Â鶹ÒùÔº

Abiotic organic synthesis during geological processes has long drawn scientific interest, as it is believed to have laid both the material and energetic groundwork for the emergence of early life on Earth.

Now, a research team co-led by Prof. Guo Shun and Prof. Liu Jingbo from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), in partnership with Zhejiang Ocean University, has presented direct geological proof that the infiltration of silicic fluids rich in ferrous chloride into dolomitic marble triggers decarbonation reactions—generating hydrogen gas (H₂) and magnetite, which in turn act as catalysts for abiotic organic synthesis.

The findings were recently in the Proceedings of the National Academy of Sciences.

On Earth's surface and in shallow subsurface environments, serpentinization of mafic-ultramafic rocks drives a chemical process: ferrous ions (Fe²⁺) are oxidized to ferric ions (Fe³⁺), while water (H₂O) is reduced to H₂. This produced H₂ then fuels abiotic organic synthesis when catalysts are present.

Recent theoretical and experimental studies have suggested that high-pressure and high-temperature (HP–HT) conditions—far more extreme than surface environments—may be even more conducive to this synthesis. However, natural geological observations supporting this hypothesis have remained scarce, leaving critical questions unaddressed: Under HP-HT extremes, how is H₂ generated? What mechanisms govern abiotic organic synthesis? And how are these products preserved?

To fill this knowledge gap, the research team conducted a comprehensive petrological analysis of metasomatic marbles collected from the Sulu ultrahigh-pressure metamorphic belt. These rocks form through the infiltration of silica-rich fluids or silicate melts and include three key types: dolomitic marble (the original "protolith" rock), olivine marble (weakly altered by metasomatism), and diopsidite (strongly metasomatized).

The team's analysis showed that decarbonation of these marbles occurred approximately 218 million years ago, under conditions of 670–800°C and pressures exceeding 1.0 gigapascal (GPa)—conditions consistent with the retrograde granulite-facies metamorphism of regional eclogites. Crucially, these carbon-rich rocks have undergone deep subduction into Earth's interior and subsequent exhumation, making them ideal natural samples to study abiotic organic synthesis under HP-HT metamorphic conditions.

A key breakthrough came from high-resolution Raman mapping of 5,000 zircon-hosted inclusions within the metasomatic rocks. This advanced imaging technique revealed the presence of multiple organic compounds, including whewellite (CaC₂O₄ • H₂O), disordered carbonaceous material (characterized by aromatic rings with aliphatic hydrocarbon chains), and methane (CH₄).

Significantly, these organic species were found in close association with volatile substances (H₂, carbon monoxide/CO, carbon dioxide/CO₂, and H₂O) and minerals formed during decarbonation (olivine, phlogopite, calcite, magnetite, and diopside). This association confirms that both H₂ and abiotic organic matter were produced during the metamorphic decarbonation process.

Further detailed petrographic analyses and mass-balance calculations provided additional critical data. The team found that progressive metasomatism of dolomitic marble led to a substantial increase in the bulk-rock iron (Fe) content—evidence that significant external Fe was introduced via fluids rich in ferrous chloride.

Modeling based on these data constrained a minimum water-to-rock ratio of 2–9 for the process, and estimated that metasomatism produced approximately 72–142 millimoles of H₂ per kilogram of metasomatic rock. This H₂ yield is comparable to that generated during the serpentinization of ultramafic rocks, which typically produces 13–315 millimoles of H₂ per kilogram.

The researchers also identified the mechanism driving organic synthesis: Magnetite formed during decarbonation acted as a catalyst for Fischer–Tropsch-type reactions, where H₂ reacts with CO₂ (a byproduct of decarbonation) to produce CH₄. The presence of whewellite further indicated that more complex organic structures—specifically carboxylic groups—were also formed during this process.

This study demonstrates that the infiltration of ferrous chloride-rich silicic fluids into dolomitic marble induces decarbonation reactions that generate H₂ and magnetite, ultimately facilitating abiotic organic synthesis under high-grade metamorphic conditions.

These results underscore the critical role of aqueous Fe in producing H₂ and magnetite, and offer new insights into potential geological hydrogen resources and the geochemical pathways relevant to the origin of early life on Earth.