The Abiotic Earth

From the Lesson

THE ABIOTIC EARTH — SERPENTINIZATION & THE LIVING ROCK

Beneath the thin crust we inhabit, the Earth is alive in a way most people never consider. Not merely molten — but chemically active, perpetually generating heat, gas, and complex hydrocarbons through a process called serpentinization. This lesson dismantles the assumption that petroleum is a "fossil" product and reveals the planet itself as a self-renewing chemical engine.

THE OLIVINE-TO-SERPENTINE REACTION

The story begins with olivine — a dense, green mineral of the nesosilicate group, with the general formula (Mg,Fe)₂SiO₄, that constitutes approximately 50–60% of the Earth's upper mantle by volume (Ringwood 1975). Olivine is the most abundant mineral in the Earth by mass. When olivine encounters water at moderate temperatures (200–300°C), it undergoes a dramatic and highly exothermic transformation first described systematically by Bowen and Tuttle (1949):

2Mg₂SiO₄ + 3H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂ (Forsterite) + (Water) → (Serpentine/Chrysotile) + (Brucite)

3Fe₂SiO₄ + 2H₂O → 2Fe₃O₄ + 3SiO₂ + 2H₂ (Fayalite) + (Water) → (Magnetite) + (Silica) + (Hydrogen gas)

The combined reaction for a typical mantle olivine (Fo₉₀, meaning 90% magnesium end-member) is:

(Mg₀.₉Fe₀.₁)₂SiO₄ + nH₂O → Serpentine + Brucite + Magnetite + H₂(g)

The thermodynamics are extraordinary. McCollom and Bach (2009, Geochimica et Cosmochimica Acta 73:856–875) calculated the enthalpy of the serpentinization reaction at approximately ΔH = −40 to −80 kJ/mol of olivine reacted, depending on composition and conditions. At the Atlantis Massif on the Mid-Atlantic Ridge, heat-flow measurements by Lowell and Rona (2002, Geophysical Research Letters 29:1531) detected thermal anomalies of 100–200 mW/m² attributable to serpentinization alone — not magmatic activity.

This means the mantle is not merely cooling down from some primordial formation event; it is actively generating new thermal energy through ongoing chemical reactions. The Earth heats itself. Kelley et al. (2001, Nature 412:145–149) documented that the Lost City Hydrothermal Field on the Mid-Atlantic Ridge (30°N) has been sustained for at least 120,000 years by serpentinization heat alone, with vent fluid temperatures of 40–91°C — entirely independent of any volcanic magma source.

THE HYDROGEN YIELD: QUANTITATIVE DATA

The hydrogen produced by serpentinization is not trivial. Experimental work by Marcaillou et al. (2011, Journal of Geophysical Research 116:B02203) measured H₂ concentrations of 1–16 mmol/kg in serpentinization fluids, while field measurements at Lost City by Kelley et al. (2005, Science 307:1428–1434) recorded dissolved H₂ concentrations up to 15 mmol/kg — among the highest measured in any natural aqueous system on Earth.

For comparison, the equilibrium H₂ concentration in seawater at the ocean surface is approximately 4 × 10⁻⁴ mmol/kg. Serpentinization fluids are enriched in hydrogen by a factor of 10,000–40,000 relative to surface water. This hydrogen is chemically hungry, seeking carbon atoms to bond with. And carbon is abundant in the deep Earth — dissolved in mantle fluids as CO₂ and CO, present in carbonates, carried down by subducting ocean plates. The estimated total carbon content of the mantle is 30–300 ppm by mass (Dasgupta and Hirschmann 2010, Earth and Planetary Science Letters 298:1–13), corresponding to 1.2–12 × 10²² kg of carbon — dwarfing the 3.5 × 10¹⁵ kg of carbon in the entire surface biosphere.

FISCHER-TROPSCH SYNTHESIS: THE MANTLE'S REFINERY

When hydrogen meets carbon dioxide or carbon monoxide under the high pressures and temperatures of the deep crust and upper mantle, Fischer-Tropsch type (FTT) reactions occur. These were first described industrially by Franz Fischer and Hans Tropsch (1926, Brennstoff-Chemie 7:97–104) and have since been demonstrated under geological conditions by numerous experimentalists:

CO₂ + 4H₂ → CH₄ + 2H₂O (Sabatier reaction, ΔG° = −130.8 kJ/mol) nCO + (2n+1)H₂ → CₙH₂ₙ₊₂ + nH₂O (general FTT synthesis) 2CO + 4H₂ → C₂H₆ + 2H₂O (ethane synthesis)

McCollom and Seewald (2006, Earth and Planetary Science Letters 243:74–84) demonstrated that FTT reactions proceed readily at 250°C and 325 bar in the presence of native iron or magnetite catalysts — conditions ubiquitous in serpentinizing oceanic crust. Their experiments produced methane, ethane, propane, and butane from CO₂ and H₂ in proportions matching the Anderson-Schulz-Flory (ASF) distribution: