Abstract
Serpentinization produces molecular hydrogen (H2) that is capable of supporting communities of microorganisms in hydrothermal fields, which suggests that serpentinization may be closely related to the origin of life at the early history of the Earth and possibly other planets. In this study, serpentinization experiments were performed at 300 °C and 3.0 kbar with natural olivine and peridotite as starting reactants to quantify the influence of acidic and alkaline solutions on the processes of serpentinization. The results reveal that acidic and alkaline solutions greatly influence molecular hydrogen (H2) generation and the rates of serpentinization. Acidic (pH = 2.50) and alkaline solutions (pH = 13.5) increased H2 production and the rates of peridotite serpentinization. Highly acidic solutions (2 M HCl), however, decreased the production of H2 after peridotite serpentinization by around two orders of magnitude. The decrease in H2 production was associated with a sharp decline in the rates of reaction; e.g., when peridotite was reacted with neutral solutions (0.5 M NaCl), 88% of reaction progress was achieved after an experimental duration of 27 days, and the reaction extent decreased by ~50% for experiments with highly acidic solutions (2 M HCl) over the same period. In contrast, for experiments with solely olivine, highly acidic solutions (2 M HCl) promoted the rates of olivine serpentinization and H2 production. The contrasting effect of highly acidic solutions (2 M HCl) on the processes of olivine and peridotite serpentinization may reflect the influence of pyroxene minerals, which could release SiO2 during peridotite serpentinization and, consequently, hydrogen generation and reaction rates may decrease. The experimental results of this study suggest that H2 production and the rates of serpentinization can be greatly influenced by acidic and alkaline solutions and co-existing minerals (e.g., pyroxene).
Highlights
Fluids issuing from serpentinizing ultramafic rocks are typically enriched in volatiles, especially molecular hydrogen (H2 ) and methane (CH4 ) [1,2,3]
Secondary minerals were identified by X-ray diffraction (XRD), SEM, and Fourier transform infrared spectroscopyOlivine (FTIR) (Figures 1 and 2), and they were quantified by FTIR (Figure S1)
The present study has revealed that hydroxyl alkalinity increases H2 production after peridotite serpentinization, and highly acidic solutions (2 M HCl) significantly decrease H2 production by around 1–2 orders of magnitude
Summary
Fluids issuing from serpentinizing ultramafic rocks are typically enriched in volatiles, especially molecular hydrogen (H2 ) and methane (CH4 ) [1,2,3]. These volatiles are capable of supporting communities of microorganisms in serpentinite-hosted hydrothermal vent fields and the deep subsurface [1,2,3]. Minerals 2019, 9, 661 solutions with pH ranging from
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