Supra-subduction zone ophiolites retain hydrogen generation potential after 300 million years

Submitted Version / preprint Donzé Truche DOI: 1 Supra-subduction zone ophiolites retain hydrogen generation potential after 300 million years Frédéric V. Donzé*¹ and Laurent Truche¹ *Corresponding author e‑mail: [email protected] Affiliations ¹Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, 38000 Grenoble, France Abstract The Ural Mountains host some of the earliest documented natural hydrogen (H₂) emissions, yet the Paleozoic ultramafic complexes are 250–400 Ma old and would normally be expected to have exhausted their serpentinization potential. We revisit overlooked Soviet observations from the Kempirsay chromitite district and compare them with recent data from the younger, Jurassic Bulqizë ophiolite in Albania. In both massifs, H₂-rich seeps (>80–90 vol% H₂) occur within ~300 m of podiform chromitite bodies. At Kempirsay, degassing takes place at low temperatures (14–30 °C), and experiments on Kempirsay rocks show that Fe-bearing minerals can generate H₂ at near-ambient conditions. New radiocarbon data from Bulqizë methane (3.76 ± 0.06 pMC, apparent age ~26 kyr) demonstrate that associated CH₄–H₂ inventories are renewed on 10⁴-year timescales. We interpret chromitite bodies and their damage zones as catalytic and hydraulic hubs embedded in a supra-subduction-zone ophiolitic architecture that localizes serpentinization and preserves reactive peridotite. This chromitite-centered architecture implies that chromitite-bearing mantle slabs can retain hydrogen generation potential for hundreds of millions of years and constitute priority targets for natural hydrogen exploration and stimulated geological hydrogen production. Submitted Version / preprint Donzé Truche DOI: 2 Keywords Natural hydrogen seeps; Chromitite-bearing ophiolites; Low-temperature serpentinization; Chromitite-hosted H₂–CH₄ fluids; Geological hydrogen systems Main Text Introduction Recent mining accidents in Kazakhstan's chromite mines have brought renewed attention to the persistent hydrogen hazard in these ancient geological formations. In February and May 2025, fires at the "Bolashak" mine injured several workers, forcing operational suspensions and highlighting a phenomenon that has plagued these mines for decades (Interfax, 2025a, b; Kursiv, 2025, ISSSource, 2025; Yermaganbetova, 2025). These incidents echo observations dating back to 1979 from the same mining district, when hydrogen-rich gas ignited underground, creating what miners described as "calm flames over 12 m high" (Ukhanov et al., 1987). The first documented observations of H₂-rich gas (66.5-81 % H₂) in Ural mines date from 1925-1931 and were reported by Zavaritsky, Tchepennikovs and Vernadsky (Lidin et al., 1982; Bohdanowicz, 1934), in the Nizhny Tagil dunite massif during drilling operations for chromite exploration at ~600 m depth. Soviet research, often overlooked in Western literature, provided the following additional observations. Lidin et al. (1982) noted that hydrogen emissions at the Kempirsay massif occurred exclusively within 300 meters of chromitite bodies and were never reported in barren ultramafic units. The isotopic signatures were also unusual: Ukhanov et al. (1987) measured some of Earth's most negative δD values (-744 to -766 ‰), which they interpreted as evidence for low-temperature processes. Devirtz et al. (1992) experimentally demonstrated hydrogen production from Kempirsay ultramafic rocks at ambient temperatures of 17-24 °C, suggesting that serpentinization reactions may be faster than commonly thought in chromite-rich geological environments. The persistence of hydrogen degassing from Paleozoic ophiolites in the Ural Mountains highlights gaps in our understanding of natural hydrogen systems. These ultramafic complexes, 250–400 Ma old, should have exhausted their serpentinization potential long ago, through migration, microbial or abiotic consumption, yet they still release hydrogen at rates that rival much younger systems. These observations closely parallel those from