Water-Limited Serpentinization Drives Abiotic Hydrocarbons Production in Ultramafic Environments

Water-Limited Serpentinization Drives Abiotic Hydrocarbons Production in Ultramafic Environments | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Water-Limited Serpentinization Drives Abiotic Hydrocarbons Production in Ultramafic Environments Hugo Dutoit, Laurent Truche, Frédéric Donzé, Christophe Monnin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5972145/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Abiotic methane and light hydrocarbons generated through serpentinization have attracted growing attention as potential energy sources for the deep biosphere and as pathfinders of natural H₂ production. Although Fischer–Tropsch-type (FTT) reactions are central to the abiotic reduction of oxidized carbon by H₂, key uncertainties persist regarding catalytic mechanisms and the influence of water. Here, we show that ultramafic rocks (chromitite and harzburgite) readily produce CH₄, and minor gaseous hydrocarbons upon reaction with H₂ and CO₂ at 200–250 °C under dry conditions. Kinetic analyses, mass balance calculations, and Raman microspectroscopy indicate that solid carbonaceous compounds constitute the dominant products, while CO acts as a short-lived intermediate. Additional tests reveal that water vapor inhibits CO₂ hydrogenation, implying that efficient FTT processes require spatial or temporal decoupling from active serpentinization. Yet serpentinization consumes water and generates H₂, ultimately sustaining hydrocarbon formation and pointing to a dynamic interplay between hydration reactions and abiotic organic synthesis in ultramafic settings. Earth and environmental sciences/Biogeochemistry Earth and environmental sciences/Solid Earth sciences/Geochemistry Earth and environmental sciences/Solid Earth sciences/Mineralogy Figures Figure 1 Figure 2 Figure 3 Figure 4 Full Text Geological H₂ systems attract increasing attention for their role in fueling the deep biosphere and for their potential as low-carbon energy sources ( Truche et al., 2020; Ménez, 2020; Templeton et al., 2024 ). Serpentinizing environments are particularly relevant, as they generate H 2 which is often found alongside CH₄ and other organic compounds. The formation of these compounds involves a complex interplay of biotic and abiotic processes ( Reeves and Fiebig, 2020; Etiope and Sherwood Lollar, 2023 ). Several lines of evidence indicate that only a few reduced carbon species are unambiguously recognized as abiotic ( Etiope and Oze, 2022; Xia and Gao, 2022 ), prompting efforts to clarify how these compounds form and persist in ultramafic settings. Methane receives particular attention because it is commonly considered a product of CO₂ hydrogenation—often referred in geological literature as Fischer-Tropsch-Type (FTT) reactions ( Reeves and Fiebig, 2020 ). This pathway involves the Sabatier reaction: Numerous experimental studies have explored this reaction under water-saturated conditions intended to simulate active serpentinization, yet these conditions typically yield only trace amounts of CH₄ even at temperatures up to 300 °C ( McCollom, 2013; McCollom, 2016; Barbier et al., 2020; Ueda et al., 2021; Reeves and Seewald, 2024 ). This observation questions the feasibility of significant abiotic methanation in serpentinizing systems, especially in ophiolites settings where temperatures rarely exceed 150 °C. Various minerals (for example spinels) and metal alloys (including iron, nickel, and platinum group elements: PGE) may act as catalysts ( Etiope et al., 2018; Pappalardo et al., 2024 ), and it is recognized that many catalytic processes become effective above 200 °C ( Horita and Berndt, 1999; Foustoukos and Seyfried Jr, 2004; Preiner et al., 2020 ). Additionally, the catalytic availability of Ni and PGE in ultramafic rocks may also be influenced by the rock matrix and by water, which can shield or deactivate these reaction sites ( Reeves and Fiebig, 2020 ). Recent experiments underline the possibility that CO₂ methanation occurs below 200 °C through gas–rock interactions in relatively dry conditions ( Jacquemin et al., 2010; Etiope and Ionescu, 2015; Medina et al., 2000; Ruiz et al., 2021 ). Certain approaches rely on synthetic catalysts or flow-through reactors that provide valuable insights into reaction kinetics albeit experimental conditions often significantly differ from natural environments. In parallel, the influence of water on CO₂ hydrogenation and the broader distribution of reduced carbon products remain undocumented so far. This present study addresses these key points by examining how temperature, ultramafic rock lithology, mineral paragenesis, and water content govern CO₂ hydrogenation. Our results demonstrate that CH₄ production and the formation of complex abiotic organic compounds occur at temperatures as low as 200 °C under dry conditions, thereby shedding new light on the fundamental parameters that drive abiotic hydrocarbons synthesis in serpentinizing environments. Impact of mineral assemblage and temperature on reaction rate and selectivity Chromitites and harzburgites were selected to explore how mineral assemblages influence the abiotic synthesis of reduced carbon species under varying thermal conditions. Chromite-bearing rocks are of particular interest because they host metals (e.g., Fe, Ni, Co, and PGE) that could act as catalysts in CO₂ hydrogenation reactions ( Zaccarini et al., 2018; González-Jiménez et al., 2017 ). Experiments involving olivine- or chromite-rich lithologies have already highlighted the role of serpentinization and associated metal phases in generating H₂ and potentially promoting abiotic organic synthesis ( Barbier et al., 2020 ). Building on these insights, we used two chromitites (from the Chamrousse ophiolite in France and the Bulqizë ophiolite in Albania) and one harzburgite (also from Bulqizë) to assess the role of mineral parageneses on reaction pathways ( Supplementary Table S1 ). Both chromitites contain 55–58 wt% chromite with high Cr content relative to Al³⁺ and Fe³⁺ (Cr# > 70), although they differ markedly in their phyllosilicate assemblages. The Chamrousse chromitite is enriched in saponite (32 wt%), whereas the Bulqizë chromitite contains more serpentine minerals (12 wt% lizardite, 