Equilibrium and kinetic controls on molecular hydrogen abundance and hydrogen isotope fractionation in hydrothermal fluids
Introduction
Molecular hydrogen (H2) is one of the major gas constituents of hydrothermal fluids. It is considered to have predominant inorganic origins, i.e., deriving from reduction of water during the oxidation of Fe-bearing minerals and magma degassing at low pressures (e.g., Aiuppa et al., 2011; Klein et al., 2020). In magmatic gases, H2 may account for up to 1-2 mol% of the total gas (e.g., Taran and Giggenbach, 2003), whereas in hydrothermal fluids with temperatures of ∼100-350 °C, the fugacity of H2 (fH2) varies over orders of magnitude (<0.001 to >1 bar; e.g., Stefánsson, 2017). It is generally accepted that fH2 increases with increasing temperature and is controlled by fluid-mineral equilibria involving various Fe-bearing minerals, sulfides, sulfates and aluminum silicates (Chiodini and Marini, 1998). Recent findings suggest, however, that such overall fluid-mineral equilibria may not prevail. Instead, concentrations may rather reflect metastable equilibria or source-controlled non-equilibrium conditions influenced by elemental fluxes, reaction pathways and progress often influenced by reaction kinetics (Stefánsson, 2017).
Molecular hydrogen concentrations in hydrothermal fluids may be highly dependent on their formation reactions and the factors influencing these reactions. However, equilibrium isotope fractionation between H2 and H2O is independent of concentration but highly dependent on temperature (Pester et al., 2018). At >200 °C the D-H exchange rate between H2 and H2O is fast, minutes to hours, whereas at 100 °C the rates are much slower, days to years (Pester et al., 2018). In comparison, while hydrothermal reservoir fluid residence times range from few years to hundreds of years (Hayba and Ingebritsen, 1997; Kadko et al., 2007; Stefánsson et al., 2015), fluid ascent to the surface is much faster, minutes for boreholes (Mubarok and Zarrouk, 2017) and several days for hot springs and fumaroles (Sturchio et al., 1993). Hence, D/H isotope equilibrium between H2 and H2O is likely to be attained for hydrothermal reservoir fluids, whereas for surface emissions, like hot springs and fumaroles, the degree of re-equilibration is controlled by the interplay between hydrogen isotope exchange rate, fluid velocity and the temperature gradient along the flow path (Pester et al., 2018).
To date, only a few studies have addressed hydrogen isotope exchange in terrestrial hydrothermal systems (Árnason, 1977; Lyon and Hulston, 1984; Kiyosu, 1983; Taran et al., 1992, 2010), and a systematic investigation between fast discharging well fluids and slower flowing fumarole fluids is missing. Here, we report H2 abundances and δD of H2 and H2O for hydrothermal fluids of variable temperature in terrestrial volcanic arc and rift systems sourced by seawater and meteoric water. Wherever possible, discharges from both wells and surface fumaroles were sampled. Hydrogen abundances and isotopic compositions are compared with geochemical modeling to infer the factors controlling H2 concentrations and δD-H2.
Section snippets
Sampling and analysis
Samples of hydrothermal fluids (n = 46) were collected in Iceland, New Zealand, Kenya and Greece (Tables S1, S2). Samples were collected of the vapor and liquid phases from well discharges and vapor fumaroles. The sample locations reflect variable geological environments (rift and arc settings) and source fluid (meteoric and seawater) (Table S1).
For fumaroles, a titanium tube was inserted into the vapor outlet and connected to the sampling gas bottles using glass or silicon tubes. For liquid
Chemical and isotopic composition of hydrothermal fluids
The measured fluid sampling temperatures are 97-250 °C and the calculated fluid reservoir temperatures based on solute and gas geothermometry are 226-359 °C (Tables 1, S2). The fluids are in all cases dominated by H2O (Fig. 1, Table 1). Molecular hydrogen is among the major gases with vapor concentrations ranging from to mol% and from to mol% for meteoric water sourced MOR and ARC systems, respectively. For the investigated seawater sourced systems, abundances of H
Metastable mineral-fluid equilibria control hydrogen concentration in hydrothermal fluids
The fH2 values of the hydrothermal fluids studied span over 3 orders of magnitude, ranging from close to MH to about an order of magnitude greater than FMQ equilibrium buffer values, providing inconclusive information on if and which redox buffers control the H2 concentrations (Fig. 4). The modeling results (Figs. 2, S3-S18) suggest the concentration of H2 to be controlled by the availability of redox sensitive aqueous species (Fe2+, , H2S, HS−, , CH4, ) that are affected by
Conclusion
A comparison of chemical and isotopic data from well and fumarolic fluid discharges from eleven hydrothermal systems hosted in rift and arc settings and sourced by meteoric water and seawater with geochemical modeling was used to assess the control on the fugacity and isotopic composition of molecular hydrogen. The sampled fluids display a wide range in reservoir temperature (226-359 °C), fH2 values and δD values of H2 and H2O from 0.002 to 3.3 bar, -646 to -391‰ and -94.1 to +11.3‰,
CRediT authorship contribution statement
Andrea Ricci: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Validation, Visualization, Writing – original draft, Writing – review & editing. Barbara I. Kleine: Conceptualization, Data curation, Formal analysis, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. Jens Fiebig: Conceptualization, Investigation, Validation, Writing – review & editing. Jóhann Gunnarsson-Robin: Investigation,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by DFG grants FI 948/8-1 and FI 948/10-1 as well as The Icelandic Centre for Research grant number #152680-05. We thank Ríkey Kjartansdóttir, Patrick Beaudry, Jeemin Rhim, Shuhei Ono, Sergio Calabrese, Walter D'Alessandro, Guendalina Pecoraino, Kyriaki Daskalopoulou, Franco Tassi, and Sven Hofmann for their help in the collection and analysis of the samples. We thank Samuel Warren Scott for proofreading the manuscript. We are especially grateful to HS Orka, Landsvirkjun,
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