This session aims to integrate geochemical, petrological, experimental, computational, and modeling approaches to advance insights into the behavior of C and H within the silicate Earth. We welcome contributions addressing: (i) the speciation and fractionation of C and H in minerals, melts, and fluids under varying redox conditions; (ii) the budgets and cycling of C–H in the bulk silicate Earth; (iii) the roles of C and H throughout Earth’s history; and (iv) implications for the formation of C–H-bearing minerals and energy resources.
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.
Presented herein is a novel online platform (https://efs.idsse.ac.cn/) that provides free access to the calculation of diverse thermodynamic properties of major terrestrial fluid systems. The platform is built upon our decades-long body of work—encompassing equations of state published over the past thirty years and complementary computer simulations (e.g., Duan et al., 2025, 2026). These equations have been widely adopted across an array of research fields, including COH cycle modeling, carbon sequestration, fluid-rock interactions, fluid inclusion analysis, marine biogeochemistry, gas hydrate studies, and petroleum/gas geochemistry. As documented on the platform’s citation page (https://efs.idsse.ac.cn/module1/citation.html), the equations have earned endorsement or positive citation from 688 leading universities worldwide or institutions, involving contributions from over 10,000 scientists. Designed as a dynamic, continuously updated tool, this platform streamlines and simplifies geochemical computations, empowering researchers to execute critical thermodynamic analyses with enhanced efficiency.
- Duan ZH, Cheng NF, Zhang ZG, Chou IM, Sun HR (2026) Molecular dynamics simulation and equation of state of the NaCl-H2O system from 573 to 1573 K, 1 to 30 kbar, and 0 to 1 m fraction of NaCl. Geochimi. Cosmochimi. Acta. 412(2026) 127-141
- Duan ZH and Cheng NF. （2025）Vapor-liquid and liquid-liquid phase equilibria in the CO2-CH4-N2 system: A Gibbs ensemble Monte Carlo simulation. Chem. Geol. 693 (2025) 122983
How to cite: Duan, Z.: Introducing a New Web-based Platform Calculating Thermodynamic Properties of Earth’s Fluid Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3665, https://doi.org/10.5194/egusphere-egu26-3665, 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-rich2-4, 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, Fe5O12Hx (x≥9) and Fe7O12Hx (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 Fe5O12Hx 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, 2026.
Hydrogen (H2) is an important clean energy source, and its geological production is closely linked to serpentinization. Serpentinization is a low-temperature (≤500 °C) hydrothermal alteration of ultramafic rocks (typically komatiites and peridotite), where olivine and pyroxene of ultramafic rocks are transformed into serpentine, (±) talc, and (±) brucite. Serpentinization occurs in diverse geological settings, including the ocean floor, mid-ocean ridges, and subduction zones. Critically, the process generates hydrogen, which sustains microbial communities in hydrothermal ecosystems. The H2 originates from the reduction of water-derived H⁺, driven by the oxidation of ferrous iron (Fe²⁺) in olivine and pyroxene to ferric iron (Fe³⁺).
In spite of its significance, hydrogen predicted based on thermodynamic models is around 1-3 orders of magnitude higher compared to hydrogen formed in experimental studies (McCollom and Bach, 2009; McCollom et al., 2016). This gap indicates that serpentinization kinetics may greatly influence H2 formation. Consistently, most previous experiments were performed using olivine, with sluggish rates of serpentinization and very low H2 concentrations, e.g., 2.8% of serpentine was produced at 300 °C and 500 bar after a reaction period of 111 days (McCollom et al., 2016). The concentrations of dissolved H2 in fluids were only 11 mmol/kg, significantly lower than the maximum H2 concentrations predicted under the same P-T conditions (~350 mmol/kg). The close association between serpentinization kinetics and H2 formation has not been systematically investigated.
