Publication Date:
2020-02-24
Description:
The occurrence and origin of methane (CH4) generated by serpentinization of ultramafic rocks is of current
timely interest in planetary geology, astrobiology and energy resource exploration, as it may contribute, in
particular, to decipher the source of methane on Mars, the origin of life and the potential of abiotic hydrocarbon
synthesis. Methane of dominant abiotic origin in serpentinized peridotites on continents (ophiolites or
igneous intrusions) has been documented so far, with complete carbon and hydrogen isotope composition,
in six countries, in the Philippines, Turkey, Oman, New Zealand, Japan and Italy. We report the discovery of
two new sites in Greece, at Archani and Ekkara, located in the Othrys ophiolite massif. Portable sensors
based on Fourier Transform InfraRed spectrometry (FTIR) and Tunable Diode Laser Absorption Spectroscopy
(TDLAS) allowed to realize that out of 21 ophiolitic springs, methane is released only by four hyperalkaline
(pH from 10.7 to 11.3) and calcium hydroxide (Ca–OH) type waters; all other 17 springs with pH b 8.7
and magnesium-rich waters in the Pindos, Vourinos and Veria ophiolites, do not show methane. This correlation
between gas occurrence and water type seems to occur worldwide; accordingly, CH4 production appears
to be intimately related to the depth and residence time of the circulating meteoric waters. Methane is emitted
into the atmosphere also from the soil surrounding the hyperalkaline springs, with fluxes of the same order of
magnitude (~102–103 mg m−2 day−1) of seepage typically observed over conventional petroleum systems.
Othrys CH4 has an isotopic composition (δ13C from −27‰ to −37.3‰ VPDB, δ2H from −250‰ to −311‰
VSMOW) similar to that reported in ultramafic rocks in New Zealand and Japan, and in Precambrian crystalline
shields, which were considered dominantly abiotic and probably derived from Fischer–Tropsch Type reactions.
The paucity of CO2, which is the norm in hyperalkaline waters, and of other hydrocarbons prevents from evaluating
possible mixing of gas of different sources, including microbial methanogenesis. Also the H2 content is trivial,
notwithstanding it being a typical product of serpentinization; this could be due to complete H2 consumption
by CO2 reduction in a limited or decreased H2 production system due, for example, to a late stage of increased
silica activity, as suggested by preliminary petrographic observations. The low geothermal gradient of the area
and the present-day serpentinization imply that,whatever the CH4 production mechanism, it took place at temperatures
below those traditionally considered for the origin of abiotic methane in hydrothermal systems.
Description:
Published
Description:
161-174
Description:
7A. Geofisica di esplorazione
Description:
JCR Journal
Description:
restricted
Keywords:
Abiotic methane, serpentinization, ophiolite
;
03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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