Kinetics of low-temperature H2 production in ultramafic rocks by ferroan brucite oxidation
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Abstract
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Figure 1 Back-scattered images of (a) the starting ferroan brucite and (b–f) its reaction products. (b) Caps#9; 378 K, 2.5 days. (c, d) SP#3; 473 K, 8 days. Euhedral micrometre-sized magnetite is visible. (e) Caps#15; 573 K, 2 days. Note that ferroan brucite platelets have recrystallised. (f) SP#6; 378 K, 69 days. ∼100 nm wide euhedral ferroan brucite platelets are observed. f-brc, ferroan brucite; mag, magnetite. | Figure 2 Kinetics of ferroan brucite alteration. (a) Number of moles of produced H2 as a function of time at 378 K (Run SP#6; black diamonds). (b) Number of moles of produced H2 as a function of time at 423 K (Runs caps#t5 to caps#t12; black circles). The dashed lines in (a) and (b) correspond to a fit of the data with Equation 3. (c) ln(r0) vs. 1/T. The linear regression of r0 is displayed with a black dashed line (R2 = 0.87) and the grey dashed lines are linear regressions for r0 with λ values of 1.01 and 1.20. | Figure 3 Comparison of experimentally determined rate of H2 production for ferroan brucite (r0, this study) and olivine serpentinisation (Neubeck et al., 2014; McCollom and Donaldson, 2016; McCollom et al., 2016). (a) Reaction rate per mass of starting material (r0). (b) Reaction rate per reactive surface area (r0/A) with A the specific surface area either measured with the BET method or calculated with the relationship provided in Brantley and Mellott (2000). | Figure 4 Numerical modelling of the contribution of ferroan brucite alteration and olivine serpentinisation to H2 production as a function of time in a partly altered peridotite (see Supplementary Information for details). (a) General view and (b) incipient stage simulation in a closed system (no H2 leak associated with diffusion or fluid advection). (c) Simulation by considering advection of a fluid at a rate of 1.4 × 10−4 kg water/day/kg rock. Dashed line: H2 production associated with ferroan brucite alteration. Solid line: H2 production associated with olivine serpentinisation. |
Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
Hydrothermal circulation across upper mantle rocks at mid-ocean ridges promotes serpentinisation reactions. In the course of these reactions, olivine reacts with water to form serpentine, magnetite and ferroan brucite, (Mg,Fe)(OH)2, along with abiotic hydrogen (Moody, 1976
Moody, J.B. (1976) Serpentinization: a review. Lithos 9, 125–138. https://doi.org/10.1016/0024-4937(76)90030-X
). As observed in ophiolites (Neal and Stanger, 1983Neal, C., Stanger, G. (1983) Hydrogen generation from mantle source rocks in Oman. Earth and Planetary Science Letters 66, 315–320. https://doi.org/10.1016/0012-821X(83)90144-9
; Abrajano et al., 1990Abrajano, T.A., Sturchio, N.C., Kennedy, B.M., Lyon, G.L., Muehlenbachs, K., Bohlke, J.K. (1990) Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines. Applied Geochemistry 5, 625–630. https://doi.org/10.1016/0883-2927(90)90060-I
; Leong et al., 2023Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H2 and CH4 outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008
), ultramafic rocks can still produce H2 at low temperature (i.e. at T < 423 K), even if they are extensively serpentinised. The extrapolation of experimental kinetic data collected in the 473–623 K range (e.g., McCollom et al., 2016McCollom, T.M., Klein, F., Robbins, M., Moskowitz, B., Berquó, T.S., Jöns, N., Bach, W., Templeton, A. (2016) Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine. Geochimica et Cosmochimica Acta 181, 175–200. https://doi.org/10.1016/j.gca.2016.03.002
) indicates that serpentinisation of olivine with a grain size of 500 μm should reach a reaction progress above 90 % in at least 10,000 yr at temperatures below 423 K.The serpentinisation of olivine produces secondary minerals, including ferroan brucite, which are Fe2+-rich and which can further react to produce H2 + magnetite at low temperature:
where [Fe(OH)2]brucite represents the Fe component of ferroan brucite.
Petrographic data on ophiolite and dredge seafloor samples (Jöns et al., 2017
Jöns, N., Kahl, W.-A., Bach, W. (2017) Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography. Lithos 268–271, 274–284. https://doi.org/10.1016/j.lithos.2016.11.014
; Klein et al., 2020Klein, F., Humphris, S.E., Bach, W. (2020) Brucite formation and dissolution in oceanic serpentinite. Geochemical Perspectives Letters 16, 1–5. https://doi.org/10.7185/geochemlet.2035
; Ellison et al., 2021Ellison, E.T., Templeton, A.S., Zeigler, S.D., Mayhew, L.E., Kelemen, P.B., Matter, J.M., The Oman Drilling Project Science Party (2021) Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite. Journal of Geophysical Research: Solid Earth 126, e2021JB021981. https://doi.org/10.1029/2021JB021981
) seem to indicate that Reaction 1 could proceed at sub-surface conditions in partly serpentinised ultramafic rocks. H2 production was achieved in hydrothermal experiments carried out on serpentinised peridotite at 373 K and was attributed to magnetite formation at the expense of ferroan brucite (Miller et al., 2017Miller, H.M., Mayhew, L.E., Ellison, E.T., Kelemen, P., Kubo, M., Templeton, A.S. (2017) Low temperature hydrogen production during experimental hydration of partially-serpentinized dunite. Geochimica et Cosmochimica Acta 209, 161–183. https://doi.org/10.1016/j.gca.2017.04.022
).In order to test the potential of Reaction 1 to produce H2 at temperatures below 423 K in ultramafic rocks, the kinetics and thermodynamics of Reaction 1 were investigated experimentally here using synthetic (Mg1−x,Fex)(OH)2 of grain size (40–100 nm) and composition (x from 0.156 to 0.205) relevant to natural ferroan brucite (Malvoisin et al., 2020
Malvoisin, B., Zhang, C., Müntener, O., Baumgartner, L.P., Kelemen, P.B., Oman Drilling Project Science Party (2020) Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project. Journal of Geophysical Research: Solid Earth 125, e2019JB018877. https://doi.org/10.1029/2019JB018877
).top
Materials and Methods
Ferroan brucite (Mg1−x,Fex)(OH)2, with x ranging from 0.156 to 0.205, was synthesised under ambient conditions from a stoichiometric solution of dissolved Fe(II) and Mg chlorides, as described in Carlin et al. (2023)
Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) FeIII-substituted brucite: Hydrothermal synthesis from (Mg0.8FeII0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845
. The ferroan brucite obtained by this method formed platelets 40 to 100 nm across (Fig. 1a). It was loaded under an Ar atmosphere with degassed ultrapure water either in welded shut gold capsules (‘caps’ experiments) or in 50 mL Parr 5500 series titanium reactors (‘SP’ experiments; see details in Tables S-2 and S-3). The capsules were run either in horizontal cold seal pressure vessels at temperatures from 348 to 573 K at 20 MPa (caps#1 to #15) or in an oven at temperatures of 378 and 423 K at the liquid-vapour equilibrium pressure (Psat, caps#t1 to #t12). Titanium reactor experiments, SP#3 to SP#5, were conducted in the 423 to 473 K range, also at Psat. SP#6 was run at 378 K with an initial Ar pressure of ∼5 MPa. Pressure evolution in SP experiments was monitored at ±2 kPa with a Keller pressure sensor PA-33X. After the experiments, the H2 produced and trapped in the gold capsule was sampled using the protocol described in Malvoisin et al. (2013)Malvoisin, B., Brunet, F., Carlut, J., Montes-Hernandez, G., Findling, N., Lanson, M., Vidal, O., Bottero, J.-Y., Goffé, B. (2013) High-purity hydrogen gas from the reaction between BOF steel slag and water in the 473–673 K range. International Journal of Hydrogen Energy 38, 7382–7393. https://doi.org/10.1016/j.ijhydene.2013.03.163
and analysed by gas chromatography. H2 leakage through the gold capsule walls is negligible at the temperatures investigated here (Malvoisin et al., 2013Malvoisin, B., Brunet, F., Carlut, J., Montes-Hernandez, G., Findling, N., Lanson, M., Vidal, O., Bottero, J.-Y., Goffé, B. (2013) High-purity hydrogen gas from the reaction between BOF steel slag and water in the 473–673 K range. International Journal of Hydrogen Energy 38, 7382–7393. https://doi.org/10.1016/j.ijhydene.2013.03.163
). The amount of H2 produced in titanium reactors was quantified either on the gas sampled at the run conditions, and/or at the end of the experiment. No H2 was detected in two blank experiments performed at 378 and 473 K over more than 40 days with capsules containing only degassed ultrapure water (100 μL). Before experiments, the titanium reactors were heated to 523 K during one day in air to extract any H2 potentially solubilised in the reactor wall (Louthan and Derrick, 1975Louthan Jr., M.R., Derrick, R.G. (1975) Hydrogen transport in austenitic stainless steel. Corrosion Science 15, 565–577. https://doi.org/10.1016/0010-938X(75)90022-0
), and to ensure Ti surface oxidation prior to reaction. No hydrogen production was detected in these blank experiments. Details of the sample characterisation techniques are provided in the Supplementary Information.top
Results and Discussion
Ferroan brucite oxidation reaction. In all experiments, ferroan brucite partly decomposed into magnetite + H2 (Table S-2). The H2 yield increased from 4 × 10−3 to 220 μmol of H2 per gram of starting material when the temperature increased from 348 to 573 K. In eight experiments (Table S-2), pyroaurite was detected (<20 wt. %), indicating the presence of minor CO2 in the reacting medium. Pyroaurite does not involve significant H2 production (Carlin et al., 2023
Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) FeIII-substituted brucite: Hydrothermal synthesis from (Mg0.8FeII0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845
). Its impact on H2 production rate is, thus, mainly associated with the lowering of the amount of ferroan brucite available for H2 production. However, for experiments used for determining kinetic and thermodynamic parameters, we estimated that a maximum of 3.4 wt. % of the initial ferroan brucite was consumed to form pyroaurite. This leads to an error on the H2 production rate that is small compared to the error associated with H2 measurement of ∼11 %. The Fe content (XFe(OH)2) of ferroan brucite was determined from the refined unit cell parameters (Table S-2) based on Vegard’s law (Carlin et al., 2023Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) FeIII-substituted brucite: Hydrothermal synthesis from (Mg0.8FeII0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845
). It was found to vary from 0.1 to 0.2, i.e. equal or slightly below XFe(OH)2 in the ferroan brucite starting material. Recrystallisation of ferroan brucite as platelets was observed in the highest temperature experiments (Fig. 1). Unless metastable growth occurred, this observation suggests that ferroan brucite with XFe(OH)2 < 0.2 is a stable reaction product in these experiments.Altogether, the experimental results revealed that brucite partly reacted during the experiments according to Reaction 1, leading to the overall reaction:
where x is the initial XFe(OH)2 and y is a parameter related to reaction progress such as 0 ≤ y ≤ x, which reflects that only a fraction, y/x, of the Fe(OH)2 component in ferroan brucite has reacted. Indeed, thermodynamic equilibrium may well be achieved for y < x.
H2 production rate and brucite oxidation rate. The amount of produced H2 was used to retrieve the progress of Reaction 2. Magnetite was not used to infer reaction progress because, due to the presence of minor Fe3+ in the starting material, part of the magnetite product may form independently of Reaction 2, i.e. without H2 production (Carlin et al., 2023
Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) FeIII-substituted brucite: Hydrothermal synthesis from (Mg0.8FeII0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845
). The amount of H2 produced in the SP experiments did not induce pressure changes significant enough to retrieve isothermal H2-production rate laws directly from pressure monitoring. However, the qualitative pressure evolution was used to determine the overall duration of the H2 production stage, which, in turn, was used to model the reaction kinetics (see Supplementary Information for details). H2 production kinetics at 378 K could be accurately retrieved from Run SP#6, where gas was regularly sampled (Fig. 2a). In addition, a series of experiments in gold capsules (caps#t5 to #t12) were stopped after different durations and H2 was analysed in order to further constrain the kinetics of Reaction 2 (Fig. 2b). These experimental data were fitted to a kinetic law (Lasaga, 1998Lasaga, A.C. (1998) Kinetic Theory in the Earth Sciences. Princeton University Press, Princeton, NJ.
