Fig. 5: Proposed scenario to account for the chemical and structural diversity of the different types of carbonaceous material in microreactor-like fluid inclusions hosted in serpentinizing olivine from the deep oceanic lithosphere.

a During Stage 1, the trapped fluid cools down to 400 °C at 2 kbar and its speciation evolves as depicted by the gray area in diagram b, for a plausible range of initial redox conditions (Supplementary Fig. 3). The path followed by the most reduced fluids (Log fH₂ ≥ FMQ) cross-cut the pyrene-CO₂ curve between 400 °C and 450 °C, allowing the early formation of pyrene, analogous to the most aromatic PACM observed on olivine walls (PACM2). In these fluids, CO₂ can also partially convert to CH₃SH and CH₄, and N₂ to NH₃, before reaching 400 °C; i.e., before serpentinization initiates. The vertical orange area depicts the main serpentinization field (stage 2). c During stage 2, for T < 400 °C (2 kbar), water becomes liquid and olivine highly reactive with an expected major stage of serpentinization at T between 300 and 400 °C that produces serpentine, brucite, magnetite, and H₂. Serpentinization advancement rapidly shifts fH₂ and pH  of the solution toward the field of organic acids as schematically drawn by the orange arrow in the speciation diagram, d, (350 °C-2kbar, methane and methanol suppressed, See Methods and Supplementary Fig. 4), with concomitant carbonate precipitation (calcite or magnesite). This hydration reaction dries out the system leading to condensation of the fluid and formation of PACM1 that wets product minerals and displays varied functional groups bearing O, H, ± S heteroatoms, in agreement with Raman, TEM, and XPS data. nD (PACM3) can metastably form from the amorphous PACM1 and PACM2 during this serpentinization stage¹¹⁴. Then, the chemical and structural characteristics of PACMs are expected to evolve with time and during cooling, and contribute to the formation of CH₄ that was kinetically limited so far. PACM polyaromatic carbonaceous material, aH₂O water activity, set to 1 or 0.1.