This study involved serpentinization experiments on peridotite at 300 °C and 24-300 MPa under conditions of varying fluid pH, salinity, and the addition of N2 and CO2. The results demonstrate that H2 yield is strongly controlled by the relative serpentinization rates of olivine versus pyroxene. Higher H₂ production correlates with conditions that accelerate olivine reaction relative to pyroxene, such as the addition of acidic, alkaline, or low-salinity (0.5 M NaCl) fluids. Conversely, in high-salinity fluids (1.5 M and 3.3 M NaCl), pyroxene reacts faster than olivine, and H₂ production is significantly suppressed (Huang et al., 2023). The addition of N2 and CO2 enhances the serpentinization of pyroxene but decreases H2 production (Shang et al., 2023). We conclude that faster relative rates of olivine serpentinization enhance H₂ generation, whereas faster rates of pyroxene inhibit it. This inhibition is likely due to silica released during pyroxene alteration, which is known to suppress H₂ formation (Huang et al., 2024). Our study establishes the coupled kinetics of olivine and pyroxene as a key factor controlling the efficiency of natural hydrogen generation.
References:
Huang, R. F. et al. (2023). Journal of Geophysical Research: Solid Earth, https:doi.org/10.1029/2022JB025218.
Huang, R. F. et al. (2024) Science China: Earth Sciences, 67, 222-233.
McCollom, T. M. and Bach, W. (2009) Geochimica et Cosmochimica Acta, 73, 856-875.
McCollom, T. M. et al. (2016). Geochimica et Cosmochimica Acta, 181, 175-200.
Shang, X.Q et al. (2023) Science Bulletin, 68, 1109-1112
How to cite: Huang, R., Li, W., Zhang, M., and Huang, X.: Hydrogen (H2) production and coupled serpentinization kineticsduring peridotite hydrothermal alteration, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8964, 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, 2026.
Metamorphic fluids in subduction zones play a critical role in arc volcanism, seismicity, and deep carbon cycling. However, the speciation and evolution of carbon during subduction and exhumation remain poorly constrained. This study investigates fluid inclusions in eclogites from the Western Tianshan HP–UHP metamorphic belt, China. Abundant CH₄-rich fluid inclusions are identified in high- to ultrahigh-pressure eclogites and associated veins. Petrological features and C–H isotopic data confirm an abiotic origin for this methane. Reconstruction of P–T–fO₂–fluid trajectories, combined with Deep Earth Water modeling, demonstrates that prograde metamorphism at 50–120 km depth promotes large-scale CH₄ synthesis via redox reactions during slab dehydration. In contrast, retrograde exhumation leads to CO₂-dominated fluid production. Quantitative flux estimates highlight eclogite-hosted methane as a globally significant yet previously overlooked abiotic CH₄ source.
Furthermore, two distinct types of fluid inclusions are identified both in eclogites and their veins: Type‑I (water-rich, with CH₄ vapor) and Type‑II (CH₄-rich, with little or no H₂O). Their coexistence indicates fluid immiscibility under high-pressure conditions. Quantitative 3D Raman spectroscopy analysis of CH₄:H₂O ratios underscores an evolutionary transition in C–O–H fluids during decompression and exhumation, driven by progressive immiscibility between CH₄ and H₂O. This phase separation enhances carbon transfer from the subducting slab to the mantle wedge, improves decarbonation efficiency, and may contribute to the formation of abiogenic natural gas accumulations.
How to cite: Zhang, L.: The Fate of Subduction Zone C-O-H Fluids Revealed by Eclogite-Hosted Fluid Inclusions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16843, 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, 2026.
Siderite (FeCO3) is rare in Mesoproterozoic strata, and the reasons for this remain unclear. However, abundant siderites discovered in the Xiamaling Formation of the North China Craton (NCC) provide valuable insights into the iron and carbon cycles, as well as ecosystem evolution in ancient oceans. In this study, we present new petrographic, elemental, zircon U-Pb dating, and C-O and Re-Os isotopic data for the Xiamaling siderites. The results show that siderites are mainly found in Units IV to VI of the Xiamaling Formation, exhibiting stratiform, nodular, and irregular morphologies. The mineral crystals range from euhedral to subhedral ellipsoidal, rhombohedral, and oolitic, or anhedral rosette-like shapes. The siderite aggregates, with irregular structures, are most abundant in Unit V, showing alternating light and dark rims in backscattered electron images. During the deposition of these siderites, submarine volcanism was active in response to the final breakup of the NCC from the Columbia supercontinent. From Units IV to VI of the Xiamaling Formation, the initial 187Os/188Os ratios decreased to nearly 0.2, indicating an increase in mantle-derived magmatism, which supplied iron and carbon, contributing up to 30-40%. This period coincides with the disappearance of positive Eu anomalies and a decrease in the influence of hydrothermal fluids. By Unit V, active rifting triggered the release of methane-rich submarine gas hydrates, leading to the alteration of siderites due to dissimilatory iron reduction, resulting in irregular structures. Anaerobic methane oxidation also occurred, causing the siderites to exhibit lower δ13C values. The formation of siderite marks the onset of mantle magmatism prior to the peak of the Large Igneous Province at 1.38 Ga, the intensification of chemical weathering, and an increased supply of nutrient elements, which stimulated primary productivity and triggered transient fluctuations in atmospheric oxygen levels.