; see Supplementary Information for details) with the reaction rate (r) as:where with k0 a kinetic constant, A the Fe(OH)2 specific surface area, Ea the activation energy, R the gas constant and T the temperature. Q and K are the reaction quotient and the equilibrium constant of Reaction 1, respectively. The standard state is defined here with unit activity for pure minerals and water at any temperature and pressure, as well as unit fugacity for ideal gas at 1 bar of pressure and any temperature. Q was approximated to with PH2 the H2 partial pressure at the conditions of the experiment and XFe(OH)2 the molar fraction of Fe(OH)2 in ferroan brucite, by assuming ideal behaviour for H2 and Fe(OH)2 in the gas phase and in the brucite solid-solution, respectively. XFe(OH)2 was calculated from Equation 2, based on the number of moles of produced H2 (nH2). PH2 was derived from nH2 considering the amount of H2 dissolved in the solution as calculated using PHREEQC (Parkhurst and Appelo, 2013
Parkhurst, D.L., Appelo, C.A.J. (2013) Description of Input and Examples for PHREEQC Version 3—A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations. USGS Techniques and Methods 6–A43, U.S. Geological Survey, Denver, CO. https://pubs.usgs.gov/tm/06/a43/
). K was estimated with the same procedure as Q, by considering that equilibrium H2 pressure equals the H2 partial pressure in the gas at the last measurement multiplied by a factor, λ, slightly above 1 (see Supplementary Information for details on this factor). Fitted r0 values are displayed in Figure 2c as a function of reciprocal temperature. The slope in the linear fit corresponds to an activation energy of 145 ± 1 kJ/mol. The intercept of the fit provides a k0 × A quotient of 8.97 × 107 mol s−1 g−1.Experimental constraints on thermodynamic properties of ferroan brucite. Based on the kinetic law derived in the previous section, 17 experiments reached equilibrium (Table S-2). These experiments were, thus, used to constrain the equilibrium constant (K) of Reaction 1.
As discussed above, a set of K values can be calculated based on nH2, measured at the end of each experiment. A pair of ΔfH° and S° values for the Fe(OH)2 end member was retrieved by least square regression through this set of K values (see Supplementary Information for thermodynamic calculation details). ΔfH°Fe(OH)2 and S°Fe(OH)2 of −581.3 ± 2.9 kJ/mol and 86.4 ± 6.3 J/mol/K were obtained, respectively. These thermodynamic values are only relevant for calculations using the same standard states as those used here, as well as the same assumption of unit activity and fugacity coefficients for ferroan brucite solid solution and H2 in the gas phase, respectively. They fall in the range of published values for Fe(OH)2 (Table S-4, Fig. S-3). The ΔfH°Fe(OH)2 value is consistent with the value by Ziemniak et al. (1995)
Ziemniak, S.E., Jones, M.E., Combs, K.E.S. (1995) Magnetite solubility and phase stability in alkaline media at elevated temperatures. Journal of Solution Chemistry 24, 837–877. https://doi.org/10.1007/BF00973442
and departs by 1.2 % from ΔfH°Fe(OH)2, tabulated in the NIST-JANAF database (Chase, 1998Chase, M.W. (1998) NIST-JANAF Thermochemical Tables. Fourth Edition, American Chemical Society, Washington, D.C.
). The values of the NIST-JANAF database only differ by 0.1 % from those commonly used for thermodynamic modelling of fluid–rock interactions in ultramafic rocks (McCollom and Bach, 2009McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032
). H2 production prediction for our experiments is overestimated by more than one order of magnitude with the McCollom and Bach’s (2009)McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032
database (Fig. S-4).Implications for low-T H2 production in ultramafic systems. Olivine serpentinisation (e.g., McCollom et al., 2016
McCollom, T.M., Klein, F., Robbins, M., Moskowitz, B., Berquó, T.S., Jöns, N., Bach, W., Templeton, A. (2016) Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine. Geochimica et Cosmochimica Acta 181, 175–200. https://doi.org/10.1016/j.gca.2016.03.002
) and ferroan brucite alteration (Miller et al., 2017Miller, H.M., Mayhew, L.E., Ellison, E.T., Kelemen, P., Kubo, M., Templeton, A.S. (2017) Low temperature hydrogen production during experimental hydration of partially-serpentinized dunite. Geochimica et Cosmochimica Acta 209, 161–183. https://doi.org/10.1016/j.gca.2017.04.022
; Ellison et al., 2021Ellison, E.T., Templeton, A.S., Zeigler, S.D., Mayhew, L.E., Kelemen, P.B., Matter, J.M., The Oman Drilling Project Science Party (2021) Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite. Journal of Geophysical Research: Solid Earth 126, e2021JB021981. https://doi.org/10.1029/2021JB021981
) are the two main processes that have been proposed to account for H2 production at low temperature (T < 423 K) in ultramafic rocks. Excluding kinetic experiments interpreted as being distorted by artefacts (McCollom and Donaldson, 2016McCollom, T.M., Donaldson, C. (2016) Generation of Hydrogen and Methane during Experimental Low-Temperature Reaction of Ultramafic Rocks with Water. Astrobiology 16, 389–406. https://doi.org/10.1089/ast.2015.1382
), a maximum of 0.028 nmol H2/g olivine/day was proposed for H2 production by olivine serpentinisation at 363 K (Fig. 3a). In comparison, the rate of H2 production measured here during ferroan brucite reaction (r0) is approximately three orders of magnitude higher (Fig. 3a). When weighted by the specific surface area of the powders used in the various experiments, however, the reaction rates are rather well aligned in an Arrhenius plot with an activation energy of ∼81 kJ/mol (Fig. 3b). The difference in grain sizes between olivine used in the experiments depicted in Figure 3 (38–212 μm) and the synthetic ferroan brucite used here (∼50 nm) probably plays a key role in their difference of reactivity.The respective contribution of olivine and Fe-brucite alteration to the H2 production rate in natural systems was evaluated using a numerical approach. A fluid–serpentinised dunite system composed of olivine (grain size of 500 μm; Malvoisin et al., 2017
Malvoisin, B., Brantut, N., Kaczmarek, M.-A. (2017) Control of serpentinisation rate by reaction-induced cracking. Earth and Planetary Science Letters 476, 143–152. https://doi.org/10.1016/j.epsl.2017.07.042
