The rocky road to organics needs drying

The study addresses how simple abiotic organics in hydrothermal settings could evolve into more complex, potentially prebiotic molecules. Historically, abiotic organics detected in H2-rich hydrothermal systems—chiefly methane with minor short-chain hydrocarbons and organic acids—have been limited in diversity and strongly diluted in fluids, challenging a unifying hydrothermal origin-of-life scenario. Recent findings of low-temperature abiotic aromatic amino acids via clay-catalyzed reactions and of carbonaceous matter in ancient oceanic lithosphere suggest that more diverse abiotic organics may form within subseafloor rocks and not solely in fluids via Fischer-Tropsch-type (FTT) processes. The authors hypothesize that mineral microcavities in olivine-rich rocks, influenced by magmatic degassing and subsequent water-consuming serpentinization reactions, foster a drying environment that drives the formation and diversification of condensed organic phases, including polyaromatic carbonaceous materials (PACMs) and nanodiamonds, alongside reduced gases (H2, CH4, N2, CH3SH). The work targets deep mid-ocean ridge rocks (Atlantis Massif, MAR) to test this mechanism and evaluate its implications for carbon cycling and prebiotic chemistry.

The paper situates its contribution within research showing: (1) abiotic methane and limited short-chain organics in serpentinization-associated hydrothermal systems; (2) proposed FTT reactions in fluids to explain short-chain hydrocarbons; (3) evidence for low-temperature abiotic synthesis of aromatic amino acids via Friedel-Crafts reactions catalyzed by iron-rich saponite clays; and (4) discovery of abiotic carbonaceous matter in ancient oceanic lithosphere. These lines of evidence imply the presence of aromatic precursors and diverse abiotic organics that prior fluid-based models did not fully account for. The authors argue that processes inside rocks, mediated by mineral surfaces, microcavities, and dehydration during serpentinization, provide alternative and complementary pathways to FTT, better explaining complex, condensed organic phases observed in oceanic lithosphere and potentially aligning with extraterrestrial observations of diverse organic matter in meteorites and comets.