How to cite: Zhang, C. and Tian, W.: Petrology, geochemistry of Mesoproterozoic siderites in the North China Craton and its implications for planetary habitability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2403, https://doi.org/10.5194/egusphere-egu26-2403, 2026.
High-mature to overmature source rocks constitute a critical material basis for deep oil and gas exploration in ancient petroliferous basins. However, intense thermal evolution induces extensive kerogen cracking and depletion of residual organic matter, resulting in the failure of conventional evaluation indicators and thereby severely limiting the accurate assessment of source rock effectiveness and resource potential.Taking the high-mature to overmature source rocks (with vitrinite reflectance Ro > 1.3%, locally reaching up to 3.0%) from the Fengcheng Formation and Lower Urho Formation of the Permian System in the Junggar Basin as the research objects, this study is grounded in geological constraints. Firstly, regional geological surveys, outcrop observations, and limited drilling core analyses were conducted to provide basic parameters for original organic carbon restoration. Meanwhile, preliminary method selection was performed by integrating the geological-geochemical differences among various formations and sags. On this premise, three methods—namely the degradation rate method, forward modeling method, and chemical kinetics method—were applied to restore the original organic carbon of the two sets of source rocks (Fengcheng Formation and Lower Urho Formation). The original organic matter types were inferred from the maceral characteristics of kerogen, which was employed to compare the adaptability and restoration accuracy of the three methods. Eventually, the original organic carbon restoration coefficients corresponding to each formation and sag were determined.Additionally, utilizing single-well logging data, a logging TOC (Total Organic Carbon) prediction model was established based on linear regression. Combined with the optimized restoration coefficients, the original organic matter abundance of single wells was calculated, realizing the accurate assessment of the original organic matter abundance of high-mature source rocks. The results indicate that after restoration, the TOC, hydrocarbon generation potential, and HI (Hydrogen Index) of the two sets of source rocks in the study area are significantly enhanced compared with those before restoration. Specifically, for the Lower Urho Formation in the Fukang Sag, the average post-restoration TOC is 1.73%, representing a 10% increase from the pre-restoration value; for the Fengcheng Formation in the Shawan Sag, the average post-restoration TOC is 0.99%, a 67% rise compared with the pre-restoration level. This evaluation method clarifies the original organic matter abundance and hydrocarbon generation potential of high-quality source rocks in sparsely explored sags, and effectively addresses the key challenges in the assessment of high-mature to overmature source rocks.
Keywords: High-mature to overmature source rocks; Organic carbon restoration; Logging prediction; Junggar Basin; Permian
How to cite: Liu, Y., Liu, H., and Cheng, B.: Restoration of Original Organic Carbon and Evaluation of Hydrocarbon Generation Potential for High-Mature to Overmature Source Rocks: A Case Study of the Permian Source Rocks in the Junggar Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3079, https://doi.org/10.5194/egusphere-egu26-3079, 2026.
Dense Hydrous Magnesium Silicates (DHMSs), as potential important water reservoirs in the Earth's interior, are critical for understanding the global water cycle, mantle composition and dynamic processes. Superhydrous phase B (shy-B), as a member of the DHMSs family, is considered an important hydrous phase in the mantle transition zone (MTZ) and even in the uppermost lower mantle (ULM) due to its high water content. Given the abundance of aluminum (Al) and iron (Fe) in the mantle, investigating their effects on the physical properties of shy-B better approximates realistic Earth's interior conditions. In this study, structural characterization and physical property analysis of synthesized Al-bearing shy-B, Fe-bearing shy-B and Fe-Al-bearing shy-B were performed under high pressure using a diamond anvil cell combined with synchrotron radiation single-crystal X-ray diffraction, infrared absorption spectroscopy and Raman scattering spectroscopy. Based on the Birch-Murnaghan (BM) equation of state fitting, the effects of different Al and Fe contents on the bulk modulus of shy-B were quantified, and the variations of density and bulk velocity with pressure were further derived, providing constraints for modeling the geodynamic processes of water subduction and transport.