), ferroan brucite (grain size of 50 nm; Malvoisin et al., 2021Malvoisin, B., Auzende, A.-L., Kelemen, P.B., the Oman Drilling Project Science Party (2021) Nanostructure of serpentinisation products: Importance for water transport and low-temperature alteration. Earth and Planetary Science Letters 576, 117212. https://doi.org/10.1016/j.epsl.2021.117212
) and water was modelled with H2 being only produced by alteration of the latter minerals. A range of escape rates of H2 (advection and/or diffusion) was defined in order to simulate hydrothermal activity at mid-ocean ridges, water infiltration in an ophiolitic unit or sub-stagnant hydraulic conditions in a deep aquifer. The temperature was set to 363 K, relevant to low-T serpentinisation (Fig. 4; see model details in Supplementary Information).The model shows that ferroan brucite is the first mineral to react with a rate that is three orders of magnitude faster than that of olivine, leading to a rapid H2 production in the first year of the reaction (Fig. 4b). In a closed system, ferroan brucite reaction rapidly stops due to the attainment of thermodynamic equilibrium (Q/K = 1). The adjustment of the thermodynamic parameters of Fe(OH)2 proposed here (1.8 % and 1.2 % for S°Fe(OH)2 and ΔfH°Fe(OH)2, respectively, compared to the database of McCollom and Bach, 2009
McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032
) has a strong impact on the predicted equilibrium H2 partial pressure and, thus, on the amount of H2 that is produced. At 363 K, equilibrium is achieved for a H2 molality of 2.2 × 10−5 mol/kg, while it is two orders of magnitude higher (5.0 × 10−3 mol/kg) with the database of McCollom and Bach (2009)McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032
. At 313 K, a difference of three orders of magnitude for H2 molality between the two database is predicted. The oxygen fugacity (fO2) lies on the H2O/H2(g) equilibrium with the database of McCollom and Bach (2009)McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032
and five orders of magnitude above with the thermodynamic data derived here. Interestingly, this latter fO2 is consistent with the fO2 measured at the bottom of Holes BA1A, BA1D and BA4A during the Oman Drilling Project (Kelemen et al., 2021Kelemen, P.B., Leong, J.A., de Obeso, J.C., Matter, J.M., Ellison, E.T., Templeton, A., Nothaft, D.B., Eslami, A., Evans, K., Godard, M., Malvoisin, B., Coggon, J.A., Warsi, N.H., Pézard, P., Choe, S., Teagle, D.A.H., Michibayashi, K., Takazawa, E., Al Sulaimani, Z., The Oman Drilling Project Science Team (2021) Initial Results From the Oman Drilling Project Multi-Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon. Journal of Geophysical Research: Solid Earth 126, e2021JB022729. https://doi.org/10.1029/2021JB022729
). After ferroan brucite reaction, olivine completely reacts in the model in approximately 3 Myr and ultimately produces 3000 times more H2 than ferroan brucite (Fig. 4a).Ferroan brucite can further react if, at the same time, H2 escapes from the system at a rate exceeding that of H2 production associated with olivine serpentinisation (Fig. 4c). This corresponds to minimum H2 escape rates of 10−5 and 7 × 10−4 mol H2/day/g rock at 313 and 363 K, respectively. The estimated maximum escape rate by vertical diffusion is three orders of magnitude lower than this threshold value (see Supplementary Information for details), suggesting that vertical diffusion is not sufficient to drive ferroan brucite reaction. The threshold value is achieved by water renewal at a minimum rate of 7 × 10−6 and 3 × 10−5 kg water/day/kg rock at 313 and 363 K, respectively. The average water-to-rock ratio at mid-ocean ridges is ∼1 (Coogan et al., 2019
Coogan, L.A., Seyfried, W.E., Pester, N.J. (2019) Environmental controls on mid-ocean ridge hydrothermal fluxes. Chemical Geology 528, 119285. https://doi.org/10.1016/j.chemgeo.2019.119285
). Considering hydrothermal activity and fluid flow during a minimum of 30,000 yr (Früh-Green et al., 2003Früh-Green, G.L., Kelley, D.S., Bernasconi, S.M., Karson, J.A., Ludwig, K.A., Butterfield, D.A., Boschi, C., Proskurowski, G. (2003) 30,000 Years of Hydrothermal Activity at the Lost City Vent Field. Science 301, 495–498. https://doi.org/10.1126/science.1085582
), it can be converted into a mean water flux of 10−7 kg water/day/kg rock. In the Oman ophiolite, ferroan brucite with x = 0.28 can represent ∼50 mol % of the serpentinisation reaction products (Malvoisin et al., 2020Malvoisin, B., Zhang, C., Müntener, O., Baumgartner, L.P., Kelemen, P.B., Oman Drilling Project Science Party (2020) Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project. Journal of Geophysical Research: Solid Earth 125, e2019JB018877. https://doi.org/10.1029/2019JB018877
). Present day alteration of such ferroan brucite may occur during interaction with rainwater. H2-rich hyperalkaline fluids are found at depths >50 m (Leong et al., 2023Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H2 and CH4 outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008
), and the recharge rate of the aquifer in Oman is 18 mm/year (Dewandel et al., 2005Dewandel, B., Lachassagne, P., Boudier, F., Al-Hattali, S., Ladouche, B., Pinault, J.-L., Al-Suleimani, Z. (2005) A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach. Hydrogeology Journal 13, 708–726. https://doi.org/10.1007/s10040-005-0449-2
). Combining these values leads to a mean meteoritic water flux of 3 × 10−7 kg water/day/kg rock. Both in ophiolites and on the seafloor, mean water flux estimates are, thus, approximately one order of magnitude lower than the minimum flux necessary to drive ferroan brucite reaction. However, fluid flow in ultramafic rocks is concentrated in cracks and microcracks (Dewandel et al., 2005Dewandel, B., Lachassagne, P., Boudier, F., Al-Hattali, S., Ladouche, B., Pinault, J.-L., Al-Suleimani, Z. (2005) A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach. Hydrogeology Journal 13, 708–726. https://doi.org/10.1007/s10040-005-0449-2
; Corre et al., 2023Corre, M., Brunet, F., Schwartz, S., Gautheron, C., Agranier, A., Lesimple, S. (2023) Quaternary low-temperature serpentinization and carbonation in the New Caledonia ophiolite. Scientific Reports 13, 19413. https://doi.org/10.1038/s41598-023-46691-y
) and is, thus, expected to be, locally, several orders of magnitude higher than the mean water flux. For example, the highest water-to-rock ratio values reported in abyssal peridotites are above 105 (Snow and Reisberg, 1995Snow, J.E., Reisberg, L. (1995) Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites. Earth and Planetary Science Letters 133, 411–421. https://doi.org/10.1016/0012-821X(95)00099-X
