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.