ABSTRACT

Hydrous alteration of ultramafic rocks produces unique reducing environments accompanied by hydrogen (H₂) generation. To understand the early stages of the reaction, batch experiments were conducted at 90°C for 2 weeks using a NaNO₃ solution, natural dunite and harzburgite samples with variable degrees of serpentinization. Our results indicate that the fresh ultramafic rocks generate more H₂ than serpentinized rocks, showing that the dissolution of major minerals in fresh ultramafic rocks (i.e., pyroxene and olivine) is the dominant factor in H₂ generation. Fresh harzburgite yielded higher amounts of H₂ of up to 322.1 μmol/kg than the other ultramafic rocks. Fresher samples had higher dissolved Si and Ca concentrations in the solutions with higher H₂ generation than the serpentinized samples, which can be explained by the dissolution of pyroxene, because the main host mineral for Ca is clinopyroxene. Magnesium-bearing silicates were observed in the experiments using a fresh harzburgite, probably due to the more effective Si supply from pyroxene dissolution than olivine. Another series of experiments using a fresh harzburgite with different chemical reagents showed that the addition of Si enhanced H₂ generation, suggesting that H₂ generation was regulated by the precipitation of magnesium silicate hydrate. Thermodynamic calculations indicated that the solution chemistry during the hydration of fresh ultramafic rocks was regulated by magnesium silicate hydrate, whereas the solution chemistry during the hydration of serpentinized rocks was buffered by brucite. Our study suggests that fresh harzburgite is more favorable for H₂ generation than dunite at 90°C because of its higher pyroxene content.