; Delacour et al., 2008Delacour, A., Früh-Green, G.L., Frank, M., Gutjahr, M., Kelley, D.S. (2008) Sr- and Nd-isotope geochemistry of the Atlantis Massif (30°N, MAR): Implications for fluid fluxes and lithospheric heterogeneity. Chemical Geology 254, 19–35. https://doi.org/10.1016/j.chemgeo.2008.05.018
), corresponding to water fluxes >10−2 kg water/day/kg rock compatible with H2 production associated with ferroan brucite oxidation. The measured maximum H2 production rate in the Oman ophiolite is of 71,000 mol H2/yr for a minimal volume of altered rock of 0.05 km3 (Leong et al., 2023Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H2 and CH4 outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008
). This corresponds to a specific flux of 10−3 nmol H2/g rock/day which is consistent with the specific H2 production rate of 5 × 10−3 nmol H2/g rock/day, estimated at 313 K based on the extrapolation of the data acquired here for ferroan brucite alteration (Fig. 3). Actually, the same extrapolation for serpentinisation of olivine having a grain size of 500 μm (Fig. 3b) yields a much lower production rate of 10−5 nmol H2/g rock/day at 313 K. Ferroan brucite oxidation could thus be a main contributor to H2 production at temperatures below 423 K in ophiolites and on the seafloor. This is consistent with petrographic observations in natural samples showing ferroan brucite oxidation to form magnetite in open systems conditions (Bach et al., 2006Bach, W., Paulick, H., Garrido, C.J., Ildefonse, B., Meurer, W.P., Humphris, S.E. (2006) Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274). Geophysical Research Letters 33, L13306. https://doi.org/10.1029/2006GL025681
; Jöns et al., 2017Jöns, N., Kahl, W.-A., Bach, W. (2017) Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography. Lithos 268–271, 274–284. https://doi.org/10.1016/j.lithos.2016.11.014
).top
Acknowledgements
This work is part of a CIFRE PhD (n°2019.1672) funded by ENGIE, and performed in collaboration with Storengy. Analyses have been performed at the Geochemistry & Mineralogy platform of ISTerre (Université Grenoble Alpes, France). The editor, E.H. Oelkers, and two anonymous reviewers are thanked for their insightful comments and careful reading.
Editor: Eric Oelkers
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References
Abrajano, T.A., Sturchio, N.C., Kennedy, B.M., Lyon, G.L., Muehlenbachs, K., Bohlke, J.K. (1990) Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines. Applied Geochemistry 5, 625–630. https://doi.org/10.1016/0883-2927(90)90060-I
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As observed in ophiolites (Neal and Stanger, 1983; Abrajano et al., 1990; Leong et al., 2023), ultramafic rocks can still produce H2 at low temperature (i.e. at T < 423 K), even if they are extensively serpentinised.
View in article
Bach, W., Paulick, H., Garrido, C.J., Ildefonse, B., Meurer, W.P., Humphris, S.E. (2006) Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274). Geophysical Research Letters 33, L13306. https://doi.org/10.1029/2006GL025681
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This is consistent with petrographic observations in natural samples showing ferroan brucite oxidation to form magnetite in open systems conditions (Bach et al., 2006; Jöns et al., 2017).
View in article
Brantley, S.L., Mellott, N.P. (2000) Surface area and porosity of primary silicate minerals. American Mineralogist 8, 1767–1783. https://doi.org/10.2138/am-2000-11-1220
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(b) Reaction rate per reactive surface area (r0/A) with A the specific surface area either measured with the BET method or calculated with the relationship provided in Brantley and Mellott (2000).
View in article
Carlin, W., Malvoisin, B., Lanson, B., Brunet, F., Findling, N., Lanson, M., Magnin, V., Fargetton, T., Jeannin, L., Lhote, O. (2023) FeIII-substituted brucite: Hydrothermal synthesis from (Mg0.8FeII0.2)-brucite, crystal chemistry and relevance to the alteration of ultramafic rocks. Applied Clay Science 234, 106845. https://doi.org/10.1016/j.clay.2023.106845
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Ferroan brucite (Mg1−x,Fex)(OH)2, with x ranging from 0.156 to 0.205, was synthesised under ambient conditions from a stoichiometric solution of dissolved Fe(II) and Mg chlorides, as described in Carlin et al. (2023).
View in article
Pyroaurite does not involve significant H2 production (Carlin et al., 2023).
View in article
This leads to an error on the H2 production rate that is small compared to the error associated with H2 measurement of ∼11 %. The Fe content (XFe(OH)2) of ferroan brucite was determined from the refined unit cell parameters (Table S-2) based on Vegard’s law (Carlin et al., 2023).
View in article
Magnetite was not used to infer reaction progress because, due to the presence of minor Fe3+ in the starting material, part of the magnetite product may form independently of Reaction 2, i.e. without H2 production (Carlin et al., 2023).
View in article
Chase, M.W. (1998) NIST-JANAF Thermochemical Tables. Fourth Edition, American Chemical Society, Washington, D.C.
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They fall in the range of published values for Fe(OH)2 (Table S-4, Fig. S-3). The ΔfH°Fe(OH)2 value is consistent with the value by Ziemniak et al. (1995) and departs by 1.2 % from ΔfH°Fe(OH)2, tabulated in the NIST-JANAF database (Chase, 1998).
View in article
Coogan, L.A., Seyfried, W.E., Pester, N.J. (2019) Environmental controls on mid-ocean ridge hydrothermal fluxes. Chemical Geology 528, 119285. https://doi.org/10.1016/j.chemgeo.2019.119285
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The average water-to-rock ratio at mid-ocean ridges is ∼1 (Coogan et al., 2019).
View in article
Corre, M., Brunet, F., Schwartz, S., Gautheron, C., Agranier, A., Lesimple, S. (2023) Quaternary low-temperature serpentinization and carbonation in the New Caledonia ophiolite. Scientific Reports 13, 19413. https://doi.org/10.1038/s41598-023-46691-y
Show in context
However, fluid flow in ultramafic rocks is concentrated in cracks and microcracks (Dewandel et al., 2005; Corre et al., 2023) and is, thus, expected to be, locally, several orders of magnitude higher than the mean water flux.
