Miller, Hannah M听1听;听Matter, Juerg听2听;听Kelemen, Peter听3听;听Ellison, Eric听4听;Templeton, Alexis听5
1听University of Colorado Boulder
2听University of Southampton, UK
3听Lamont-Doherty Earth Observatory of Columbia University
4听University of Colorado at Boulder
5听University of Colorado Boulder
Hyperalkaline fluids (pH >10), rich in Ca2+, H2听and CH4, accessed from subsurface wells situated in the Samail ophiolite in Oman were studied to investigate their geochemistry, gas concentrations, and isotopic signatures. This data was integrated with analyses of recovered well chips to elucidate alteration processes affecting ultramafic rocks in the subsurface of Oman as they undergo modern water/rock reactions. All previous work on Oman serpentinizing fluids has focused on surface seeps, which quickly lose their reducing character and precipitate carbonates when they contact the atmosphere. Analyzing the geochemistry from the subsurface itself, and the rocks reacting with those fluids, provides much greater insight into the operative reactions in serpentinizing aquifers.
Wells at 33C contain around 30mM of Cl-, 3mM Ca2+, negligible Mg+听(<0.007mM) and no detectable dissolved organic carbon. The subsurface fluids contain 0.17-0.67mM H2听of -680 to -685.7 per mil 未D H2听predicted to have formed at ~50C. CH4听is also found in the fluids (0.04-1.44 mM) with heavy 未13C CH4听per mil values (2.4-3) and 未D CH4听from -205 to -232 per mil. The CH4听falls into the traditionally interpreted abiotic range based off isotopic signatures and has the most isotopically heavy carbon reported in the literature thus far. These gas-rich fluids have evolved through low temperature oxidation and hydration of reduced Fe-species present in the peridotite. Two distinct generations of serpentine are present as visualized by micro-Raman microscopy. Magnetite veins are closely associated with the generation two serpentine, and 2-10um amorphous black specks of magnetite overprint all minerals in the well chips. The extensive magnetite formation and Fe(III)-rich serpentine both may contribute to H2generation at low temperatures.
Understanding the aqueous geochemistry and mineralogy in the subsurface has provided insight into the geochemistry that can be support microbial life in the subsurface as H2听and CH4听are powerful electron donors that can be paired with nitrate-,sulfate-,and iron-reduction as well as methanogenesis and anaerobic methane oxidation. Preliminary 16S rRNA sequencing of fluids from well water reveals a mixture of anaerobic and aerobic organisms with a variety of metabolisms linked to the geochemical environment.