9 wt% chrysotile). These differences provide a range of potential catalytic sites and water-buffering capacities. To probe the influence of temperature on CO 2 methanation, bulk rock powders (0.1–1 mm) were crushed, sieved and cleaned under inert Ar atmosphere. They were then placed in stainless steel batch reactors and outgassed at 200°C under primary vacuum. The samples were reacted with H₂ and CO₂ (approximately 2 bar each, giving a molar H₂/CO₂ ratio = 1) under dry conditions between 100 and 250°C. The impact of water vapor was subsequently tested by introducing controlled amounts of moisture (see Methods for details). Throughout each run, gas composition was monitored to quantify reaction rates and identify carbon-bearing species. Special attention was directed toward the solid phases before and after experiments to determine whether additional organic materials formed on or within the mineral surfaces. A preliminary blank experiment at 250 °C (no rock powders) carried out for a month showed that the stainless-steel reactor did not promote any reactions of interest, as neither H₂ nor CO₂ concentrations changed, and no CH 4 can was detected ( see Methods ). Experiments with chromitites and harzburgite in the presence of CO₂ alone also resulted in no consumption of CO₂ or generation of detectable amounts of CH₄, demonstrating the essential role of H₂ in driving these reactions. When both H₂ and CO₂ were introduced, distinct reactivity patterns emerged among the three ultramafic rocks ( Supplementary Tables S2–S4; Fig. 1 ). At 200 °C, the Chamrousse chromitite displayed a clear and progressive consumption of both H₂ and CO₂ ( Fig. 1a ), accompanied by a steady production of CH₄ that reached a plateau around 600 hours. The near-complete depletion of the reactants coincided with the cessation of CH₄ production. In parallel, carbon monoxide (CO) was detected early in the reaction, then declined as CH₄ formed, consistent with a role for CO as a transient intermediate. This observation explains why CO has never been measured so far in natural serpentinizing systems and suggests that the overall methanation pathway proceeds via the reverse water–gas shift reaction ( Eq. 2 ), followed by CO hydrogenation to CH₄ ( Eq. 3 ). Comparable trends were observed with the Bulqizë chromitite and harzburgite ( Fig. 1b,c ), yet their overall reaction rates were significantly lower than those of the Chamrousse chromitite. With the Bulqizë chromitite, CO production remained substantial while CH₄ concentrations stayed comparatively low ( Fig. 1b ). In contrast, the Bulqizë harzburgite generated more CH₄ at 250 °C, along with a limited level of CO ( Fig. 1c ). These findings emphasize the strong dependence of both reaction kinetics and selectivity on lithological and mineralogical factors. Zero-order kinetics for CH₄ formation and H₂ consumption appeared most clearly at 200 and 250 °C. At 200 °C, only the Chamrousse chromitite produced significant CH₄, with a production rate of 0.751 ± 0.075 nmol m⁻² h⁻¹ over the first 168 hours. At 250 °C, both the Bulqizë chromitite and harzburgite generated CH₄, at rates of 0.118 ± 0.012 and 4.70 ± 0.47 nmol m⁻² h⁻¹, respectively. Attempts to explore lower temperatures (100 °C and 150 °C) with the Chamrousse chromitite did not yield detectable amounts of CO or CH₄ after 3000 hours. By assuming an activation energy of 80 kJ mol⁻¹ for CO₂ methanation ( Zimmerman and Bukur, 1990; Huff and Satterfield, 1984; Vogt et al., 2019 ) and using the 200 °C data as a reference, we estimate that CH₄ production at 150 °C would be ~800 times slower ( Supplementary Fig. S1; Supplementary Table S5 ). This decrease explains the absence of quantifiable reactivity within the timeframe investigated here. Longer experiments, larger reactive surface areas, higher pressures, or higher H₂/CO₂ ratios may help capture such low-temperature reaction regimes. Still, the rates measured here and the relevant activation energies indicate that abiotic CH₄ formation is likely to occur in ultramafic systems at lower-temperatures ( Etiope and Ionescu, 2015; Pappalardo et al., 2024 ). Comparisons of H₂ consumption by CO₂ hydrogenation with H₂ production during serpentinization ( McCollom et al., 2016 ) shed additional light on the potential significance of these processes in ultramafic settings ( Fig. 2; Supplementary Table S6 ). By extrapolating the H₂ consumption rate observed at 200 °C for the Chamrousse chromitite across a range of temperatures using the same 80 kJ mol⁻¹ activation energy, it appears that CO₂ hydrogenation can outpace serpentinization-driven H₂ generation as long as H₂ and CO₂ remain available and dry conditions persist. This observation may explain why certain ophiolitic gas seeps contain abundant CH₄ and only trace levels of H₂ ( D’Alessandro et al., 2018; Etiope et al., 2013; Etiope and Sherwood Lollar, 2013 ). The contrasted catalytic potentials of the three investigated lithologies highlight the importance of mineralogy and trace metal content. The unique abundance of saponite (32 wt%) in the Chamrousse chromitite likely enhances catalytic performance through large surface area, numerous sorption sites, and redox-active metal substitutions (Fe, Ni, Co). This smectite-group mineral has already been linked to aromatic amino acid formation in serpentinized peridotites ( Ménez et al., 2018 ) and is recognized as an effective Fischer–Tropsch-type catalyst ( Sun et al., 2017; Heller-Kallai, 2001 ). Layered silicates such as serpentine and talc, along with brucite, may also facilitate electron exchange by accommodating metal substitution in their structures. Spinel-group minerals—including Fe-chromite and magnetite—are known to catalyze Fischer–Tropsch reactions ( Mierczynski et al., 2018; Rao et al., 1994 ). Furthermore, Ni and Co concentrations can reach 3200 and 130 ppm, respectively, in the Bulqizë rocks ( Xiong et al., 2015 ), creating additional catalytic sites ( van Helden et al., 2020; Hernández Mejía et al., 2020 ). Platinum-group elements may play an analogous role ( Pappalardo et al., 2024 ) and deserve further attention in future studies. C and H mass balance and identification of solid carbonaceous compounds A detailed assessment of carbon and hydrogen consumption reveals that neither CH₄ nor CO fully accounts for the