View in article
Delacour, A., Früh-Green, G.L., Frank, M., Gutjahr, M., Kelley, D.S. (2008) Sr- and Nd-isotope geochemistry of the Atlantis Massif (30°N, MAR): Implications for fluid fluxes and lithospheric heterogeneity. Chemical Geology 254, 19–35. https://doi.org/10.1016/j.chemgeo.2008.05.018
Show in context
For example, the highest water-to-rock ratio values reported in abyssal peridotites are above 105 (Snow and Reisberg, 1995; Delacour et al., 2008), corresponding to water fluxes >10−2 kg water/day/kg rock compatible with H2 production associated with ferroan brucite oxidation.
View in article
Dewandel, B., Lachassagne, P., Boudier, F., Al-Hattali, S., Ladouche, B., Pinault, J.-L., Al-Suleimani, Z. (2005) A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach. Hydrogeology Journal 13, 708–726. https://doi.org/10.1007/s10040-005-0449-2
Show in context
Present day alteration of such ferroan brucite may occur during interaction with rainwater. H2-rich hyperalkaline fluids are found at depths >50 m (Leong et al., 2023), and the recharge rate of the aquifer in Oman is 18 mm/year (Dewandel et al., 2005).
View in article
However, fluid flow in ultramafic rocks is concentrated in cracks and microcracks (Dewandel et al., 2005; Corre et al., 2023) and is, thus, expected to be, locally, several orders of magnitude higher than the mean water flux.
View in article
Ellison, E.T., Templeton, A.S., Zeigler, S.D., Mayhew, L.E., Kelemen, P.B., Matter, J.M., The Oman Drilling Project Science Party (2021) Low-Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite. Journal of Geophysical Research: Solid Earth 126, e2021JB021981. https://doi.org/10.1029/2021JB021981
Show in context
Petrographic data on ophiolite and dredge seafloor samples (Jöns et al., 2017; Klein et al., 2020; Ellison et al., 2021) seem to indicate that Reaction 1 could proceed at sub-surface conditions in partly serpentinised ultramafic rocks. H2 production was achieved in hydrothermal experiments carried out on serpentinised peridotite at 373 K and was attributed to magnetite formation at the expense of ferroan brucite (Miller et al., 2017).
View in article
Olivine serpentinisation (e.g., McCollom et al., 2016) and ferroan brucite alteration (Miller et al., 2017; Ellison et al., 2021) are the two main processes that have been proposed to account for H2 production at low temperature (T < 423 K) in ultramafic rocks. Excluding kinetic experiments interpreted as being distorted by artefacts (McCollom and Donaldson, 2016), a maximum of 0.028 nmol H2/g olivine/day was proposed for H2 production by olivine serpentinisation at 363 K (Fig. 3a).
View in article
Früh-Green, G.L., Kelley, D.S., Bernasconi, S.M., Karson, J.A., Ludwig, K.A., Butterfield, D.A., Boschi, C., Proskurowski, G. (2003) 30,000 Years of Hydrothermal Activity at the Lost City Vent Field. Science 301, 495–498. https://doi.org/10.1126/science.1085582
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Considering hydrothermal activity and fluid flow during a minimum of 30,000 yr (Früh-Green et al., 2003), it can be converted into a mean water flux of 10−7 kg water/day/kg rock.
View in article
Jöns, N., Kahl, W.-A., Bach, W. (2017) Reaction-induced porosity and onset of low-temperature carbonation in abyssal peridotites: Insights from 3D high-resolution microtomography. Lithos 268–271, 274–284. https://doi.org/10.1016/j.lithos.2016.11.014
Show in context
Petrographic data on ophiolite and dredge seafloor samples (Jöns et al., 2017; Klein et al., 2020; Ellison et al., 2021) seem to indicate that Reaction 1 could proceed at sub-surface conditions in partly serpentinised ultramafic rocks. H2 production was achieved in hydrothermal experiments carried out on serpentinised peridotite at 373 K and was attributed to magnetite formation at the expense of ferroan brucite (Miller et al., 2017).
View in article
This is consistent with petrographic observations in natural samples showing ferroan brucite oxidation to form magnetite in open systems conditions (Bach et al., 2006; Jöns et al., 2017).
View in article
Kelemen, P.B., Leong, J.A., de Obeso, J.C., Matter, J.M., Ellison, E.T., Templeton, A., Nothaft, D.B., Eslami, A., Evans, K., Godard, M., Malvoisin, B., Coggon, J.A., Warsi, N.H., Pézard, P., Choe, S., Teagle, D.A.H., Michibayashi, K., Takazawa, E., Al Sulaimani, Z., The Oman Drilling Project Science Team (2021) Initial Results From the Oman Drilling Project Multi-Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon. Journal of Geophysical Research: Solid Earth 126, e2021JB022729. https://doi.org/10.1029/2021JB022729
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Interestingly, this latter fO2 is consistent with the fO2 measured at the bottom of Holes BA1A, BA1D and BA4A during the Oman Drilling Project (Kelemen et al., 2021).
View in article
Klein, F., Humphris, S.E., Bach, W. (2020) Brucite formation and dissolution in oceanic serpentinite. Geochemical Perspectives Letters 16, 1–5. https://doi.org/10.7185/geochemlet.2035
Show in context
Petrographic data on ophiolite and dredge seafloor samples (Jöns et al., 2017; Klein et al., 2020; Ellison et al., 2021) seem to indicate that Reaction 1 could proceed at sub-surface conditions in partly serpentinised ultramafic rocks. H2 production was achieved in hydrothermal experiments carried out on serpentinised peridotite at 373 K and was attributed to magnetite formation at the expense of ferroan brucite (Miller et al., 2017).
View in article
Lasaga, A.C. (1998) Kinetic Theory in the Earth Sciences. Princeton University Press, Princeton, NJ.
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These experimental data were fitted to a kinetic law (Lasaga, 1998; see Supplementary Information for details) with the reaction rate (r) as:
Eq. 3.
where with k0 a kinetic constant, A the Fe(OH)2 specific surface area, Ea the activation energy, R the gas constant and T the temperature. Q and K are the reaction quotient and the equilibrium constant of Reaction 1, respectively.
