Equilibrium and kinetic controls on molecular hydrogen abundance and hydrogen isotope fractionation in hydrothermal fluids

https://doi.org/10.1016/j.epsl.2021.117338Get rights and content

Highlights

  • H2 in hydrothermal fluids is formed by H2O reduction in response to iron oxidation.

  • Hydrogen abundance in hydrothermal fluids is controlled by metastable equilibria.

  • H2 fugacity depends on temperature, r/w ratio and reservoir fluid composition.

  • δD-H2O and equilibrium isotope fractionation control δD-H2 of reservoir fluids.

  • The extent of H-isotope disequilibrium reflects reservoir-to-surface travel times.

Abstract

Molecular hydrogen (H2) is among the main components in hydrothermal fluids and exerts a major role in geochemical and biological processes. Nevertheless, the understanding and quantification of factors controlling H2 abundance and isotope composition remain uncertain. Sub-aerial hydrothermal systems provide a unique opportunity to study sources and reactions of H2. Here, we present hydrogen fugacity (fH2) and δD values of H2 and H2O in hydrothermal fluids from such systems worldwide (Greece, Iceland, Kenya and New Zealand), representative of mid-ocean ridge and arc systems and sourced by seawater and meteoric water. The hydrothermal fluid temperatures are 226-359 °C, and the fH2, δD-H2 and δD-H2O values are 0.002-3.3 bar, -646 to -391‰ and -94.1 to +11.3‰, respectively. Comparison of measured data with geochemical modeling reveals that H2 is predominantly sourced from the reduction of H2O, enabled through the oxidation of aqueous Fe+II to Fe+III and precipitation of Fe+III-bearing hydrothermal minerals (e.g., epidote). We argue that fH2 in hydrothermal fluids is controlled by metastable mineral-fluid and fluid-fluid equilibria upon progressive rock alteration and depend on temperature, rock-to-water ratio (r/w), source water composition and volcanic gas input. Consequently, fH2 in these systems is not fixed by a sole redox buffer. Our data demonstrate that δD-H2 values of reservoir fluids at depth are controlled by the δD value of the source water and the equilibrium isotope fractionation characteristic of the hydrothermal reservoir temperatures. Attainment of isotopic equilibrium at reservoir conditions is supported by fast isotopic equilibration times at T > 200 °C, on the order of minutes to few hours, relative to hydrothermal fluid residence times within the reservoirs of years to decades or even longer. During its ascent to the surface, the isotopic composition of H2 can be modified due to isotopic re-equilibration with H2O. The extent of re-equilibration depends on the interplay between fluid flow velocity, cooling rate and kinetics of D-H exchange between H2 and H2O. For fast flowing well discharges (∼0.1 to >10 m/s), negligible changes occur, whereas for slowly flowing (<0.06 m/s) fumaroles, complete re-equilibration at surface temperatures (∼100 °C) is sometimes observed. Based on the extent of hydrogen isotope disequilibrium relative to reservoir conditions, fluid reservoir-to-surface travel times are estimated to range from few minutes to <3 hrs for well fluids and few hours to days for fumarolic vapor.

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 10×103 to 105×103 mol% and from 1×103 to 87×103 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+, Fe(OH)4, H2S, HS, SO42, CH4, HCO3) 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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