total amount of CO₂ and H₂ reacted. Under the conditions tested with the Chamrousse chromitite at 200 °C, the sum of the moles of CH₄ and CO remained below 15 % relative to CO₂ consumption, and CH₄ alone represented less than 10 % of the H₂ converted. Similar trends were observed in experiments with the Bulqizë ultramafic rocks, suggesting that additional carbon-bearing species formed throughout the reaction and that their relative contribution to the overall mass balance grew with increasing reaction extent ( Supplementary Fig. S2 ). High-resolution mass spectrometry of the gas phase revealed a complex distribution of hydrocarbons ( Supplementary Fig. S3 ) at concentrations three to four orders of magnitude lower than CH₄ or CO. This result indicates that a substantial fraction of gaseous CO₂ and H₂ produced liquid or solid organic phases. In addition to this inference, a marked “bitumen-like” odor and darkened material were detected on both the chromite grains and reactor walls at the end of the runs ( Supplementary Fig. S4 ). Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) showed carbon-rich coatings on the mineral surfaces with no specific affinity for a particular phase ( Fig. 3a,b ). Similar observations have been reported in chromitites from various ophiolitic settings ( Pappalardo et al., 2024; Economou-Eliopoulos et al., 2019; Pujol-Solà et al., 2018; Griffin et al., 2016; Yang et al., 2015 ). These carbonaceous deposits, often described as “coke” in industrial contexts, arise under low H₂/CO₂ ratios (< 3) and incomplete hydrogenation of CO intermediates at moderate temperatures ( Bengaard et al., 2002; Lee et al., 2021 ). Raman microspectroscopy analyses further elucidated the nature of these solid products ( Fig. 3c ). The presence of graphite bands (G band at 1583 cm⁻¹ and D bands at 1350, 1623, and around 1100 cm⁻¹) indicates structurally disordered graphitic materials ( Beyssac and Lazzeri, 2012 ). Second-order Raman features also showed typical disorder peaks consistent with partially crystalline carbon. Elevated D/G intensity ratios (~ 0.7) suggest small crystal sizes, though graphene layers may still be present. The blackish material bridging mineral grains exhibited the same G and D bands, with an additional feature near 1450 cm⁻¹ that aligns with asymmetric deformation modes of CH₃ or CH₂ groups in saturated or unsaturated hydrocarbons ( Socrates, 2004 ). Below 1300 cm⁻¹, multiple weaker peaks possibly correspond to C–C skeletal vibrations of alkanes or rocking motions of methyl and methylene groups, although their precise assignment remains tentative due to low signal intensity. Strong fluorescence from these deposits is consistent with polyaromatic hydrocarbons, resins, or asphaltenes, in line with the pronounced bitumen aroma noted during sampling. Concurrently, marked drops in system pressure at 200–250 °C suggest that gaseous reactants are continuously converted into solid phases ( Supplementary Fig. S5) . Water generated during CO₂ hydrogenation ( Eqs. 1–3 ) appears to be consumed by serpentinization, as supported by the occurrence of newly formed serpentine flakes on reacted olivine and chromite grains ( Supplementary Fig. S6 ). These textural changes imply that serpentinization can proceed within the vapor phase, further contributing to pressure reduction and carbon sequestration in solid form. Overall, these observations demonstrate that CH₄ and CO are only part of a broader suite of reaction products. The formation of graphitic-like solids and various mineralized carbonaceous compounds appears inherent to this catalytic pathway, as indicated by the observed various chemical, structural, and spectroscopic signatures observed ( Fig. 3c ). Water inhibition The influence of water was examined by introducing a controlled amount of deoxygenated water vapor into the ongoing H₂–CO₂ reaction at 200 °C with the Chamrousse chromitite ( see Methods ). The total system pressure (max 15.1 bar) remained below the water saturation threshold (15.5 bar), ensuring the absence of a liquid phase. Shortly after this injection, CH₄ production ceased abruptly ( Fig. 4 ). In parallel, CO levels dropped, whereas H₂ and CO₂ showed no further net consumption, but at the contrary their concentrations slightly increased. These observations are consistent with the water–gas shift pathway, where water vapor promotes CO₂ and H₂ production by favoring the forward reaction while limiting its reverse counterpart. There is also a possibility that water deactivates or alters catalytic surfaces. This behavior highlights the need for spatial or temporal separation of CO₂ hydrogenation sites from active serpentinization zones, which require water. Indeed, serpentinization not only generates H₂ but also consumes water, ultimately creating the dry conditions necessary for CO₂ hydrogenation. These findings align with geological observations by Andreani et al. (2023) and underscore the dual role of serpentinization in both driving H₂ production and facilitating abiotic carbon reduction processes under water-limited conditions. Furthering our understanding of low temperature CO 2 hydrogenation in serpentinizing environments Our findings highlight the importance of rock composition, temperature, and water availability in governing abiotic hydrocarbon synthesis within ultramafic environments. They also confirm that the reaction interplay between serpentinization and CO 2 hydrogenation reactions, is crucial to the overall dynamics. Water strongly limit CO 2 hydrogenation, yet serpentinization consumes water and generates H 2 , creating local conditions favoring hydrocarbons formation. As a result, CO₂ hydrogenation is likely more effective where serpentinization is spatially or temporally decoupled, or if H₂ exsolves from the aqueous phase to drive gas–rock interactions. Measured reaction rates at 200–250 °C, in combination with realistic activation energies, indicate that CH₄ and mineralized carbonaceous compounds can form in significant quantities at temperatures as low as 100–150 °C under dry conditions. This mechanism may explain the distinct isotopic signatures of CH₄ observed in ophiolitic settings. More than 80 mol% of the reduced carbon produced in our experiments occurs as graphitic material or hydrogenated solid compounds, underscoring the need to look beyond CH₄ alone. These carbonaceous solids not only provide tangible evidence of abiotic CO₂ reduction, but also serve as key pathfinders for the presence of H₂- and CH₄-rich fluids in serpentinizing systems. Future investigations focusing on their characterization in natural environments will be critical for unraveling the full extent of low-temperature abiotic hydrocarbon pathways. Declarations Acknowledgment This project has received funding from the French National Research Agency (grant agreement ANT-20-CE01-0020). H.D. acknowledges support from the French Ministry for Research and Innovation (MESRI). 