Submitted Version / preprint Donzé Truche DOI: 3 the Bulqizë mine, hosted in a Jurassic ophiolite approximately 170 Ma old, where active hydrogen seeps persist within chromitite-bearing peridotites (Truche et al., 2024, 2025; Yao et al., 2025). The fact that ophiolites spanning such a wide age range, from Paleozoic to Mesozoic, show similar degassing patterns raises the question of how chromitites control natural H₂ systems in chromitite-bearing SSZ ophiolites worldwide. Without ruling out the possibility that part of the present-day hydrogen emissions reflect older serpentinization events that are now extinct, we address this question by examining several features of these ophiolites that may support long-lived and still active H₂ systems. A first-order observation is that both Kempirsay and Bulqizë share a common geological heritage: they are suprasubduction ophiolites that host abundant podiform chromitites. Podiform chromitites form in such settings during high-degree partial melting of the mantle wedge, and concentrate chromium-rich minerals and associated phases that can strongly influence redox conditions. Similar associations between chromitite bodies and hyperalkaline H₂-CH₄ fluids have been documented in several other chromitite-bearing ophiolites worldwide, suggesting that chromitite-bearing ophiolites of this type may represent a coherent class of natural hydrogen systems. In this study, we use a comparative analysis of the Kempirsay and Bulqizë ophiolites to document this class of chromitite-bearing systems and assess their implications for natural hydrogen generation and exploration. Geological Setting and Hydrogen Occurrences in Ural and Bulqizë chromite mines The Ural Mountains exhibit numerous ophiolitic complexes that originate from the oceanic lithosphere formed during the Paleozoic evolution of the Uralian Ocean between 400-250 Ma (Brown et al., 2006; Puchkov, 2009). These complexes were obducted during the Late Devonian-Early Carboniferous collision between the Baltica and Siberia-Kazakhstan continents (Spadea & D'Antonio, 2006). Among these, the Kempirsay massif in Kazakhstan stands out as one of Earth's largest exposed ultramafic complexes, covering over 900 km². Within the Sakmara allochthon, it represents fore-arc lithosphere formed along the Devonian Magnitogorsk Island arc (Savelieva et al., 1997; Fryer & Greenough, 1992, Johnson, 2012). To understand the significance of hydrogen emissions from such ancient rocks, we can compare them with the much younger Bulqizë ophiolite in Albania (Table 1). This Jurassic complex Submitted Version / preprint Donzé Truche DOI: 4 formed 160-165 million years ago in a Neo-Tethyan setting (Beccaluva et al., 1994; Dilek & Furnes, 2009), provides an interesting contrast. Table 1. Comparative characteristics of H₂-producing ophiolitic systems (references for the values are provided in the text) Parameter Kempirsay (Urals) Bulqizë (Albania) Implications Geological features Ophiolite age 250-400 Ma 160-165 Ma 200+ Ma age difference Geothermal gradient ~12°C/km 25-30°C/km Distinct thermal regimes Temperature assessment at H₂ source depth 14-30°C 100-150°C Different generation conditions? Degree of serpentinization ~28% (historical estimate) <2% in harzburgite Substantial fresh rock remains Gas characteristics δD-H₂ (‰ VSMOW) -744 to -766 -743 ± 3 Remarkably similar signatures H₂ concentration 92-98% 84 ± 4% Both highly enriched CH₄ content 0.8-1.7% 13.2 ± 0.7% Variable secondary processes Maximum documented H₂ flux 20 m³/day ~550 m³/day Different scales or preservation? Submitted Version / preprint Donzé Truche DOI: 5 Parameter Kempirsay (Urals) Bulqizë (Albania) Implications Association with chromitite location 100% 100% Comparable spatial control Kempirsay and Bulqizë share similarities like both hosting large podiform chromitites surrounded by dunite envelopes. They both display persistent H₂ seepages, with gas compositions > 80 % H₂ (92–98 % in Kempirsay, 84 % in Bulqizë). However, they differ on some other points. Firstly, the Kempirsay ophiolitic massif exhibits