How to cite: Chen, S., Li, X., and Li, F.: Equations of State of Superhydrous Phase B with Varying Mg, Al and Fe Contents: Implications for Water Transport in the Mantle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6068, 2026.
The deep carbon cycle is crucial for understanding Earth's global carbon budget and the exchange of volatiles between the surface and the interior. Carbonates are the primary carriers of carbon into the deep mantle via subducting oceanic lithosphere, yet their stability and distribution remain subject to significant uncertainty. While current understanding of carbonate stability and elasticity is largely based on end-member minerals, the effects of complex cation mixing, which is prevalent in natural systems, are poorly constrained. In this study, we synthesized a series of multi-cation carbonates with varying cation compositions (Ca, Mg, Fe) and investigated their crystal structures and elastic properties under in situ high-pressure and high-temperature conditions using synchrotron X-ray diffraction and Brillouin scattering. Our results demonstrate that the random substitution of cations significantly modulates the phase stability and sound velocities of these carbonates. Specifically, the combined effects of Mg and Fe substitution for Ca induce distinct elastic anomalies and anisotropy variations that deviate from the behavior of end-member phases. These findings provide critical constraints for interpreting seismic observations in subduction zones and the deep mantle. By clarifying the influence of cation chemistry on carbonate elasticity, this work enhances our ability to quantify deep carbon reservoirs and understand the dynamic processes governing the global carbon cycle.
How to cite: Zhang, J., Li, X., and Li, F.: High-Pressure Stability and Elasticity of Multi-Cation Carbonates: Implications for the Deep Carbon Cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8521, 2026.
Carbon and hydrogen speciation and cycling in planetary interiors fundamentally shapes planetary structure and evolution. However, the behaviors of C-H system under extreme pressure-temperature (P-T) conditions across diverse planetary redox conditions remains poorly constrained. Through high P-T experiments spanning ice giant to terrestrial planet conditions, we investigated the C-H-O speciation, transport mechanisms, and implications for volatile budgets in differentiated bodies throughout the solar system and beyond. Our laser-heated diamond anvil cell experiments combined with X-ray diffraction up to 108 GPa and 3,410 K revealed pronounced melting-point depression of diamond in the presence of H2O, demonstrating that carbon speciation and phase stability in deep planetary interiors—from ice giants like Neptune and Uranus to carbon-rich exoplanets—are strongly influenced by volatile interactions. Multi-anvil press experiments on carbon-saturated Fe-Si alloys (4-27 wt.% Si) at 5-20 GPa constrained carbon solubility variations with silicon conent in the Fe-Si-C liquids. This relationship reveals how redox conditions controls carbon partitioning during core-mantle differentiation in reduced planetary environments like Mercury, potentially driving carbon exsolution and transport from metallic to silicate reservoirs to form distinct diamond layer on top of core-mantle boundary. For more oxidized and hydrated planetary mantle, such as the Earth’s mantle, our high P-T experiments examining the reaction of iron carbides (Fe3C, Fe7C3) with hydrous minerals such as brucite produce elemental carbon, in form of diamond, and (Mg,Fe)O, demonstrating that redox reactions between reduced carbon-bearing phases and hydrous minerals can generate diamond and redistribute carbon during magma ocean crystallization and slab-mantle interactions. These findings illuminate the speciation and cycling of C-H under varying redox conditions controls volatile budgets and distribution across planetary bodies, influences the core-mantle-crust carbon cycling through diverse planetary processes in our solar system and exoplanetary systems.
How to cite: Chen, B., Okuda, Y., Peckenpaugh, J., and Chao, K.-H.: Redox-Controlled Carbon Speciation and Cycling in Planetary Interiors: From Ice Giants to Rocky Planets, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19677, 2026.