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H. and Bukur D. B. (1990). Reaction kinetics over iron catalysts used for Fischer-Tropsch synthesis. Canadian J. Chem. Eng. 68, 292-301, Additional Declarations There is NO Competing Interest. Supplementary Files NatGeoFTTSupplementaryinformations.docx Supplementary Information Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5972145","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":493098682,"identity":"57fbffb6-6e52-4799-97b1-df4d5b67dd51","order_by":0,"name":"Hugo Dutoit","email":"data:image/png;base64,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","orcid":"","institution":"Université Grenoble Alpes","correspondingAuthor":true,"prefix":"","firstName":"Hugo","middleName":"","lastName":"Dutoit","suffix":""},{"id":493098683,"identity":"01229942-5d77-4ec9-80f7-003f9d9d7efa","order_by":1,"name":"Laurent Truche","email":"","orcid":"https://orcid.org/0000-0002-8471-1349","institution":"University Grenoble Alpes","correspondingAuthor":false,"prefix":"","firstName":"Laurent","middleName":"","lastName":"Truche","suffix":""},{"id":493098684,"identity":"4c72cd69-0d9b-4f0e-9a76-a57d6c7001c7","order_by":2,"name":"Frédéric Donzé","email":"","orcid":"","institution":"Université Grenoble Alpes - ISTerre","correspondingAuthor":false,"prefix":"","firstName":"Frédéric","middleName":"","lastName":"Donzé","suffix":""},{"id":493098685,"identity":"2082e9c3-4130-40a4-a0cb-c83c5fab4b0e","order_by":3,"name":"Christophe Monnin","email":"","orcid":"","institution":"Géosciences Environnement Toulouse","correspondingAuthor":false,"prefix":"","firstName":"Christophe","middleName":"","lastName":"Monnin","suffix":""}],"badges":[],"createdAt":"2025-02-06 09:46:42","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-5972145/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5972145/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":88778946,"identity":"1f3a2b2c-06a1-4376-a2a5-235341be0239","added_by":"auto","created_at":"2025-08-11 10:25:09","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":89932,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEvolution of H₂ and CO₂ concentrations over time, together with CO and CH₄ production, at 200 °C and 250 °C under dry conditions. Note that the x and y scales are different depending on the rock used. Panels show results for (a) Chamrousse chromitite (France), (b) Bulqizë chromitite (Albania), and (c) Bulqizë harzburgite (Albania). Each plot indicates the rock mass (m) and the Specific Surface Area (SSA) of the samples used. Error bars on the final data points correspond to maximum uncertainties recorded during the experiments.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5972145/v1/92aec7bae428b6512a37ff62.jpg"},{"id":88778947,"identity":"c186df58-ee7e-47e8-bd63-bc01486b870a","added_by":"auto","created_at":"2025-08-11 10:25:09","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":54631,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eArrhenius plot comparing H₂ consumption by CO₂ hydrogenation to H₂ production during olivine serpentinization. The blue line represents the extrapolation of the zero-order H₂ consumption rate measured at 200 °C for the Chamrousse chromitite, assuming an activation energy of 80 kJ mol⁻¹ (Zimmerman and Bukur, 1990; Huff and Satterfield, 1984; Vogt et al., 2019). The pink line depicts the H₂ production rates from hydrothermal alteration of olivine at 200–300 °C (McCollom et al., 2016). Shaded regions around each curve indicate the respective uncertainties associated with these measurements.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5972145/v1/f13fd5d791328c9cc62debc4.jpg"},{"id":88778949,"identity":"031b8104-0d70-47d7-bb6c-b8011ae31162","added_by":"auto","created_at":"2025-08-11 10:25:09","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":89080,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) SEM backscattered electron image of a chromite grain coated with carbonaceous material after 1000 hours of reaction at 200 °C with the Chamrousse chromitite. (b) Similar coating on an olivine grain from the same experiment. Each micrograph highlights the EDS spot analysis (red square), confirming the presence of carbon (samples are gold-coated). (c) Raman spectra recorded on polished sections of the black deposits formed on reacted grains and on the reactor walls. Raw, unfiltered spectra show prominent G and D bands, consistent with graphitic structures, along with additional peaks reflecting the presence of disordered carbon compounds and hydrocarbon groups.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5972145/v1/2a5921800ef61b2c7885806c.jpg"},{"id":88778948,"identity":"86db6740-8301-4448-bf18-2703a319d927","added_by":"auto","created_at":"2025-08-11 10:25:09","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":44888,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eInfluence of water vapor on the H₂–CO₂ reaction at 200 °C in the presence of the Chamrousse chromitite. A controlled injection of water vapor halts CH₄ production and reduces CO levels, while H₂ and CO₂ concentrations show a constant concentration. These observations point to a shift in reaction pathways, likely involving water–gas shift processes and a concurrent reduction in catalytic activity.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-5972145/v1/3f4c9ef53f71f08eed81e4e5.jpg"},{"id":105729041,"identity":"dd800533-0fc2-43bd-9cd7-ac3af82d4a40","added_by":"auto","created_at":"2026-03-30 11:13:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2263124,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5972145/v1/2e16860f-8c6b-45db-a3fa-c6fb4bedcbcf.pdf"},{"id":88778961,"identity":"58849378-0cb7-4303-80c8-6c4ab68344ee","added_by":"auto","created_at":"2025-08-11 10:25:09","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4809646,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"NatGeoFTTSupplementaryinformations.docx","url":"https://assets-eu.researchsquare.com/files/rs-5972145/v1/546929da0da2e372cd6ee148.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Water-Limited Serpentinization Drives Abiotic Hydrocarbons Production in Ultramafic Environments","fulltext":[{"header":"Full Text","content":"\u003cp\u003eGeological H₂ systems attract increasing attention for their role in fueling the deep biosphere and for their potential as low-carbon energy sources (\u003cstrong\u003eTruche et al., 2020; M\u0026eacute;nez, 2020; Templeton et al., 2024\u003c/strong\u003e). Serpentinizing environments are particularly relevant, as they generate H\u003csub\u003e2\u003c/sub\u003e which is often found alongside CH₄ and other organic compounds. The formation of these compounds involves a complex interplay of biotic and abiotic processes (\u003cstrong\u003eReeves and Fiebig, 2020; Etiope and Sherwood Lollar, 2023\u003c/strong\u003e). Several lines of evidence indicate that only a few reduced carbon species are unambiguously recognized as abiotic (\u003cstrong\u003eEtiope and Oze, 2022; Xia and Gao, 2022\u003c/strong\u003e), prompting efforts to clarify how these compounds form and persist in ultramafic settings. Methane receives particular attention because it is commonly considered a product of CO₂ hydrogenation\u0026mdash;often referred in geological literature as Fischer-Tropsch-Type (FTT) reactions (\u003cstrong\u003eReeves and Fiebig, 2020\u003c/strong\u003e). This pathway involves the Sabatier reaction:\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\" width=\"661\" height=\"43\"\u003e\u003c/p\u003e\n\u003cp\u003eNumerous experimental studies have explored this reaction under water-saturated conditions intended to simulate active serpentinization, yet these conditions typically yield only trace amounts of CH₄ even at temperatures up to 300 \u0026deg;C (\u003cstrong\u003eMcCollom, 2013; McCollom, 2016; Barbier et al., 2020; Ueda et al., 2021; Reeves and Seewald, 2024\u003c/strong\u003e). This observation questions the feasibility of significant abiotic methanation in serpentinizing systems, especially in ophiolites settings where temperatures rarely exceed 150 \u0026deg;C. Various minerals (for example spinels) and metal alloys (including iron, nickel, and platinum group elements: PGE) may act as catalysts (\u003cstrong\u003eEtiope et al., 2018; Pappalardo et al., 2024\u003c/strong\u003e), and it is recognized that many catalytic processes become effective above 200 \u0026deg;C (\u003cstrong\u003eHorita and Berndt, 1999; Foustoukos and Seyfried Jr, 2004; Preiner et al., 2020\u003c/strong\u003e). Additionally, the catalytic availability of Ni and PGE in ultramafic rocks may also be influenced by the rock matrix and by water, which can shield or deactivate these reaction sites (\u003cstrong\u003eReeves and Fiebig, 2020\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eRecent experiments underline the possibility that CO₂ methanation occurs below 200 \u0026deg;C through gas\u0026ndash;rock interactions in relatively dry conditions (\u003cstrong\u003eJacquemin et al., 2010; Etiope and Ionescu, 2015; Medina et al., 2000; Ruiz et al., 2021\u003c/strong\u003e). Certain approaches rely on synthetic catalysts or flow-through reactors that provide valuable insights into reaction kinetics albeit experimental conditions often significantly differ from natural environments. In parallel, the influence of water on CO₂ hydrogenation and the broader distribution of reduced carbon products remain undocumented so far. This present study addresses these key points by examining how temperature, ultramafic rock lithology, mineral paragenesis, and water content govern CO₂ hydrogenation. Our results demonstrate that CH₄ production and the formation of complex abiotic organic compounds occur at temperatures as low as 200 \u0026deg;C under dry conditions, thereby shedding new light on the fundamental parameters that drive abiotic hydrocarbons synthesis in serpentinizing environments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eImpact of mineral assemblage and temperature on reaction rate and selectivity\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eChromitites and harzburgites were selected to explore how mineral assemblages influence the abiotic synthesis of reduced carbon species under varying thermal conditions. Chromite-bearing rocks are of particular interest because they host metals (e.g., Fe, Ni, Co, and PGE) that could act as catalysts in CO₂ hydrogenation reactions (\u003cstrong\u003eZaccarini et al., 2018;\u003c/strong\u003e \u003cstrong\u003eGonz\u0026aacute;lez-Jim\u0026eacute;nez et al., 2017\u003c/strong\u003e). Experiments involving olivine- or chromite-rich lithologies have already highlighted the role of serpentinization and associated metal phases in generating H₂ and potentially promoting abiotic organic synthesis (\u003cstrong\u003eBarbier et al., 2020\u003c/strong\u003e). Building on these insights, we used two chromitites (from the Chamrousse ophiolite in France and the Bulqiz\u0026euml; ophiolite in Albania) and one harzburgite (also from Bulqiz\u0026euml;) to assess the role of mineral parageneses on reaction pathways (\u003cstrong\u003eSupplementary Table S1\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eBoth chromitites contain 55\u0026ndash;58 wt% chromite with high Cr content relative to Al\u0026sup3;⁺ and Fe\u0026sup3;⁺ (Cr# \u0026gt; 70), although they differ markedly in their phyllosilicate assemblages. The Chamrousse chromitite is enriched in saponite (32 wt%), whereas the Bulqiz\u0026euml; chromitite contains more serpentine minerals (12 wt% lizardite, 9 wt% chrysotile). These differences provide a range of potential catalytic sites and water-buffering capacities. To probe the influence of temperature on CO\u003csub\u003e2\u003c/sub\u003e methanation, bulk rock powders (0.1\u0026ndash;1 mm) were crushed, sieved and cleaned under inert Ar atmosphere. They were then placed in stainless steel batch reactors and outgassed at 200\u0026deg;C under primary vacuum. The