Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability

The Samail
ophiolite in Oman is undergoing modern hydration and carbonation of peridotite
and may host a deep subsurface biosphere. Previous investigations of
hyperalkaline fluids in Oman have focused on fluids released at surface seeps,
which quickly lose their reducing character and precipitate carbonates upon
contact with the O2/CO2–rich atmosphere. In this work,
geochemical analysis of rocks and fluids from the subsurface provides new insights
into the operative reactions in serpentinizing aquifers. Serpentinite rock and
hyperalkaline fluids (pH >10), which exhibit millimolar concentrations of Ca2+,
H2 and CH4, as well as variable sulfate and nitrate, were
accessed from wells situated in mantle peridotite near Ibra and studied to
investigate their aqueous geochemistry, gas concentrations, isotopic
signatures, mineralogy, Fe speciation and microbial community composition.
The bulk
mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite,
serpentine and magnetite.  At depth,
Fe-bearing brucite is commonly intermixed with serpentine, whereas near the
surface, olivine and brucite are lost and increased magnetite and serpentine is
detected.  Micro-Raman spectroscopy
reveals at least two distinct generations of serpentine present in drill
cuttings recovered from several depths from two wells. Fe K-edge x-ray
absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a
strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and
Fe(III) in the serpentine. Magnetite veins are also closely associated with
this second generation serpentine, and 2-10µm magnetite grains overprint all
minerals in the drill cuttings. Thus we propose that the dissolved H2
that accumulates in the subsurface hyperalkaline fluids was evolved through low
temperature oxidation and hydration of relict olivine, as well as
destabilization of pre-existing brucite present in the partially serpentinized
dunites and harzburgites.  In particular,
we hypothesize that Fe-bearing brucite is currently reacting with dissolved
silica in the aquifer fluids to generate late-stage magnetite, additional
serpentine and dissolved H2.

Dissolved CH4
in the fluids exhibits the most isotopically heavy carbon in CH4
reported in the literature thus far. The CH4 may have formed through
abiotic reduction of dissolved CO2 or through biogenic pathways
under extreme carbon limitation. The
methane isotopic composition may have also been modified by significant methane
oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an
abundance of Meiothermus,
Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters).
The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The
composition of these microbial communities suggests that low-temperature
hydrogen and methane generation, coupled with the presence of electron
acceptors such as nitrate and sulfate, sustains subsurface microbial life
within the Oman ophiolite.