Huge Variation in H₂ Generation During Seawater Alteration of Ultramafic Rocks

Abstract

Serpentinizing systems, established when aqueous planetary fluids react with ultramafic (UM) rocks, produce significant quantities of dihydrogen (H 2 ) across a large range of geochemical conditions. This H 2 actively supports chemosynthetic communities on Earth (

Variable Salinity and Hydrogen Production in Europa's Ocean

“…Ultramafic rock, which produces H 2 through reaction with seawater, can be composed of a mixture of olivine, orthopyroxene, and clinopyroxene. In addition to the fluid composition, the representative mix of these three minerals, the temperature, and the rock‐water ratio will dictate the rate of reaction (Allen & Seyfried, 2003; Ely et al., 2023; Klein et al., 2013; Leong & Shock, 2020; Palandri & Kharaka, 2004).…”

“…Utilizing a rock composition of 70% olivine within our model places the hydration reaction above the “H 2 cliff” described by Ely et al. (2023) that can occur in low water‐to‐rock ratio ultramafic systems. Regarding temperature, the production of hydrogen due to hydration of olivine is dramatically inhibited at temperatures above ∼320°C, with reductions in the forsterite abundance resulting in lower drop‐off temperatures (Klein et al., 2013).…”

“…This “H 2 cliff” is primarily seen at temperatures below 260°C. However, if temperatures are above 260°C, hydrogen production remains relatively unaffected by the composition of the ultramafic rock (Ely et al., 2023) within both high and low water‐rock ratio systems. In high water‐rock ratio systems, serpentinization of olivine and production of H 2 reduce as temperature decreases (Ely et al., 2023; McCollom et al., 2016).…”

Low-temperature hydrogen production and consumption in partially-hydrated peridotites in Oman: implications for stimulated geological hydrogen production

“…Equilibrium thermodynamic models can be used to predict potential hydrogen yields. The best targets for stimulated hydrogen production are rocks such as peridotites, which can produce 2-4 kg hydrogen/m 3 of rock, up to 4orders of magnitude more hydrogen than mafic rocks such as basalts (Bach, 2016;Leong et al, 2021a;Osselin et al, 2022;Ely et al, 2023). For serpentinization of ultramafic rocks, the integrated set of reactions that couple the oxidation of mineral-derived Fe(II) to the reduction of water to produce hydrogen gas are predicted to occur at maximum extents at temperatures between 200 and 300 °C (McCollom and Bach, 2009;Klein et al, 2013).…”

“…Another critical factor that controls H 2 generation is the waterrock ratio, which is the mass of water a given mass of rock had reacted with (Klein et al, , 2013McCollom and Bach, 2009;Ely et al, 2023). Brucite [(Mg,Fe)(OH) 2 ] and serpentine with higher Fe(II)-greenalite component are favored to form at low water-rock ratios, which buffers the fluids to lower silica and higher calcium activities, respectively McCollom and Bach, 2009;Frost et al, 2013;Leong and Shock, 2020;Tutolo et al, 2020;Ely et al, 2023). The overall Fe(III)/Fe Total of secondary phases precipitated at these conditions is low, and thus less H 2 is generated per mass of rock reacted relative to mineral assemblages formed at higher water-rock ratios (Figure 1B).…”