Yuhang Lu1,2, Renbiao Tao2, Qinqiang Meng3
(¹School of Earth and Space Sciences, Peking University, Beijing 100871, China
2Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100193,China
3Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China)
Abstract: Natural hydrogen (H2) is increasingly recognized as a potentially important low-carbon energy resource on Earth. Abiotic H2 is one of the most important contributors of natural H2 and for decades has been primarily attributed to water-rock reactions in ultramafic systems, especially serpentinization of peridotite, which is widespread at the Earth’s surface and shallow lithosphere, including the ocean floor and subduction-related settings. However, mafic to intermediate-felsic intrusive rocks emplaced in a variety of tectonic settings such as continental rift basins, magmatic arcs, and orogenic belts may also represent significant, yet comparatively underexplored ability of H2 production. During alteration, the Fe(II)-bearing silicates (e.g., pyroxene, biotite, amphibole) in those rocks can be transformed into secondary Fe-rich hydrous silicates with elevated Fe3+/ΣFe, potentially coupling H2O reduction to H2 generation.
Here we investigate chloritization of Fe-rich diabase from drill cores in the Chuxiong Basin, SW China, as a non-serpentinization pathway for abiotic H2 generation. Our study combines field relationships, petrography, mineral chemistry, Fe redox constraints, and gas geochemical observations to test whether alteration of primary Fe(II)-bearing phases, including clinopyroxene, biotite, Fe-Ti oxides, into chlorite-rich assemblages can generate or preserve molecular hydrogen. Particular attention is paid to the association of chlorite with magnetite, hematite, and sulfide phases,because these minerals recordcompetingfates ofiron: sequestrationas Fe(II)-rich silicates,oxidation to Fe(III)-bearing phases, or participation in water reduction.
The results suggest that chloritization in the Chuxiong diabase is spatially organized along fractures and reaction fronts, rather than representing uniform low-grade alteration. Such textures imply focused fluid–rock interaction and transient redox gradients, conditions favorable for localized H2 production and subsequent migration. Quantitative EPMA flank-method and FIB STEM-EELS analyses show that the newly formed chlorite is strongly Fe-rich, with FeOᵀ contents of 29–39 wt% and Fe3+/Feᵀ ratios of 0.36–0.83. These values indicate substantial oxidation of pyroxene-hosted Fe(II) during chlorite formation. Preliminary mass-balance evaluation suggests that even a low egree of chloritization could generate H2 amounts comparable to, or higher than, those expected from equivalent degrees of serpentinization. This high apparent efficiency likely reflects the unusually Fe-rich and ferric nature of the chlorite, together with fracture-controlled fluid access that localizes reaction fronts and may sustain long-term H2 production.
We propose that chloritization should be considered as part of a broader “Fe-silicate alteration–H2 system,” rather than as a subordinate alteration halo around serpentinization. This perspective expands the range of potential H₂-generating lithologies from ultramafic rocks to Fe-rich mafic and intermediate crustal reservoirs, including rift basalts, hydrothermal basement, and altered intrusive complexes.
Keywords: Natural hydrogen; Chuxiong basin; Water-rock interaction; Chlorization; Serpentinization
Author Profile (first or corresponding author):
Lu YH, Male, Student, PhD, mainly engaged in High-temperature and high-pressure experimental petrology. E-mail: yuhang.lu@hpstar.ac.cn
