Thermodynamic modeling of the serpentinization of ultramafic rocks from the Korean Peninsula: insights into natural hydrogen (H2) generation

초록

This study investigates the serpentinization of ultramafic rocks in the Korean Peninsula and its implications for natural hydrogen (H-2) generation through thermodynamic modeling. The ultramafic rocks underwent metamorphism in an active continental tectonic setting during the Late Paleozoic to Early Mesozoic, followed by serpentinization during uplift/exhumation. To constrain H-2 generation mechanisms during serpentinization, P-T-X pseudosections were applied. Modeling results indicate that H-2 generation is primarily influenced by (1) the modal proportions of ferromagnesian silicates, and (2) the bulk-rock Fe# [Fe/(Fe + Mg)] and XFe3+ [Fe2O3/(Fe2O3 + FeO)] ratio. During retrograde metamorphism under a geothermal gradient of 20-40 degrees C/km, interaction between ultramafic rocks and pure H2O fluid at 200 degrees C and 2 kbar yields H-2 concentrations of similar to 370 ppm in GM and 560-810 ppm in GB ultramafic rocks. At near-surface conditions (<3 km depth), H-2 production rates vary depending on water content, XFe3+, and CO2 activity. Peak H-2 concentrations occur when XFe3+ < 0.1 and water contents of 30-35 mole %, whereas increasing XFe3+ (>0.4) reduces H-2 yield. Furthermore, the presence of CO2 can lead to simultaneous H-2 and CH4 generation, reducing overall H-2 production. These findings provide new insights into the thermodynamic constraints on natural H-2 formation during various degree of serpentinization.

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