Mantle-derived fluids are increasingly recognized as key contributors to hydrocarbon and natural hydrogen resources, yet their phase relations, compositions, and evolutionary pathways remain poorly constrained. Petrological observations and experiments suggest that deep hydrogen reacts with carbon-bearing materials to form hydrocarbons such as methane, resulting in the coexistence of H2 and CH4 in the upper mantle. In contrast, surface natural hydrogen accumulations commonly contain >90% H2. This disparity points to significant fluid fractionation during ascent, potentially driven by phase separation. However, a lack of data at high pressure and temperature has prevented clear constraints on the phase behavior and thermodynamic properties of H2-CH4 systems under upper mantle conditions.
In this study, we investigate the structure and thermodynamic properties of the H2–CH4 system under upper mantle conditions using first-principles molecular dynamics simulations integrated with available experimental constraints. Simulations were performed for pure H2, pure CH4 and H2–CH4 mixtures over a wide range of compositions under upper mantle conditions. Long-range interactions were treated using the SCAN+rVV10 functional. The resulting simulation data were used to construct P–V–T equations of state and to develop a thermodynamic model for the H2–CH4 binary system. Our results show that at high hydrogen concentrations, H2 and CH4 exhibit fluid immiscibility, leading to the segregation of hydrogen-rich fluids. This immiscibility becomes more pronounced with decreasing pressure and temperature, consistent with conditions expected during fluid ascent. Radial distribution function analyses indicate that both components remain molecular, with no evidence for additional species formation under the investigated conditions. Large-scale simulations involving up to 1012 atoms reproduce the same immiscibility behavior, confirming the robustness of the results.
These findings place new constraints on the phase behavior of H2–CH4 fluids in the upper mantle and provide a plausible mechanism for the generation of hydrogen-rich fluids observed at Earth’s surface. The thermodynamic models developed here offer a quantitative framework for future studies of deep hydrogen cycling and mantle hydrocarbon systems.
How to cite: Gao, H., Li, Y., Zhang, Z., and Ni, H.: Phase Behavior and Immiscibility of H2–CH4 Fluids Under Upper Mantle Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4772, https://doi.org/10.5194/egusphere-egu26-4772, 2026.
Fluorine (F) and water (H2O) are critical volatiles in magmatic systems. They play a vital role in magmatism, hydrothermal metallogenesis and so on. Previous studies have studied the effects of F on melt viscosity and element diffusion. However, the impact of fluorine on electrical conductivity, and the coupled effect of F-H2O, remain poorly constrained. We performed in-situ electrical conductivity measurements on metaluminous rhyolitic melts in a piston cylinder apparatus combined with a Solartron 1260 impedance analyzer. The experimental conditions spanned 0.5–1.0 GPa and 700–1200 °C with different contents of F and H2O. The results show that H2O can significantly enhance the electrical conductivity of metaluminous rhyolitic melt, with an increase of 0.5-1.0 log units by adding ~4 wt% H2O. In contrast, adding ~4 wt% F can only increase the electrical conductivity by 0.2–0.3 log units. Moreover, the fluorine-water coupling effect is less than the sum of their independent contributions. This result indicates that the non-linear coupling mechanisms between the two volatiles must be considered when evaluating their speciation and transport behavior. Additionally, we found that the influence of F on the electrical conductivity of rhyolitic melts varies with the aluminum saturation index (ASI). The effect of fluorine becomes more pronounced with increasing ASI. Based on the measurement data, we established a general electrical conductivity model for F-H2O-bearing metaluminous rhyolitic melts, which can be applied to constrain high-conductivity anomalies in the Earth's crust. For example, the high-conductivity anomaly beneath the Gangdese belt in southern Tibet can be explained by the existence of 8–22 vol% of melt with >6 wt% H2O; and the conductivity anomaly in the upper crust of the Yellowstone volcano corresponds to 11–24 vol% of melt. This study highlights the characteristics of electrical conductivity for F-H2O-bearing melts, providing key physical constraints for understanding volatile migration in magmatic-hydrothermal systems.
How to cite: Li, S. and Guo, X.: Electrical Conductivity of F-H2O-bearing Rhyolitic Melts: Implications for High-Conductivity Anomalies in the Crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8918, 2026.
