Article,
A thermodynamic assessment of the potential synthesis of condensed hydrocarbons during cooling and dilution of volcanic gases
Journal of Geophysical Research-Solid Earth, 105 (B1): 539--559 (2000)
Abstract
The possibility for abiotic synthesis of condensed hydrocarbons in
cooling/diluting terrestrial volcanic gases has been evaluated on
the basis of the consideration of metastable chemical equilibria
involving gaseous CO, CO2, H-2 and H2O. The stabilities of n-alkanes
and polycyclic aromatic hydrocarbons (PAHs) have been evaluated for
several typical volcanic gas compositions under various conditions
for cooling/diluting of queached volcanic gas. The modeling shows
that n-alkanes and PAHs have a thermodynamic potential to form metastably
from H-2 and CO below similar to 250 degrees C within the stability
field of graphite. Despite the predominance of CO2 in volcanic gases,
synthesis of hydrocarbons from CO2 and H-2 is less favored energetically
than from CO and H-2. Both low temperature and a high H/C atomic
ratio in volcanic gas generally favor stability of hydrocarbons with
higher H/C ratios. PAHs are thermodynamically stable at temperatures
similar to 10 degrees-50 degrees C higher than large n-alkanes; however,
at lower temperatures, PAHs and n-alkanes have similar stabilities
and are likely to form metastable mixtures. Both the energetic drive
to form hydrocarbons and possible temperatures of formation increase
as the oxidation state (fO(2)) of the volcanic gases decreases and
as the cooling/dilution ratios of volcanic gases increase. Synthesis
of hydrocarbons is energetically more likely in cooling trapped gases
than in ashcloud eruptive columns. Mechanisms for hydrocarbon formation
may include Fischer-Tropsch-type synthesis catalyzed by magnetite
from solid volcanic products. On the early Earth, Mars, and Jupiter's
satellite Europa, several factors would have provided more favorable
conditions for hydrocarbon synthesis in volcanic gases than under
current terrestrial conditions and might have contributed to the
production of organic compounds required for the emergence of life.