• Study site and samples: Olivine-hosted secondary fluid inclusions (FIs) in fresh troctolites from IODP Hole U1309D, Atlantis Massif (30°N Mid-Atlantic Ridge), at depths of ~1100–1200 m.b.s.f. within a deep igneous section extending to ~1400 m.b.s.f. The rocks experienced early fluid trapping during cooling and crack-healing of olivine (~600–800 °C; P ~2 kbar), later exhumed to P < 0.3 kbar and T ~100 °C.
• Raman spectroscopy: Punctual Raman analyses on 36 closed FIs from three intervals (228R2, 235R1, 247R3). Identification of gaseous species (H2, CH4, N2, CH3SH) and secondary minerals (serpentine polymorphs, brucite, magnetite, calcite/magnesite). First-order Raman signatures of PACMs characterized by D and G bands; nanodiamonds (nD) inferred from a downshifted, broadened D band (~1313–1332 cm−1; FWHM-D 54–70 cm−1) and associated G band near 1550 cm−1.
• 3D hyperspectral Raman mapping: Conducted on two FIs (FI3, FI5) with confocal acquisition using 250 nm steps in X-Y and Z to reconstruct 3D volumes. A custom Matlab pipeline extracted D and G band parameters using Lorentzian–Breit–Wigner–Fano fitting, yielding FWHM-D, FWHM-G, peak positions (wD, wG), and intensity ratio R1 = ID/IG. Data quality was filtered by GOF metrics; thousands of spectra per inclusion were analyzed to map PACM heterogeneity.
• Focused Ion Beam–Scanning Electron Microscopy (FIB-SEM) and EDS: FI5 was opened by FIB (30 kV, 10 nA), protected by Pt deposition; cross-sections observed in high vacuum at 15 kV using EsB and SESI detectors. Elemental distributions acquired with EDS (Oxford Aztec). Thin foils (~100 nm) were lifted out for TEM.
• Transmission Electron Microscopy (TEM/HR-TEM, STEM-EDS): A JEOL 2100 (200 kV) characterized PACM textures and nanostructures. FFT analysis provided lattice spacings of nanophases; ~5 nm particles with d ≈ 0.20 nm consistent with nD (d111). STEM-EDS determined qualitative compositions, including heteroatom content (O, S, metals) and C/O ratios.
• X-ray Photoelectron Spectroscopy (XPS): PHI 5000 Versaprobe II, Al Kα (1486.6 eV), spot ~10 µm, charge neutralized; C 1s peak calibrated at 284.8 eV; light Ar etch (250 V, 1 min). Peak fitting (Multipak) yielded proportions of C–C/C=C/C–H (~80%), C–O/C–O–C (~12%), C=O/O–C=O (~5%), with minor carbonate (CaCO3) and carbide contributions. Survey spectra showed minor Si and Ti, consistent with possible carbides.
• Methane abundance and isotopes: Rock chips (1–2 mm) were vacuum-dried (60 °C), crushed under He in a hydraulic crusher; released gases trapped on Porapak Q at liquid N2 temperature, then desorbed at 150 °C. Separation on GC (Poraplot Q, −30 to 80 °C), oxidation to CO2 via NiO/CuO/Pt oven, and isotopic measurement by Thermo Delta V IRMS. CH4 concentration minimum: 143 µmol/kg rock; δ13CCH4 = −8.9 ± 0.1‰; δDCH4 = −161.4 ± 1‰.
• Thermodynamic/speciation modeling and scenario development: Considered magmatic CO2–H2O–SO2–N2 vapor exsolved at MOR conditions evolving upon cooling to ~400 °C (2 kbar), with redox paths crossing pyrene–CO2 equilibria (early aromatic formation). For T < 400 °C, modeled serpentinization-driven increases in fH2 and pH, carbonate precipitation, and lowered water activity leading to condensation and PACM formation. Kinetic inhibition of CH4 at 300–400 °C considered; metastable organics favored. Modeling tools and thermodynamic data referenced (e.g., SUPCRT92/EQ3/6 and literature databases).

• Discovery of diverse abiotic compounds within olivine-hosted microcavities: All 36 analyzed fluid inclusions contain H2 and/or CH4 and secondary serpentinization minerals (serpentine, brucite, magnetite, and carbonates). Newly documented species include N2(g), methanethiol (CH3SH), and polyaromatic carbonaceous materials (PACMs), with coexisting nanodiamonds (nD).
• Three PACM end-members identified by Raman/SEM/TEM: • PACM1: Amorphous, gel-like material wetting serpentine and brucite fibers; highly functionalized with heteroatoms (C/O ~1; includes O, S, metals), showing bands for C=O (~1735 cm−1) and C–O/C–O–C (~1100 cm−1) and a shoulder near 1200 cm−1. High structural disorder and macromolecular characteristics. • PACM2: More aromatic, mesoporous spongy texture of ~20 nm nanofilaments on olivine walls; carbon-rich (C/O ~9); co-localizes with dense, spotted nanoparticles (5–50 nm) attributed to nD and possible carbides. • PACM3: Nanodiamond-like material with D band near ~1325 cm−1 (FWHM-D 54–70 cm−1) and G near ~1550 cm−1; locally present within PACM1 and co-localized with PACM2.
• 3D Raman maps reveal micron-scale heterogeneity and a continuum from functionalized to more aromatic/ordered PACMs, overlapping trends observed in meteoritic and kerogen carbonization continua.
• XPS confirms PACMs are dominated by C–C/C=C/C–H bonds (~80%) with significant oxygenated functionalities (~17% combined C–O and C=O groups), consistent with macromolecular organic matter plus minor carbonate and carbide.
• CH4 abundance and isotopic composition: ≥143 µmol CH4 per kg rock; δ13CCH4 = −8.9 ± 0.1‰ and δDCH4 = −161.4 ± 1‰, within the abiotic range and similar to CH4 venting at nearby Lost City, supporting an abiotic origin and genetic link.
• Two-stage formation scenario: Stage 1 (≥400–450 °C, ~2 kbar): Magmatic CO2–H2O–SO2–N2-dominated fluids cool; redox evolution crosses pyrene–CO2 equilibrium enabling early deposition of aromatic carbon (analog to PACM2) and partial conversion of CO2 to CH3SH/CH4 and N2 to NH3 under reduced conditions. Carbonaceous films can condense on freshly cracked olivine during rapid cooling. Stage 2 (≤400 °C): Onset of serpentinization produces serpentine, brucite, magnetite, and H2; increases pH; induces carbonate precipitation; and crucially dries the microenvironment (lowers water activity), driving condensation of macromolecular PACM1. nD (PACM3) can form metastably from PACM1/2. Over time, evolving PACMs and redox conditions facilitate CH4 formation previously limited by kinetics.
• Implication: Drying within mineral microcavities during serpentinization is a key driver for abiotic organic synthesis and diversification in the oceanic lithosphere, broadening the catalog of abiotic carbon forms available to prebiotic chemistry.