View in article
Leong, J.A., Nielsen, M., McQueen, N., Karolytė, R., Hillegonds, D.J., Ballentine, C., Darrah, T., McGillis, W., Kelemen, P. (2023) H2 and CH4 outgassing rates in the Samail ophiolite, Oman: Implications for low-temperature, continental serpentinization rates. Geochimica et Cosmochimica Acta 347, 1–15. https://doi.org/10.1016/j.gca.2023.02.008
Show in context
As observed in ophiolites (Neal and Stanger, 1983; Abrajano et al., 1990; Leong et al., 2023), ultramafic rocks can still produce H2 at low temperature (i.e. at T < 423 K), even if they are extensively serpentinised.
View in article
Present day alteration of such ferroan brucite may occur during interaction with rainwater. H2-rich hyperalkaline fluids are found at depths >50 m (Leong et al., 2023), and the recharge rate of the aquifer in Oman is 18 mm/year (Dewandel et al., 2005).
View in article
The measured maximum H2 production rate in the Oman ophiolite is of 71,000 mol H2/yr for a minimal volume of altered rock of 0.05 km3 (Leong et al., 2023).
View in article
Louthan Jr., M.R., Derrick, R.G. (1975) Hydrogen transport in austenitic stainless steel. Corrosion Science 15, 565–577. https://doi.org/10.1016/0010-938X(75)90022-0
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Before experiments, the titanium reactors were heated to 523 K during one day in air to extract any H2 potentially solubilised in the reactor wall (Louthan and Derrick, 1975), and to ensure Ti surface oxidation prior to reaction.
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Malvoisin, B., Brunet, F., Carlut, J., Montes-Hernandez, G., Findling, N., Lanson, M., Vidal, O., Bottero, J.-Y., Goffé, B. (2013) High-purity hydrogen gas from the reaction between BOF steel slag and water in the 473–673 K range. International Journal of Hydrogen Energy 38, 7382–7393. https://doi.org/10.1016/j.ijhydene.2013.03.163
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After the experiments, the H2 produced and trapped in the gold capsule was sampled using the protocol described in Malvoisin et al. (2013) and analysed by gas chromatography.
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H2 leakage through the gold capsule walls is negligible at the temperatures investigated here (Malvoisin et al., 2013).
View in article
Malvoisin, B., Brantut, N., Kaczmarek, M.-A. (2017) Control of serpentinisation rate by reaction-induced cracking. Earth and Planetary Science Letters 476, 143–152. https://doi.org/10.1016/j.epsl.2017.07.042
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A fluid–serpentinised dunite system composed of olivine (grain size of 500 μm; Malvoisin et al., 2017), ferroan brucite (grain size of 50 nm; Malvoisin et al., 2021) and water was modelled with H2 being only produced by alteration of the latter minerals.
View in article
Malvoisin, B., Zhang, C., Müntener, O., Baumgartner, L.P., Kelemen, P.B., Oman Drilling Project Science Party (2020) Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project. Journal of Geophysical Research: Solid Earth 125, e2019JB018877. https://doi.org/10.1029/2019JB018877
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In order to test the potential of Reaction 1 to produce H2 at temperatures below 423 K in ultramafic rocks, the kinetics and thermodynamics of Reaction 1 were investigated experimentally here using synthetic (Mg1−x,Fex)(OH)2 of grain size (40–100 nm) and composition (x from 0.156 to 0.205) relevant to natural ferroan brucite (Malvoisin et al., 2020).
View in article
In the Oman ophiolite, ferroan brucite with x = 0.28 can represent ∼50 mol % of the serpentinisation reaction products (Malvoisin et al., 2020).
View in article
Malvoisin, B., Auzende, A.-L., Kelemen, P.B., the Oman Drilling Project Science Party (2021) Nanostructure of serpentinisation products: Importance for water transport and low-temperature alteration. Earth and Planetary Science Letters 576, 117212. https://doi.org/10.1016/j.epsl.2021.117212
Show in context
A fluid–serpentinised dunite system composed of olivine (grain size of 500 μm; Malvoisin et al., 2017), ferroan brucite (grain size of 50 nm; Malvoisin et al., 2021) and water was modelled with H2 being only produced by alteration of the latter minerals.
View in article
McCollom, T.M., Bach, W. (2009) Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks. Geochimica et Cosmochimica Acta 73, 856–875. https://doi.org/10.1016/j.gca.2008.10.032
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The values of the NIST-JANAF database only differ by 0.1 % from those commonly used for thermodynamic modelling of fluid–rock interactions in ultramafic rocks (McCollom and Bach, 2009).
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H2 production prediction for our experiments is overestimated by more than one order of magnitude with the McCollom and Bach’s (2009) database (Fig. S-4).
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At 363 K, equilibrium is achieved for a H2 molality of 2.2 × 10−5 mol/kg, while it is two orders of magnitude higher (5.0 × 10−3 mol/kg) with the database of McCollom and Bach (2009).
View in article
The oxygen fugacity (fO2) lies on the H2O/H2(g) equilibrium with the database of McCollom and Bach (2009) and five orders of magnitude above with the thermodynamic data derived here.
View in article
McCollom, T.M., Donaldson, C. (2016) Generation of Hydrogen and Methane during Experimental Low-Temperature Reaction of Ultramafic Rocks with Water. Astrobiology 16, 389–406. https://doi.org/10.1089/ast.2015.1382
Show in context
Olivine serpentinisation (e.g., McCollom et al., 2016) and ferroan brucite alteration (Miller et al., 2017; Ellison et al., 2021) are the two main processes that have been proposed to account for H2 production at low temperature (T < 423 K) in ultramafic rocks. Excluding kinetic experiments interpreted as being distorted by artefacts (McCollom and Donaldson, 2016), a maximum of 0.028 nmol H2/g olivine/day was proposed for H2 production by olivine serpentinisation at 363 K (Fig. 3a).
View in article
Comparison of experimentally determined rate of H2 production for ferroan brucite (r0, this study) and olivine serpentinisation (Neubeck et al., 2014; McCollom and Donaldson, 2016; McCollom et al., 2016).
