Publication Date:
2020-12-09
Description:
We report a new case of methane (CH4) of apparent abiotic origin in continental serpentinized ultramafic
rocks. Multiple analytical techniques, on-site and in the laboratory, revealed methane and ethane
degassing from hyperalkaline (pH 〉 11) Ca2þ
eOH mineral waters in boreholes drilled in the Alter-do-
Chão igneous intrusion, at Cabeço de Vide, in mainland Portugal. The C and H isotopic composition of CH4
(d13C w 20&; d2H: 283&) suggests a dominant abiotic origin, although minor thermogenic contributions
cannot be excluded. Similarly, low methane-to-ethane ratios suggest a predominantly nonmicrobial
source, consistent with previous microbiological data showing the lack of methanogenic
archaea in these waters. Heavier hydrocarbons, CO2 and H2 are below detection limits. This case study
confirms that CH4 from serpentinized ultramafic rocks can be transported by hyperalkaline fluids linked
to deep circulation of meteoric waters. Maximum depth of Cabeço de Vide serpentinized rocks is less
than 1 km, and present temperatures are likely lower than 50 C. Serpentinization and related gas formation
may have occurred at any time during thermal evolution of the igneous intrusion, so gas formation
temperature cannot be easily determined. This case is an opportunity to test thermometry
provided by CH4 isotopologue analyses. The existence of methane in continental serpentinized igneous
rocks is more widespread than previously thought and petroleum systems with similar serpentinized
ultramafics in reservoir rocks may have traces of the observed 13C-enriched CH4
Description:
Published
Description:
12-16
Description:
7A. Geofisica di esplorazione
Description:
JCR Journal
Description:
restricted
Keywords:
Abiotic methane, serpentinization
;
03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
