Identifying the processes that govern the hydrogen isotopic composition of H2 and CH4 in fumarolic emissions: a case study from Nisyros, Greece
Final Report Abstract
In this project the applicability of the H2O-CH4-H2 geothermometer to volcanic-hydrothermal systems has been investigated. For this purpose, the hydrogen isotopic compositions of H2, H2O and CH4 in fumaroles (Nisyros, Greece; Iceland) and well discharges (Iceland, New Zealand, Kenya) have been analyzed. In addition to bulk stable isotope data, clumped isotope data on methane was obtained for fumaroles from Nisyros as well as for fumaroles and well discharges from Iceland. Methane predominantly derives from the thermal decomposition of organic matter, whereas molecular hydrogen originates from the reduction of water by aqueous Fe+II. The isotopic H2O-H2 geothermometer is able to record hydrothermal reservoir temperatures prevailing at depth, but only if sampled from rapidly ascending well discharges, i.e. characterized by reservoir-tosurface travel times < 1h. If the ascent of the discharges takes longer, significant isotopic reequilibration of H2 occurs. This, in general, is the case for fumaroles from Nisyros, Greece, and from Iceland, for which travel times between 6h and 1 month are indicated by the deviation of apparent hydrogen isotopic temperature from reservoir temperature. Contrary to molecular hydrogen, there occurs no significant isotopic re-equilibration between methane and water during the ascent of the gases. At locations with reservoir temperatures < 300°C, methane in fumaroles and well discharges is observed to be in homo- and heterogeneous isotopic disequilibrium with respect to reservoir temperatures and water composition. Hydrogen isotope exchange between water and methane may become significant at temperatures exceeding 300°C. At Nisyros, both the apparent CH4-H2O isotope fractionation and the D13CDH3 clumped isotope composition closely match equilibrium values expected for the measured reservoir temperature of 340°C. However, attainment of full isotopic equilibrium underneath Nisyros is not supported by preliminary D12CD2H2 data. Considering that CH4 and H2 have different origins and are characterized by slow and fast isotopic re-equilibration rates with water, respectively, the H2-CH4 geothermometer cannot be applied to volcanic-hydrothermal systems to derive methane formation and/or reservoir temperatures. If reservoir temperatures are independently known, the H2O-H2 geothermometer can be applied to determine fluid ascent rates.
Publications
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Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons. Geochemical Perspectives Letters, 23-27.
Fiebig, J.; Stefánsson, A.; Ricci, A.; Tassi, F.; Viveiros, F.; Silva, S.; Lopez, T.M.; Schreiber, C.; Hofmann, S. & Mountain, B.W.
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High temperature generation and equilibration of methane in terrestrial geothermal systems: Evidence from clumped isotopologues. Geochimica et Cosmochimica Acta, 309, 209-234.
Beaudry, Patrick; Stefánsson, Andri; Fiebig, Jens; Rhim, Jeemin H. & Ono, Shuhei
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Equilibrium and kinetic controls on molecular hydrogen abundance and hydrogen isotope fractionation in hydrothermal fluids. Earth and Planetary Science Letters, 579, 117338.
Ricci, Andrea; Kleine, Barbara I.; Fiebig, Jens; Gunnarsson-Robin, Jóhann; Mativo, Kamunya Kennedy; Mountain, Bruce & Stefánsson, Andri
