Dehydration of Serpentinized Ultramafic Rocks as a Candidate Deep Crustal Source of Natural Hydrogen and Hydrocarbons: Experimental Evidence from the Lesser Caucasus

Natural hydrogen generated by serpentinization — the hydration of ultramafic rocks, in which ferrous iron is oxidised and molecular hydrogen is liberated from water — has recently been shown to proceed at rates of only 0.1 to 0.5 tonnes H₂ yr⁻¹ km⁻³ of reactive rock, implying that economically significant accumulations would require 10³ to 10⁴ years to develop [1]. A complementary process capable of releasing fluid abruptly, rather than gradually, would relax this constraint. This paper reports experimental evidence for such a process: the dehydration (deserpentinization) of already serpentinized ultramafic rocks carried to greater crustal depths. Serpentinites and serpentinized ultramafic rocks of the ophiolite belts of the Lesser Caucasus were studied at pressures to 20 kbar, and at 5–6 × 10⁸ Pa with heating to 900 °C, in solid-media apparatus with continuous monitoring of compressional-wave travel time [2, 3]. Dehydration proceeds over the interval 500–700 °C, the interval differing between samples (550–700 °C and 600–750 °C in the two apoperidotite serpentinites) according to serpentine variety, serpentine content and rock fabric, and is accompanied by step-like changes in wave velocity and by volume changes reaching 30%. Petrographic and X-ray diffraction analysis records the breakdown of chrysotile, lizardite, antigorite and brucite to forsterite and enstatite, with redistribution and recrystallization of magnetite and the formation of iron hydroxides. Iron and chromium oxides constitute 5–8% of serpentinized ultramafic rocks throughout the belts. Gas chromatographic analysis of the released fluid identified alkanes in both fractions: the first fluid sample contained low-molecular-weight alkanes C₁₆–C₂₁ with a maximum at C₂₀, while the second was dominated by high-molecular-weight alkanes C₂₃–C₃₅ with trace C₄₅ [4]. Molecular hydrogen was not measured. The experimental programme was conducted before natural hydrogen became a subject of interest, and no gas analysis for H₂ was performed. Hydrogen is identified here as a predicted product of the reaction on mineralogical and thermodynamic grounds, and its measurement is the principal outstanding requirement. Serpentinized ultramafic rocks exhibit densities of 2.53–2.77 g/cm³ and compressional velocities of 5.05–5.77 km/s, against 2.98–3.19 g/cm³ and 6.45–7.43 km/s for unaltered pyroxenites, permitting buried serpentinized bodies to be identified as low-velocity, low-density layers. Such layers were predicted at 35–50 km beneath Armenia from laboratory data in 1975 [5] and have since been repeatedly detected on deep seismic sounding profiles [6, 7]. Implications for hydrogen exploration in orogenic belts containing relics of palaeo-oceanic lithosphere are discussed