samples were reacted with H₂ and CO₂ (approximately 2 bar each, giving a molar H₂/CO₂ ratio = 1) under dry conditions between 100 and 250\u0026deg;C. The impact of water vapor was subsequently tested by introducing controlled amounts of moisture (see \u003cstrong\u003eMethods\u003c/strong\u003e for details). Throughout each run, gas composition was monitored to quantify reaction rates and identify carbon-bearing species. Special attention was directed toward the solid phases before and after experiments to determine whether additional organic materials formed on or within the mineral surfaces. A preliminary blank experiment at 250 \u0026deg;C (no rock powders) carried out for a month showed that the stainless-steel reactor did not promote any reactions of interest, as neither H₂ nor CO₂ concentrations changed, and no CH\u003csub\u003e4\u003c/sub\u003e can was detected (\u003cstrong\u003esee Methods\u003c/strong\u003e). Experiments with chromitites and harzburgite in the presence of CO₂ alone also resulted in no consumption of CO₂ or generation of detectable amounts of CH₄, demonstrating the essential role of H₂ in driving these reactions.\u003c/p\u003e\n\u003cp\u003eWhen both H₂ and CO₂ were introduced, distinct reactivity patterns emerged among the three ultramafic rocks (\u003cstrong\u003eSupplementary Tables S2\u0026ndash;S4; Fig. 1\u003c/strong\u003e). At 200 \u0026deg;C, the Chamrousse chromitite displayed a clear and progressive consumption of both H₂ and CO₂ (\u003cstrong\u003eFig. 1a\u003c/strong\u003e), accompanied by a steady production of CH₄ that reached a plateau around 600 hours. The near-complete depletion of the reactants coincided with the cessation of CH₄ production. In parallel, carbon monoxide (CO) was detected early in the reaction, then declined as CH₄ formed, consistent with a role for CO as a transient intermediate. This observation explains why CO has never been measured so far in natural serpentinizing systems and suggests that the overall methanation pathway proceeds via the reverse water\u0026ndash;gas shift reaction (\u003cstrong\u003eEq. 2\u003c/strong\u003e), followed by CO hydrogenation to CH₄ (\u003cstrong\u003eEq. 3\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\" width=\"649\" height=\"70\"\u003e\u003c/p\u003e\n\u003cp\u003eComparable trends were observed with the Bulqiz\u0026euml; chromitite and harzburgite (\u003cstrong\u003eFig. 1b,c\u003c/strong\u003e), yet their overall reaction rates were significantly lower than those of the Chamrousse chromitite. With the Bulqiz\u0026euml; chromitite, CO production remained substantial while CH₄ concentrations stayed comparatively low (\u003cstrong\u003eFig. 1b\u003c/strong\u003e). In contrast, the Bulqiz\u0026euml; harzburgite generated more CH₄ at 250 \u0026deg;C, along with a limited level of CO (\u003cstrong\u003eFig. 1c\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eThese findings emphasize the strong dependence of both reaction kinetics and selectivity on lithological and mineralogical factors. Zero-order kinetics for CH₄ formation and H₂ consumption appeared most clearly at 200 and 250 \u0026deg;C. At 200 \u0026deg;C, only the Chamrousse chromitite produced significant CH₄, with a production rate of 0.751 \u0026plusmn; 0.075 nmol m⁻\u0026sup2; h⁻\u0026sup1; over the first 168 hours. At 250 \u0026deg;C, both the Bulqiz\u0026euml; chromitite and harzburgite generated CH₄, at rates of 0.118 \u0026plusmn; 0.012 and 4.70 \u0026plusmn; 0.47 nmol m⁻\u0026sup2; h⁻\u0026sup1;, respectively. Attempts to explore lower temperatures (100 \u0026deg;C and 150 \u0026deg;C) with the Chamrousse chromitite did not yield detectable amounts of CO or CH₄ after 3000 hours. By assuming an activation energy of 80 kJ mol⁻\u0026sup1; for CO₂ methanation (\u003cstrong\u003eZimmerman and Bukur, 1990; Huff and Satterfield, 1984; Vogt et al., 2019\u003c/strong\u003e) and using the 200 \u0026deg;C data as a reference, we estimate that CH₄ production at 150 \u0026deg;C would be ~800 times slower (\u003cstrong\u003eSupplementary Fig. S1; Supplementary Table S5\u003c/strong\u003e). This decrease explains the absence of quantifiable reactivity within the timeframe investigated here. Longer experiments, larger reactive surface areas, higher pressures, or higher H₂/CO₂ ratios may help capture such low-temperature reaction regimes. Still, the rates measured here and the relevant activation energies indicate that abiotic CH₄ formation is likely to occur in ultramafic systems at lower-temperatures (\u003cstrong\u003eEtiope and Ionescu, 2015; Pappalardo et al., 2024\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eComparisons of H₂ consumption by CO₂ hydrogenation with H₂ production during serpentinization (\u003cstrong\u003eMcCollom et al., 2016\u003c/strong\u003e) shed additional light on the potential significance of these processes in ultramafic settings (\u003cstrong\u003eFig. 2; Supplementary Table S6\u003c/strong\u003e). By extrapolating the H₂ consumption rate observed at 200 \u0026deg;C for the Chamrousse chromitite across a range of temperatures using the same 80 kJ mol⁻\u0026sup1; activation energy, it appears that CO₂ hydrogenation can outpace serpentinization-driven H₂ generation as long as H₂ and CO₂ remain available and dry conditions persist. This observation may explain why certain ophiolitic gas seeps contain abundant CH₄ and only trace levels of H₂ (\u003cstrong\u003eD\u0026rsquo;Alessandro et al., 2018; Etiope et al., 2013; Etiope and Sherwood Lollar, 2013\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eThe contrasted catalytic potentials of the three investigated lithologies highlight the importance of mineralogy and trace metal content. The unique abundance of saponite (32 wt%) in the Chamrousse chromitite likely enhances catalytic performance through large surface area, numerous sorption sites, and redox-active metal substitutions (Fe, Ni, Co). This smectite-group mineral has already been linked to aromatic amino acid formation in serpentinized peridotites (\u003cstrong\u003eM\u0026eacute;nez et al., 2018\u003c/strong\u003e) and is recognized as an effective Fischer\u0026ndash;Tropsch-type catalyst (\u003cstrong\u003eSun et al., 2017; Heller-Kallai, 2001\u003c/strong\u003e). Layered silicates such as serpentine and talc, along with brucite, may also facilitate electron exchange by accommodating metal substitution in their structures. Spinel-group minerals\u0026mdash;including Fe-chromite and magnetite\u0026mdash;are known to catalyze Fischer\u0026ndash;Tropsch reactions (\u003cstrong\u003eMierczynski et al., 2018; Rao et al., 1994\u003c/strong\u003e). Furthermore, Ni and Co concentrations can reach 3200 and 130 ppm, respectively, in the Bulqiz\u0026euml; rocks (\u003cstrong\u003eXiong et al., 2015\u003c/strong\u003e), creating additional catalytic sites (\u003cstrong\u003evan Helden et al., 2020; Hern\u0026aacute;ndez Mej\u0026iacute;a et al., 2020\u003c/strong\u003e). Platinum-group elements may play an analogous role (\u003cstrong\u003ePappalardo et al., 2024\u003c/strong\u003e) and deserve further attention in future studies.