Kinetics of D/H isotope fractionation between molecular hydrogen and water

Journal Article · Sat Dec 01 00:00:00 EST 2018 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2018.09.015· OSTI ID:1570230

Pester, Nicholas J. ^[1]; Conrad, Mark E. ^[2]; Knauss, Kevin G. ^[2]; DePaolo, Donald J. ^[1]

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1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Earth and Environmental Sciences Area; Univ. of California, Berkeley, CA (United States). Dept. of Earth and Planetary Science
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Earth and Environmental Sciences Area

At equilibrium, the D/H isotope fractionation factor between H₂ and H₂O (α_H2O-H2(eq)) is a sensitive indicator of temperature, and has been used as a geothermometer for natural springs and gas discharges. However, δD_H2 measured in spring waters may underestimate subsurface temperatures of origin due to partial isotopic re-equilibration during ascent and cooling. In this work, we present new experimental data on the kinetics of D–H exchange for H₂ dissolved in liquid water at temperatures below 100 °C. Comparing these results with published exchange rates obtained from gas phase experiments (100–400 °C), we derive a consistent activation energy of 52 kJ/mol, and the following rate expressions; ln k =9.186 - 6298/T and k₁ = 9764.61[H₂O] e-6298/T where T is absolute temperature (K), k is the universal rate constant ([L/mol]/hr), and k₁ is a pseudo-first-order constant (hr^-1) applicable to water-dominated terrestrial systems by constraining [H₂O] as the density of H₂O (in mol/L) at the P-T of interest. The density-dependent rate constant accounts for the kinetic disparity of D–H exchange with H₂ when dissolved in liquid H₂O relative to a gas/steam phase, exemplifed by 1/k₁ at 100 °C of ~2 days in liquid, versus ~7 yrs in saturated steam. This difference may explain the high variability of δDH₂ observed in fumarolic gases. Fluids convecting in the crust frequently reach T > 225 °C, where isotopic equilibrium is rapidly attained (<1 hr). We compare fractionation factors measured in natural fluids (α_OBS) with values expected for equilibrium at the T of acquisition. Where these values differ, we use kinetic models to estimate cooling rates during upward advection that account for the observed disequilibrium. Models fit to fluids from Yellowstone Park and the Lost City (deep-sea) vent field, both recovered at ~90 °C, require respective transit times of ~7 hrs and ~11 days between higher temperature reaction zones and the surface. Using estimates of subsurface depths of origin, however, suggests similar mean fluid flow rates (10 s of meters/hr). Additional complications must be considered when interpreting the δDH₂ of lower-temperature effluent. When applied to data from deep-sea hydrothermal systems, our kinetic models indicate microbial catalysis accelerates D–H exchange once fluids cool below ~60 °C. The H₂ measured in both continental alkaline springs and fracture fluids from Precambrian shield rock is likely produced at T < 100 °C, through processes such as serpentinization. In these settings, δDH₂ values appear closer to equilibrium with H₂O than those from geothermal systems. Considering kinetic isotope effects may yield H₂ that is out of equilibrium when generated at lower temperatures, we calculate maximum (isothermal) times to apparent isotopic equilibrium of 1.3 yrs at 50 °C, 9 yrs at 25 °C, and 35 yrs at 10 °C. A similar calculation applied to Antarctic brines (-13 °C), where measured δDH₂ is far from equilibrium, yields ~350 yrs. This time is shorter than the fluids have been isolated (2.8 ka), suggesting kinetic isotope effects associated with H₂ destruction or loss via diffusion may also be possible.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1570230
• Alternate ID(s): OSTI ID: 1635903
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 242, Issue C; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
hydrogen isotopes
isotope exchange kinetics
hydrothermal system
Lost City
volcanic gas
serpentinization
microbial catalysis
geothermometer