Abiotic synthesis during the interaction of ferrous chloride–rich silicic fluids with marble under high-grade metamorphic conditions

Ferrous chloride-rich silicic fluid and melt infiltration led to the decarbonation of dolomitic marble in the Chinese Sulu ultrahigh-pressure metamorphic terrain under temperatures ranging from 670 to 800 °C, pressures from the aragonite + albite to calcite stability fields and the oxygen fugacities between the hematite-magnetite and pyrite-pyrrhotite-magnetite buffers, resulting in the formation of olivine marble and diopsidite. The inclusions in zircons trapped during the decarbonation process suggest that the H₂-producing reaction 3FeCl₂ (aq) + 3CaCO₃ + H₂O = Fe₃O₄ (magnetite) + 3CaCl₂ (aq) + 3CO₂ + H₂ occurred and that it induced magnetite-catalyzed Fischer-Tropsch-type synthesis, as indicated by the presence of whewellite, disordered carbonaceous material, CH₄, and CO in the inclusions. The results of this study highlight the role of aqueous Fe in generating H₂ and magnetite and have far-reaching implications for carbon speciation and solubility in deep fluids and for endogenic abiotic synthesis, which may be pivotal in the prebiotic occurrence of organic compounds on Earth.