exceptionally low surface heat flow values of 25-35 mW m⁻² (mean ~30 mW m⁻²), significantly below the global continental average of ~65 mW m⁻² (Kukkonen et al., 1997; Brown et al., 2006; Golovanova, 2005; Krabbendam, 2001). Direct temperature-depth profiles from boreholes and the URSEIS (Urals Reflection Seismic Experiment and Integrated Studies project) geothermal model indicate a present-day crustal thermal gradient of ~12°C km⁻¹ (10-15°C km⁻¹ in the upper 2 km) (Mikhailov et al., 2002). This anomalously cold geothermal regime reflects the combined effects of low crustal heat production (~0.45 μW m⁻³), thick lithosphere inherited from Paleozoic collision, and residual Quaternary glacial cooling. By contrast, the Bulqizë ophiolitic massif displays higher surface heat flow values of 45-50 mW m⁻², above the Albanian national average of 30-40 mW m⁻² (Frashëri, 2015). Direct measurements inside the Bulqizë mine indicate steep thermal gradients of 25-30°C km⁻¹ in the upper 2-3 km (Goskolli, 2022), corresponding to temperatures higher than ~100 °C at depths of a few kilometers (Yao et al., 2025). Secondly, the degree of serpentinization could be quite different in the two mines. Historical mapping suggested that ~28 % of the Kempirsay peridotite was serpentinized (Lidin et al., 1982). Subsequent core drilling and bulk-rock LOI (“Loss On Ignition”) measurements, indicate that the upper-most kilometer is now almost completely serpentinized (70–100 %, LOI = 10–17 wt %), with fresh lherzolite restricted to depths > 1 km (Saveliev et al., 2022). At Bulqizë, petrographic mapping shows that most harzburgite and dunite are still largely unaltered (< 2 % serpentine) on the first km, with hydration restricted to chromitite-bearing fault corridors (Xiong et al., 2015). Inside these corridors, the alteration intensity rises from dunite envelopes to massive chromitite, reflecting possible multiple serpentinization pulses and late fluid percolation (Junge et al., 2021). Submitted Version / preprint Donzé Truche DOI: 6 The spatial distribution of hydrogen emissions provides insights into the controlling factors of these ancient systems. At the Kempirsay massif, detailed underground mapping by Soviet geologists documented a well-defined pattern: all hydrogen occurrences clustered in the immediate vicinity of chromitite ore bodies. The Molodezhnaya mine exemplified this relationship, producing sustained flows reaching 20 m³/day from discrete fracture systems adjacent to chromitite lenses. Gas composition showed extreme hydrogen enrichment (9298%) with minor methane (0.8-1.7%) and nitrogen (0.4-0.6%), plus trace helium, similar to the 84% H₂ observed at Bulqizë (Truche et al., 2025) despite the vast differences in age and temperature of the ophiolite environment. Strong support for low-temperature generation comes from Soviet experimental work that has received insufficient attention in recent literature. Devirtz et al. (1992) subjected Kempirsay rock samples to controlled laboratory conditions, demonstrating hydrogen production at both 100 °C and at ambient temperatures between 17 and 24 °C. Similar lowtemperature hydrogen generation has been reported from other serpentinizing systems (Mayhew et al., 2013; Neubeck et al., 2014; Ellison et al., 2021), though the specific role of chromitites as catalytic enhancers remains underexplored. The detection of radiocarbon in methane from Bulqizë represents an additional constraint on the timescales of these systems (Truche et al., 2025). The measured value of 3.76 ± 0.06 pMC, corresponding to an apparent age of ~26 000 years, shows that at least part of the CH₄ inventory is renewed on 10⁴-year timescales rather than the 10⁶-year scales often assumed for ophiolitic systems. The young age of CH₄ implies that the associated H₂ required for its synthesis must also be recently produced, indicating active, ongoing hydrogen generation rather than simple release of ancient gas reservoirs. This suggests that chromitite-bearing SSZ ophiolites may play a more dynamic role in the global carbon and hydrogen cycles than previously