The distribution of water in Earth’s deep interior critically influences planetary differentiation and long-term geodynamics. However, the water content of the lower mantle is poorly constrained, as its estimation depends on complex, redox-sensitive partitioning processes under extreme pressure–temperature conditions during magma ocean crystallization.
To address this, we perform large-scale simulations of magma ocean crystallization using a machine learning interatomic potential—trained on first-principles data and specifically optimized for bridgmanite and silicate melt. This approach enables efficient sampling of a vast parameter space, including pressures, temperatures, melt water contents, and oxygen fugacities relevant to the early lower mantle. We use these simulations to quantify the water partition coefficient between bridgmanite and melt and to assess redox controls on iron partitioning between the mantle and core.
Our results reveal that water is highly incompatible in bridgmanite, with its partitioning strongly modulated by redox state. Numerical models based on our partition data indicate that upon lower mantle crystallization, a substantial portion of Earth’s deep water was sequestered into a long-lived basal magma ocean, leaving the overlying solid mantle relatively dry. Furthermore, we find that oxygen fugacity profiles remained largely stable throughout this process. Our analysis suggests Earth’s water was predominantly accreted during its early formation stages, with only a limited addition post mantle differentiation—a budget that could be supplied by a small mass fraction of a late veneer with CI chondrite–like composition.
These findings provide novel quantitative constraints on deep-Earth water storage and redox evolution, offering pivotal insights into the coupled chemical and thermal history of the early Earth and the dynamics of magma ocean crystallization.
How to cite: Wan, L., Li, Y., and Ni, H.: Redox-State Dependent Water Partitioning and the Sequestration of Earth’s Deep Water in a Basal Magma Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5194, https://doi.org/10.5194/egusphere-egu26-5194, 2026.
The ability to reconstruct the thermal history of a basin is essential in modeling reservoir and source rock quality. In carbonate rocks, conventional thermal indicators could not be used, for apatites and vitrinite are hard to be found. The clumped isotope palaeothermometer and U-Pb dating are promising technique for constraining the thermal history of basins. In this study we test if carbonate clumped isotope thermometry could be used to explore the thermal histories.
This paper collects the drill cores of Cambrian and Ordovician carbonate rocks in Awati-Keping region, Tarim Basin, northwest China. The calcites and dolomites are relatively homogeneous. The Carbonate components of Ordovician strata yield statistically indistinguishable clumped isotope temperatures(TΔ47), ranging from 108.4 to 189°C. The U-Pb ages are from 483 to 525Ma. The thermal evolution paths of the carbonate clumped isotope temperature (TΔ47 ) in the Awati-Keping region are simulated using the first order rate approximation model.
From this study , it shows that the thermal history evolution of the Awati-Keping region can be divided into three stages: ① Slow heat flow decline stage: During the Cambrian-Ordovician periods, the heat flow values (45-55 mW/m²). As the Tarim Basin evolved into a craton, the terrestrial heat flow gradually decreased to 41-45 mW/m² by the Carboniferous. ② Rapid heat flow evolution stage: Influenced by Early Permian tectonic-thermal events, a transient peak heat flow (60-70 mW/m²) occurred in the Awati-Keping region. ③ Heat flow recession stage: During the Mesozoic-Cenozoic, heat flow values in the Awati-Keping region declined to 35-45 mW/m². The Early Permian high-temperature strata in the Awati-Keping region resulted from combined effects of burial depth and anomalous high heat flow events. Both magmatic eruptions and mantle plume upwelling contributed to the anomalous high heat flow event during the Early Permian.
The maturity evolution of the Yuertus Formation source rocks in the Awati-Keping region can be divided into three stages since deposition: gradual maturation during Cambrian-Early Carboniferous, rapid maturation acceleration during Late Carboniferous-Late Permian, and maturation stagnation from Triassic to present. The Shunbeixi area exhibits lower thermal evolution degree of source rocks, which is favorable for hydrocarbon accumulation.
How to cite: Xu, Q.: Reconstructing Thermal Histories in Carbonate Basins: A Clumped Isotope and U-Pb Dating Thermometry Approach from the Tarim Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4206, https://doi.org/10.5194/egusphere-egu26-4206, 2026.