The findings demonstrate that deep-seated olivine microcavities at mid-ocean ridges can trap magmatic-derived, C–O–S–H–N fluids whose evolution during cooling and serpentinization yields a rich suite of abiotic organics. The coexistence of reduced gases (H2, CH4, N2, CH3SH) with condensed PACMs and nD indicates that rock-hosted, mineral-mediated processes provide an alternative to fluid-only FTT mechanisms. The proposed two-stage pathway reconciles the presence of both aromatic and functionalized macromolecular carbons: early aromatic deposition on olivine surfaces during high-temperature cooling, followed by low-temperature serpentinization that consumes water, raises pH and fH2, precipitates carbonates, and drives condensation of heterogeneous PACMs. This drying is crucial, as reduced water activity favors condensation/polymerization and metastable organic formation despite slow CH4 kinetics at 300–400 °C. The isotopic signature and abundances of CH4 connect the microcavity processes to observed abiotic methane in nearby hydrothermal systems (Lost City). Collectively, the results enlarge the recognized diversity of abiotic organic carbon in hydrothermal settings, suggest microreactor-like roles for fluid inclusions and mineral surfaces in prebiotic chemistry, and highlight mechanisms likely operative on other planetary bodies hosting cooled and hydrated olivine-rich magmas.

This work identifies drying during serpentinization within olivine-hosted microcavities as a pivotal process for abiotic organic synthesis and diversification in the oceanic lithosphere. The authors document, for the first time in present-day oceanic crust, the co-occurrence of N2, CH3SH, diverse PACMs, and nanodiamonds with serpentinization minerals and reduced gases, and propose a two-stage magmatic-to-hydrothermal pathway that explains their formation. These insights expand the inventory of abiotic carbon forms available to hydrothermal ecosystems and prebiotic chemistry and suggest that similar processes should occur on other worlds with olivine-rich magmatic systems that cool and hydrate. Future studies could quantify reaction kinetics and water activity effects, track the time-evolution of PACMs toward methane and other organics, and assess the mobility and bioavailability of these materials during later fluid-flow episodes at shallower crustal levels.

• Sample and context limitations: The study examines 36 fluid inclusions from three troctolites at a single site (Atlantis Massif), which may not capture full spatial variability.
• Analytical constraints: XPS lacks spatial resolution to separate PACM1 vs PACM2 contributions within individual inclusions; Raman-based identification of nanodiamonds relies on size-dependent spectral features whose interpretation can be complex. FIB-thinned foils of PACM2 were thicker, limiting HR-TEM structural characterization.
• Modeling assumptions: The speciation path and two-stage scenario use equilibrium considerations and simplified systems (e.g., suppressing CH4 to explore metastable organics), while actual kinetics and fluid residence times in inclusions are uncertain.
• Kinetic factors: Methane formation is kinetically inhibited at ≤400 °C; inferred later CH4 production from PACMs is not directly observed but is mechanistically reasoned.
• Generalizability: While the mechanism is likely in olivine-rich systems, its prevalence across different ridge settings or subduction-related environments requires broader sampling.