F., Nestola, F., Novella, D., and Pamato, M. G.: A comparison between hydrogen and helium diffusion in diamondat Earth’s mantle conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16731, https://doi.org/10.5194/egusphere-egu26-16731, 2026.

Precious gases (mainly helium and molecular hydrogen therein) belong to critically strategic resources. According to the series genetic identification methods and detailed geological-geochemical analysis, the formation and enrichment are classified into two types: tectonomagmatic active basins (the Songliao and Bohai Bay basins) and stable cratonic basins (the Ordos Basin). For tectonomagmatic active basins, the origins of precious gases are represented by crust-mantle mixing, primarily linked to mantle degassing, water-rock reactions in mafic ultramafic magmatic rocks, and water radiolysis. In contrast, for stable cratonic basins, precious gases are mainly derived from crustal degassing. Specifically, helium originates through radioactive decay in sedimentary and basement rocks, while natural hydrogen is formed from reactions of water and rock in crystalline basements, radiolysis of water, and thermal evolution of hydrocarbon source rocks. Based on the detailed analysis of several cases discovered in the sedimentary basins, the favorable conditions for precious gases enrichment include sufficient gas flux, favorable migration pathways and tectonic positions, and effective seals. The enrichment process of precious gas is primarily controlled by generation timing, geological temperatures, and accumulation-dispersion efficiency. Accordingly, we identify potential enrichment formations for precious gas: the Shahejie Formation of Boxing Subsag (Bohai Bay Basin), Huoshiling Formation of the Changling and Xujiaweizi Fault Depressions (Songliao Basin), and Lower Shihezi and Majiagou Formations (Ordos Basin).

How to cite: Liu, Q., Li, P., Wei, Y., Zhu, D., and Jin, Z.: Formation and enrichment mechanisms of precious gas insedimentary basins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21864, https://doi.org/10.5194/egusphere-egu26-21864, 2026.