Conventional models attribute anomalously high Poisson’s ratios in Earth’s interior to the presence of fluids or melts. Here, we propose a fundamentally different mechanism, which is rooted in the unique crystal chemistry of lawsonite (CaAl2Si2O7(OH)2·H2O). Our high-pressure experiments reveal anomalous changes in the elastic tensor of lawsonite at ~4 GPa and ~9 GPa, which are linked to phase transitions involving hydrogen-bond reorganization. Compared to typical mantle minerals, lawsonite exhibits moderately low P-wave velocity (VP) and very low S-wave velocity (VS). More notably, it is characterized by exceptionally high isotropic aggregate Poisson’s ratio (0.32–0.38) and VP/VS ratio (1.92–2.27), which serve as diagnostic identifiers in the interpretation of seismic models. In hot subduction zones, such as Cascadia and Southwest Japan, lawsonite provides a key mineralogical mechanism for the high-Poisson’s-ratio anomalies observed at 20–50 km depth, presenting a viable explanation distinct from conventional models that invoke overpressured fluid-saturated oceanic crust. In colder subduction systems such as NE Japan, the presence of 20–40 vol.% lawsonite can account for the regional-scale seismic anomalies observed at 50–90 km depth. Furthermore, we find that the localized ultra-low shear-wave velocity zone at 50–60 km depth in oceanic crust is most likely caused by lawsonite enrichment. The seismologically unique signature of lawsonite and its compatibility with seismic models underscore how this mineral could have a critical role in facilitating water transport into the deep mantle.
How to cite: Li, X.: Single-crystal elasticity of lawsonite at high pressure: Implications for high Poisson's ratio and VP/VS zones in subduction zones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10976, 2026.
Coal represents a crucial energy form within the carbon cycle of the Earth's lithosphere and serves as a significant reservoir for hydrocarbon energy sources, such as natural gas. As a plastic stratum, coal rock governs the formation of structural fractures under intense stress conditions in deep strata, thereby influencing the accumulation of oil and gas. In recent years, China National Petroleum Corporation (CNPC) has extended its coalbed methane exploration from depths shallower than 1,000 meters to those exceeding 2,000 meters, achieving large-scale production in the Junggar Basin and the Ordos Basin. This endeavor has become a pivotal business segment for the company in enhancing its reserves and production capacity.
The coal reservoirs in the Middle-Lower Jurassic of the Kuqa Depression in the Tarim Basin are buried at depths ranging from 0 to 8,000 meters. The cumulative thickness of coal seams is 40 to 260 meters, with a single layer thickness of 1 to 19.5 meters. The coal reservoirs buried at depths of 1,500 to 5,000 meters are relatively continuously distributed, covering an area of 5,730 km². Based on experimental analysis such as CT scanning, scanning electron microscopy, nuclear magnetic resonance, and nitrogen adsorption, outcrop and drilling samples have been conducted. It is reported that the coal reservoirs in this area have developed cleats, which are distributed in a linear and networked manner. There are 8 to 15 surface cleats per 10 cm and 13 to 24 end cleats per 10 cm. The reservoir spaces are connected, with microfractures being developed, indicating good reservoir conditions. The microfractures are mainly vertical and interlayer fractures, with good connectivity. The surface porosity ranges from 2.32% to 12.32%, and the connected porosity is 5.12%.Scanning electron microscopy reveals the development of cellular pores, primary pores, and hydrocarbon-generating pores. The matrix porosity is 10.4% to 15.5%, with the development of micro-pores (less than 10 nm) and mesopores (10 to 1,000 nm), which is conducive to the occurrence of adsorbed gas and free gas. Micropores (less than 2 nm) account for 63.4%, serving as the main adsorption space, while macropores with diameters ranging from 100 to 3,000 μm are the main occurrence space for free gas. Under the later strong thrust nappe action, the concentrated development area of coal is a regional differential structural deformation and strike-slip transformation zone, which controls the generation of fractures and the development intensity of structural fractures, and is a potential sweet-spot for reservoir development.
How to cite: Wang, J., Xu, X., and Zhang, H.: Characteristics of deep coal reservoirs and structural fractures controlling in Middle-Lower Jurassic of the Kuqa Depression in the Tarim Basin, China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16263, 2026.
