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Methane origin in the Samail ophiolite: Comment on “Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability” [Geochim. Cosmochim. Acta 179 (2016) 217–241]
Language
English
Obiettivo Specifico
6A. Geochimica per l'ambiente
Status
Published
JCR Journal
JCR Journal
Title of the book
Issue/vol(year)
/197 (2017)
Pages (printed)
467-470
Issued date
2017
Abstract
Miller et al. (2016) report a new study of fluids in the
peridotites of the Samail ophiolite in Oman related to modern
serpentinization (olivine hydration), a process that can
provide energy and raw materials for chemosynthetic
microbial life. The authors, in particular, report an isotopic
composition for methane (CH4) in groundwater near
Ibra (up to 1.4 mM) that is unusually 13C-enriched
(d13CCH4 +2.4 and +3‰ VPDB), and consider the gas
origin to be uncertain, i.e., abiotic or microbial, and to be
modulated by significant fractionation due to oxidation or
diffusion. The purpose of this comment is to clarify and correct
a few points concerning the possible origin of the
d13CCH4 values, with the intention to promote a fruitful
and constructive debate, considering the interest that there
is for serpentinization and the associated formation of various
gases.
The CH4 data from Miller et al. are re-examined in a global
context of gas in serpentinized peridotites and, in particular,
by considering published data (isotope composition of
CH4 and concentrations of C2+ alkanes) also obtained from
the Samail ophiolite, data neglected by the authors. These
data significantly impact the interpretation of Miller et al.
concerning the possibility that methane can be microbial.
Potential isotopic fractionations by oxidation or diffusion,
evaluated considering d13CCH4–d2HCH4 correlated variations,
the occurrence of significant amounts of ethane and
propane in the Oman ophiolite aquifers and Rayleigh fractionation
analysis suggest that methane can hardly be considered
microbial. Isotopic fractionations, however, are not
necessary to explain the unusual d13CCH4 values: an alternative
hypothesis is that methane carbon may derive from
13C-enriched carbonates occurring below the Samail ophiolite
nappe, a hypothesis not considered by Miller et al.
peridotites of the Samail ophiolite in Oman related to modern
serpentinization (olivine hydration), a process that can
provide energy and raw materials for chemosynthetic
microbial life. The authors, in particular, report an isotopic
composition for methane (CH4) in groundwater near
Ibra (up to 1.4 mM) that is unusually 13C-enriched
(d13CCH4 +2.4 and +3‰ VPDB), and consider the gas
origin to be uncertain, i.e., abiotic or microbial, and to be
modulated by significant fractionation due to oxidation or
diffusion. The purpose of this comment is to clarify and correct
a few points concerning the possible origin of the
d13CCH4 values, with the intention to promote a fruitful
and constructive debate, considering the interest that there
is for serpentinization and the associated formation of various
gases.
The CH4 data from Miller et al. are re-examined in a global
context of gas in serpentinized peridotites and, in particular,
by considering published data (isotope composition of
CH4 and concentrations of C2+ alkanes) also obtained from
the Samail ophiolite, data neglected by the authors. These
data significantly impact the interpretation of Miller et al.
concerning the possibility that methane can be microbial.
Potential isotopic fractionations by oxidation or diffusion,
evaluated considering d13CCH4–d2HCH4 correlated variations,
the occurrence of significant amounts of ethane and
propane in the Oman ophiolite aquifers and Rayleigh fractionation
analysis suggest that methane can hardly be considered
microbial. Isotopic fractionations, however, are not
necessary to explain the unusual d13CCH4 values: an alternative
hypothesis is that methane carbon may derive from
13C-enriched carbonates occurring below the Samail ophiolite
nappe, a hypothesis not considered by Miller et al.
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