View in article
McCollom, T.M., Klein, F., Robbins, M., Moskowitz, B., Berquó, T.S., Jöns, N., Bach, W., Templeton, A. (2016) Temperature trends for reaction rates, hydrogen generation, and partitioning of iron during experimental serpentinization of olivine. Geochimica et Cosmochimica Acta 181, 175–200. https://doi.org/10.1016/j.gca.2016.03.002
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The extrapolation of experimental kinetic data collected in the 473–623 K range (e.g., McCollom et al., 2016) indicates that serpentinisation of olivine with a grain size of 500 μm should reach a reaction progress above 90 % in at least 10,000 yr at temperatures below 423 K.
View in article
Olivine serpentinisation (e.g., McCollom et al., 2016) and ferroan brucite alteration (Miller et al., 2017; Ellison et al., 2021) are the two main processes that have been proposed to account for H2 production at low temperature (T < 423 K) in ultramafic rocks. Excluding kinetic experiments interpreted as being distorted by artefacts (McCollom and Donaldson, 2016), a maximum of 0.028 nmol H2/g olivine/day was proposed for H2 production by olivine serpentinisation at 363 K (Fig. 3a).
View in article
Comparison of experimentally determined rate of H2 production for ferroan brucite (r0, this study) and olivine serpentinisation (Neubeck et al., 2014; McCollom and Donaldson, 2016; McCollom et al., 2016).
View in article
Miller, H.M., Mayhew, L.E., Ellison, E.T., Kelemen, P., Kubo, M., Templeton, A.S. (2017) Low temperature hydrogen production during experimental hydration of partially-serpentinized dunite. Geochimica et Cosmochimica Acta 209, 161–183. https://doi.org/10.1016/j.gca.2017.04.022
Show in context
Petrographic data on ophiolite and dredge seafloor samples (Jöns et al., 2017; Klein et al., 2020; Ellison et al., 2021) seem to indicate that Reaction 1 could proceed at sub-surface conditions in partly serpentinised ultramafic rocks. H2 production was achieved in hydrothermal experiments carried out on serpentinised peridotite at 373 K and was attributed to magnetite formation at the expense of ferroan brucite (Miller et al., 2017).
View in article
Olivine serpentinisation (e.g., McCollom et al., 2016) and ferroan brucite alteration (Miller et al., 2017; Ellison et al., 2021) are the two main processes that have been proposed to account for H2 production at low temperature (T < 423 K) in ultramafic rocks. Excluding kinetic experiments interpreted as being distorted by artefacts (McCollom and Donaldson, 2016), a maximum of 0.028 nmol H2/g olivine/day was proposed for H2 production by olivine serpentinisation at 363 K (Fig. 3a).
View in article
Moody, J.B. (1976) Serpentinization: a review. Lithos 9, 125–138. https://doi.org/10.1016/0024-4937(76)90030-X
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In the course of these reactions, olivine reacts with water to form serpentine, magnetite and ferroan brucite, (Mg,Fe)(OH)2, along with abiotic hydrogen (Moody, 1976).
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Neal, C., Stanger, G. (1983) Hydrogen generation from mantle source rocks in Oman. Earth and Planetary Science Letters 66, 315–320. https://doi.org/10.1016/0012-821X(83)90144-9
Show in context
As observed in ophiolites (Neal and Stanger, 1983; Abrajano et al., 1990; Leong et al., 2023), ultramafic rocks can still produce H2 at low temperature (i.e. at T < 423 K), even if they are extensively serpentinised.
View in article
Neubeck, A., Duc, N.T., Hellevang, H., Oze, C., Bastviken, D., Bacsik, Z., Holm, N.G. (2014) Olivine alteration and H2 production in carbonate-rich, low temperature aqueous environments. Planetary and Space Science 96, 51–61. https://doi.org/10.1016/j.pss.2014.02.014
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Comparison of experimentally determined rate of H2 production for ferroan brucite (r0, this study) and olivine serpentinisation (Neubeck et al., 2014; McCollom and Donaldson, 2016; McCollom et al., 2016).
View in article
Parkhurst, D.L., Appelo, C.A.J. (2013) Description of Input and Examples for PHREEQC Version 3—A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations. USGS Techniques and Methods 6–A43, U.S. Geological Survey, Denver, CO. https://pubs.usgs.gov/tm/06/a43/
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The standard state is defined here with unit activity for pure minerals and water at any temperature and pressure, as well as unit fugacity for ideal gas at 1 bar of pressure and any temperature. Q was approximated to with PH2 the H2 partial pressure at the conditions of the experiment and XFe(OH)2 the molar fraction of Fe(OH)2 in ferroan brucite, by assuming ideal behaviour for H2 and Fe(OH)2 in the gas phase and in the brucite solid-solution, respectively. XFe(OH)2 was calculated from Equation 2, based on the number of moles of produced H2 (nH2). PH2 was derived from nH2 considering the amount of H2 dissolved in the solution as calculated using PHREEQC (Parkhurst and Appelo, 2013).
View in article
Snow, J.E., Reisberg, L. (1995) Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites. Earth and Planetary Science Letters 133, 411–421. https://doi.org/10.1016/0012-821X(95)00099-X
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For example, the highest water-to-rock ratio values reported in abyssal peridotites are above 105 (Snow and Reisberg, 1995; Delacour et al., 2008), corresponding to water fluxes >10−2 kg water/day/kg rock compatible with H2 production associated with ferroan brucite oxidation.
View in article
Ziemniak, S.E., Jones, M.E., Combs, K.E.S. (1995) Magnetite solubility and phase stability in alkaline media at elevated temperatures. Journal of Solution Chemistry 24, 837–877. https://doi.org/10.1007/BF00973442
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They fall in the range of published values for Fe(OH)2 (Table S-4, Fig. S-3). The ΔfH°Fe(OH)2 value is consistent with the value by Ziemniak et al. (1995) and departs by 1.2 % from ΔfH°Fe(OH)2, tabulated in the NIST-JANAF database (Chase, 1998).
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Supplementary Information
The Supplementary Information includes:
- Characterisation Techniques
- Time Needed to Reach Thermodynamic Equilibrium in Titanium Reactors
- H2 Production Rate-law
- Retrieval of ΔfH° and S° of the Fe(OH)2 End Member: Calculation Method
- Numerical Modelling of H2 Production During Serpentinised Dunite Alteration
- Tables S-1 to S-4
- Figures S-1 and S-4
- Supplementary Information References
Download the Supplementary Information (PDF)