recognized. These observations are consistent with the idea that low-temperature serpentinization can sustain hydrogen generation under appropriate geochemical conditions, but they do not by themselves explain why active seeps in both massifs are strictly confined to the vicinity of chromitite ore bodies; this is the question we now address in the following discussion. The Chromitite Catalytic Factory The association between hydrogen emissions and chromitite bodies at Kempirsay and Bulqizë reflects more than spatial coincidence. In both massifs, all documented H₂-rich seeps are located within a few hundred meters of podiform chromitite, whereas no comparable Submitted Version / preprint Donzé Truche DOI: 7 degassing has been reported within the same type of ophiolites without chromitite despite similar ultramafic host rocks (Donzé et al., 2024). This spatial pattern suggests that chromitites and their surrounding damage zones play a central role in both H₂ production and migration. First, chromitite bodies can act as mechanically stiff inclusions that focus fracturing and maintain damage zones within a more deformable peridotite–serpentinite matrix, thereby providing permeable pathways for infiltrating meteoric water and for the escape of produced gas. In addition to this mechanical role, chromitite-bearing peridotites also provide favorable redox and catalytic conditions for hydrogen generation. At the mineral–fluid interface, Fe(II)-rich spinel surfaces (chromite, magnetite) can catalyze hydrogen generation by transferring electrons from structural and adsorbed Fe(II) to water molecules and protons adsorbed on the spinel surface, thereby reducing water to H₂ while oxidizing Fe(II) to Fe(III) (Mayhew et al., 2013). Sustained H₂ production can occur as dissolution of neighboring Fe(II)-bearing silicates replenishes aqueous Fe²⁺ that sorbs onto spinel surfaces and is repeatedly oxidized, until secondary Fe(III)-(hydr)oxide or silica-rich coatings passivate the spinel and inhibit further electron transfer. Similar associations have been reported in other chromitite-bearing ophiolites worldwide (Figure 1). Beyond Kempirsay and Bulqizë, similar associations between chromitite bodies and hyperalkaline H₂–CH₄ fluids have been documented in at least a dozen ophiolitic massifs, spanning the Dinarides–Hellenides belt (Krivaja–Konjuh, Ozren, Borja, Othrys, Argolida), the eastern Mediterranean (Troodos, Tekirova, Kızıldağ, Tișovița), and further afield (Zambales, Cedars, Elba) (Etiope et al., 2013, 2017, 2018; D'Alessandro et al., 2018; Neal & Shand, 2002; Sciarra et al., 2022; Morrill et al., 2013; Aquino et al., 2025). Although the chromitite-H₂ system connection has not been directly documented in the Oman ophiolite, this case provides additional insights through numerous hyperalkaline springs where H₂-rich gas bubbles and podiform chromite deposits serve as indicators, with some chromitite bodies being closely associated with H₂ emission points (Neal and Stanger, 1983; Augé, 1987; Rollinson, 2005; Leong et al., 2021; Pasquet et al., 2024; Templeton et al., 2024). Occluded H₂ and CH₄ have also been measured directly within chromitite samples from Bulqizë and Othrys ophiolites (Truche et al., 2024; Etiope, 2024; Pappalardo et al., 2025). In many of these systems, CH₄ is interpreted as a secondary product of Fischer–Tropsch-type reactions fueled by H₂, further emphasizing the central role of chromitite-hosted catalytic interfaces. All these observations indicate that the association between chromitite occurrence and reduced H₂bearing fluids is not restricted to our two case studies, but appears recurrent in chromitite- Submitted Version / preprint Donzé Truche DOI: 8 bearing SSZ ophiolites. However, Kempirsay and Bulqizë remain, to our knowledge, the only systems where the spatial relationship between active H₂ seeps and mapped podiform chromitite bodies has been quantified at local (10²–10³ m) scale; systematic surveys in other ophiolites will be required to test how general this tight coupling is. Figure 1. Global distribution of