H2-rich fluids from serpentinization: geochemical and biotic implications

Abstract

Metamorphic hydration and oxidation of ultramafic rocks produces serpentinites, composed of serpentine group minerals and varying amounts of brucite, magnetite, and/or FeNi alloys. These minerals buffer metamorphic fluids to extremely reducing conditions that are capable of producing hydrogen gas. Awaruite, FeNi3, forms early in this process when the serpentinite minerals are Fe-rich. Olivine with the current mantle Fe/Mg ratio was oxidized during serpentinization after the Moon-forming impact. This process formed some of the ferric iron in the Earth's mantle. For the rest of Earth's history, serpentinites covered only a small fraction of the Earth's surface but were an important prebiotic and biotic environment. Extant methanogens react H2 with CO2 to form methane. This is a likely habitable environment on large silicate planets. The catalytic properties of FeNi3 allow complex organic compounds to form within serpentinite and, when mixed with atmospherically produced complex organic matter and waters that circulated through basalts, constitutes an attractive prebiotic substrate. Conversely, inorganic catalysis of methane by FeNi3 competes with nascent and extant life.

Fig. 1.

Molality of H_2(aq) for SBM buffer (reaction 3) as a function of temperature at 500 bars total fluid pressure. Isopleth S_0.1BM represents high-Fe serpentinite, with X_Fe = 0.1 in crysotile and X_Fe = 0.07 in brucite, and S_0.025BM represents low-Fe serpentinite, with X_Fe = 0.025 in crysotile and 0.0175 in brucite. QFM buffer and hydrogen solubility bars are shown.

Fig. 2.

Phase diagram as in Fig. 1 showing isopleths of SBM buffer equilibria for serpentinites with X_Fe in cryostile ranging from 0.1 to 0.025. H₂ partial pressure contours are thick gray lines.

Fig. 3.

Hydrogen partial pressures and X_Fe in serpentine in equilibrium with brucite, magnetite, and awaruite as a function of X_Fe in awaruite and temperature. The two-phase region (awaruite plus Ni solid solution) is shaded.

Fig. 4.

Examples of two paths of fluid composition on phase diagram as in Fig. 2. The path A–F represents venting of fluid in a deep system on land that is initially in equilibrium with serpentinite at point A. The second path (R–S–A–B–C–R) represents circulation beneath a massive H₂ atmosphere on the early Earth.