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eC and H mass balance and identification of solid carbonaceous compounds\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA detailed assessment of carbon and hydrogen consumption reveals that neither CH₄ nor CO fully accounts for the total amount of CO₂ and H₂ reacted. Under the conditions tested with the Chamrousse chromitite at 200 \u0026deg;C, the sum of the moles of CH₄ and CO remained below 15 % relative to CO₂ consumption, and CH₄ alone represented less than 10 % of the H₂ converted. Similar trends were observed in experiments with the Bulqiz\u0026euml; ultramafic rocks, suggesting that additional carbon-bearing species formed throughout the reaction and that their relative contribution to the overall mass balance grew with increasing reaction extent (\u003cstrong\u003eSupplementary Fig. S2\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eHigh-resolution mass spectrometry of the gas phase revealed a complex distribution of hydrocarbons (\u003cstrong\u003eSupplementary Fig. S3\u003c/strong\u003e) at concentrations three to four orders of magnitude lower than CH₄ or CO. This result indicates that a substantial fraction of gaseous CO₂ and H₂ produced liquid or solid organic phases. In addition to this inference, a marked \u0026ldquo;bitumen-like\u0026rdquo; odor and darkened material were detected on both the chromite grains and reactor walls at the end of the runs (\u003cstrong\u003eSupplementary Fig. S4\u003c/strong\u003e). Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) showed carbon-rich coatings on the mineral surfaces with no specific affinity for a particular phase (\u003cstrong\u003eFig. 3a,b\u003c/strong\u003e). Similar observations have been reported in chromitites from various ophiolitic settings (\u003cstrong\u003ePappalardo et al., 2024; Economou-Eliopoulos et al., 2019; Pujol-Sol\u0026agrave; et al., 2018; Griffin et al., 2016; Yang et al., 2015\u003c/strong\u003e). These carbonaceous deposits, often described as \u0026ldquo;coke\u0026rdquo; in industrial contexts, arise under low H₂/CO₂ ratios (\u0026lt; 3) and incomplete hydrogenation of CO intermediates at moderate temperatures (\u003cstrong\u003eBengaard et al., 2002; Lee et al., 2021\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eRaman microspectroscopy analyses further elucidated the nature of these solid products (\u003cstrong\u003eFig. 3c\u003c/strong\u003e). The presence of graphite bands (G band at 1583 cm⁻\u0026sup1; and D bands at 1350, 1623, and around 1100 cm⁻\u0026sup1;) indicates structurally disordered graphitic materials (\u003cstrong\u003eBeyssac and Lazzeri, 2012\u003c/strong\u003e). Second-order Raman features also showed typical disorder peaks consistent with partially crystalline carbon. Elevated D/G intensity ratios (~ 0.7) suggest small crystal sizes, though graphene layers may still be present. The blackish material bridging mineral grains exhibited the same G and D bands, with an additional feature near 1450 cm⁻\u0026sup1; that aligns with asymmetric deformation modes of CH₃ or CH₂ groups in saturated or unsaturated hydrocarbons (\u003cstrong\u003eSocrates, 2004\u003c/strong\u003e). Below 1300 cm⁻\u0026sup1;, multiple weaker peaks possibly correspond to C\u0026ndash;C skeletal vibrations of alkanes or rocking motions of methyl and methylene groups, although their precise assignment remains tentative due to low signal intensity. Strong fluorescence from these deposits is consistent with polyaromatic hydrocarbons, resins, or asphaltenes, in line with the pronounced bitumen aroma noted during sampling.\u003c/p\u003e\n\u003cp\u003eConcurrently, marked drops in system pressure at 200\u0026ndash;250 \u0026deg;C suggest that gaseous reactants are continuously converted into solid phases (\u003cstrong\u003eSupplementary Fig. S5)\u003c/strong\u003e. Water generated during CO₂ hydrogenation (\u003cstrong\u003eEqs. 1\u0026ndash;3\u003c/strong\u003e) appears to be consumed by serpentinization, as supported by the occurrence of newly formed serpentine flakes on reacted olivine and chromite grains (\u003cstrong\u003eSupplementary Fig. S6\u003c/strong\u003e). These textural changes imply that serpentinization can proceed within the vapor phase, further contributing to pressure reduction and carbon sequestration in solid form.\u003c/p\u003e\n\u003cp\u003eOverall, these observations demonstrate that CH₄ and CO are only part of a broader suite of reaction products. The formation of graphitic-like solids and various mineralized carbonaceous compounds appears inherent to this catalytic pathway, as indicated by the observed various chemical, structural, and spectroscopic signatures observed (\u003cstrong\u003eFig. 3c\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eWater inhibition\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe influence of water was examined by introducing a controlled amount of deoxygenated water vapor into the ongoing H₂\u0026ndash;CO₂ reaction at 200 \u0026deg;C with the Chamrousse chromitite (\u003cstrong\u003esee Methods\u003c/strong\u003e). The total system pressure (max 15.1 bar) remained below the water saturation threshold (15.5 bar), ensuring the absence of a liquid phase. Shortly after this injection, CH₄ production ceased abruptly (\u003cstrong\u003eFig. 4\u003c/strong\u003e). In parallel, CO levels dropped, whereas H₂ and CO₂ showed no further net consumption, but at the contrary their concentrations slightly increased. These observations are consistent with the water\u0026ndash;gas shift pathway, where water vapor promotes CO₂ and H₂ production by favoring the forward reaction while limiting its reverse counterpart. There is also a possibility that water deactivates or alters catalytic surfaces.\u003c/p\u003e\n\u003cp\u003eThis behavior highlights the need for spatial or temporal separation of CO₂ hydrogenation sites from active serpentinization zones, which require water. Indeed, serpentinization not only generates H₂ but also consumes water, ultimately creating the dry conditions necessary for CO₂ hydrogenation. These findings align with geological observations by \u003cstrong\u003eAndreani et al. (2023)\u003c/strong\u003e and underscore the dual role of serpentinization in both driving H₂ production and facilitating abiotic carbon reduction processes under water-limited conditions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFurthering our understanding of low temperature CO\u003csub\u003e2\u003c/sub\u003e hydrogenation in serpentinizing environments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOur findings highlight the importance of rock composition, temperature, and water availability in governing abiotic hydrocarbon synthesis within ultramafic environments. They also confirm that the reaction interplay between serpentinization and CO\u003csub\u003e2\u003c/sub\u003e hydrogenation reactions, is crucial to the overall dynamics. Water strongly limit CO\u003csub\u003e2\u003c/sub\u003e hydrogenation, yet serpentinization consumes water and generates H\u003csub\u003e2\u003c/sub\u003e, creating local conditions favoring hydrocarbons formation. As a result, CO₂ hydrogenation is likely more effective where serpentinization is spatially or temporally decoupled, or if H₂ exsolves from the aqueous phase to drive gas\u0026ndash;rock interactions. Measured reaction rates at 200\u0026ndash;250 \u0026deg;C, in combination with realistic activation energies, indicate that CH₄ and mineralized carbonaceous compounds can form in significant quantities at temperatures as low as 100\u0026ndash;150 \u0026deg;C under dry conditions. This mechanism may explain the distinct isotopic signatures of CH₄ observed in ophiolitic settings. More than 80 mol% of the reduced carbon produced in our experiments occurs as graphitic material or hydrogenated solid compounds, underscoring the need to look beyond CH₄ alone. These carbonaceous solids not only provide tangible evidence of abiotic CO₂ reduction, but also serve as key pathfinders for the presence of H₂- and CH₄-rich fluids in serpentinizing systems. Future investigations focusing on their characterization in natural environments will be critical for unraveling the full extent of low-temperature abiotic hydrocarbon pathways.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgment\u003c/h2\u003e\u003cp\u003eThis project has received funding from the French National Research Agency (grant agreement ANT-20-CE01-0020). H.D. acknowledges support from the French Ministry for Research and Innovation (MESRI). L.T. acknowledges support from the Institut Universitaire de France\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eThe data supporting the findings of this study are available within the paper and its Supplementary Information.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAnders, E. (1989). Pre-biotic organic matter from comets and asteroids. \u003cem\u003eNature\u003c/em\u003e,\u0026nbsp;\u003cem\u003e342\u003c/em\u003e(6247), 255-257,\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eAndreani, M., Montagnac, G., Fellah, C., Hao, J., Vandier, F., Daniel, I., ... \u0026amp; M\u0026eacute;nez, B. (2023). The rocky road to organics needs drying. \u003cem\u003eNature Communications\u003c/em\u003e,\u0026nbsp;\u003cem\u003e14\u003c/em\u003e(1), 347,\u0026nbsp;\u003c/li\u003e\n \u003cli\u003e\u0026nbsp;Atreya, S. K., Mahaffy, P. R., \u0026amp; Wong, A. S. (2007). 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Eng.\u0026nbsp;\u003c/em\u003e68, 292-301, \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5972145/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5972145/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAbiotic methane and light hydrocarbons generated through serpentinization have attracted growing attention as potential energy sources for the deep biosphere and as pathfinders of natural H₂ production. Although Fischer–Tropsch-type (FTT) reactions are central to the abiotic reduction of oxidized carbon by H₂, key uncertainties persist regarding catalytic mechanisms and the influence of water. Here, we show that ultramafic rocks (chromitite and harzburgite) readily produce CH₄, and minor gaseous hydrocarbons upon reaction with H₂ and CO₂ at 200–250 °C under dry conditions. Kinetic analyses, mass balance calculations, and Raman microspectroscopy indicate that solid carbonaceous compounds constitute the dominant products, while CO acts as a short-lived intermediate. Additional tests reveal that water vapor inhibits CO₂ hydrogenation, implying that efficient FTT processes require spatial or temporal decoupling from active serpentinization. Yet serpentinization consumes water and generates H₂, ultimately sustaining hydrocarbon formation and pointing to a dynamic interplay between hydration reactions and abiotic organic synthesis in ultramafic settings.\u003c/p\u003e","manuscriptTitle":"Water-Limited Serpentinization Drives Abiotic Hydrocarbons Production in Ultramafic Environments","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-11 10:25:04","doi":"10.21203/rs.3.rs-5972145/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c375cb38-14c1-4081-87f4-46ea06fa9505","owner":[],"postedDate":"August 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":52359911,"name":"Earth and environmental sciences/Biogeochemistry"},{"id":52359912,"name":"Earth and environmental sciences/Solid Earth sciences/Geochemistry"},{"id":52359913,"name":"Earth and environmental sciences/Solid Earth sciences/Mineralogy"}],"tags":[],"updatedAt":"2026-03-30T09:33:55+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-11 10:25:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5972145","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5972145","identity":"rs-5972145","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}