Neoproterozoic and Phanerozoic ophiolite belts, modified after Vaughan & Scarrow (2003). Supra-subduction-zone ophiolites that host podiform chromitites and reduced H₂–CH₄-rich fluids discussed in this study are highlighted, including the Kempirsay massif in the Urals (Kazakhstan), the Bulqizë ophiolite (Albania) and other chromitite-bearing peridotite massifs cited in the text. Beyond this first-order picture, several features of podiform chromitites may further enhance hydrogen generation. During fluid-related alteration, chromite (FeCr₂O₄) commonly develops Fe-rich ferritchromite and Cr-rich magnetite rims, reflecting progressive Fe enrichment and oxidation of the spinel (Ahmed & Surour, 2016). More generally, in serpentinizing ultramafic rocks, oxidation of ferrous iron in Fe(II)-bearing minerals is widely recognized to generate H₂-rich fluids (e.g. Holm et al., 2015; Barbier et al., 2020; Huang et al., 2016; Preiner et al., 2018). Chromitites are also enriched in platinum-group elements by one to two orders of magnitude relative to surrounding peridotites (Melcher et al., 1997), creating metal-rich interfaces where catalytic water reduction and Fischer–Tropsch-type reactions may be promoted (Dutoit et al., 2025; Truche et al., 2025; Oze et al., 2007). Quantifying the contribution of these mineralogical and catalytic effects, however, will require dedicated experimental and field studies beyond the scope of this work. Submitted Version / preprint Donzé Truche DOI: 9 Chromitite bodies thus operate as local catalytic and hydraulic hubs. Their role can be understood within the broader architecture of supra-subduction-zone ophiolites, focusing on how their inherited architecture controls both the early localization of serpentinization and the long-term preservation of reactive mantle domains. Mineralogical and structural drivers of chromite-hydrogen coupling in SSZ ophiolites Supra-subduction-zone (SSZ) ophiolites create a distinctive mantle architecture in which podiform chromitites, strongly depleted peridotites and inherited permeability structures are closely intertwined (Pearce et al., 1984; Dilek & Furnes, 2011). High-degree melting and melt–rock interaction in fore-arc settings produce harzburgite–dunite domains that are both olivine-dominated and Fe²⁺-bearing, providing a chemically favorable substrate for H₂producing reactions once hydration occurs (Pearce et al., 1984; Dilek & Furnes, 2011). At the same time, SSZ magmatism focuses Cr-rich melts into deforming peridotite to form podiform chromitites, while faulting and dyke emplacement imprint a heterogeneous network of fractures, magmatic contacts and damage zones that control where seawater can initially penetrate the lithospheric mantle (Dilek & Furnes, 2011; Parkinson, 1998; Maulana et al., 2015). In this configuration, serpentinization is expected to be strongly localized along a subset of these inherited high-permeability structures, rather than proceeding uniformly throughout the mantle section (Mével, 2003; Rouméjon et al., 2015; Aupart et al., 2021). Early hydration along faults, shear zones, gabbro–peridotite contacts and dyke swarms, creates serpentinite corridors that become mechanically weak and highly permeable, concentrating both deformation and fluid flow during subsequent intra-oceanic subduction and obduction (Rouméjon et al., 2015; Cox et al., 2021; Tarling et al., 2019). By contrast, intervening blocks of harzburgite–dunite and gabbro that remain less fractured and only weakly hydrated behave as more competent panels and can be translated upward largely as coherent blocks, with limited additional serpentinization during exhumation (Evans et al., 2021; Tarling et al., 2019). This behavior, for example well-illustrated by field relationships in East Sulawesi, shows that the preservation of relatively “dry” ophiolitic fragments to the surface is not the absence of serpentinization, but rather the consequence of its early localization along discrete shear zones (Parkinson, 1998; Maulana et al., 2015). 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