¹Rebecca N. Greenberger,¹John F. Mustard,²Edward A. Cloutis,³Lisa M. Pratt,³Peter E. Sauer,²Paul Mann,⁴Kathryn Turner,⁵M. Darby Dyar,³David L. Bish
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook St, Box 1846, Providence, RI 02912, USA
²Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, R3B 2E9, Canada
³Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405-1405, USA
⁴Department of Physics, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, R3B 2E9, Canada
⁵Mount Holyoke College, Department of Astronomy, 50 College St., South Hadley, MA 01075, USA

Molecular hydrogen produced through iron oxidation during formation of serpentine and magnetite can sustain terrestrial subsurface ecosystems. The Fe3+ in serpentine partitions into octahedral and tetrahedral sites differently as serpentinization proceeds, and tetrahedral Fe3+ is present toward the end of serpentinization. We map Fe oxidation states in a serpentinite to determine the degree to which serpentinization progressed and where hydrogen production has been maximized to assess habitability at an abandoned chrysotile mine in Norbestos, Quebec, in association with the Canadian Space Agency’s Mars Methane Analogue Mission. We also analyzed stable isotopes of carbon and oxygen in carbonates to constrain the conditions of water–rock interaction during serpentinization. Iron oxidation and coordination was determined through field imaging of rock walls with a visible hyperspectral imager (420–720 nm), and samples collected from imaged rocks and elsewhere in the mine were imaged in the laboratory (420–1100 nm). Sample chemistry, mineralogy, and oxidation state were determined with laboratory measurements of visible through mid-infrared reflectance spectra, major element chemistry, mineralogy, and Mössbauer spectroscopy. Mapping with hyperspectral imaging of outcrops and hand samples shows that tetrahedral Fe3+ is common in serpentinites at this site, and results are confirmed through other measurements. Major element chemistry and mineralogy are consistent with serpentine plus minor carbonate. Carbonate samples show an exceptional range in δ13C (−13.14 to +16.12‰+16.12‰ VPDB) and δ18O (−15.48 to −3.20‰−3.20‰ VPDB) that vary with location in the mine. Carbonates south of a shear zone (δ13C more positive) likely formed during periods of serpentinization in a carbon-limited reservoir closed to carbon addition but open to methane escape. Carbonates in a shear zone (δ13C more negative) probably formed later at low temperatures through CO2-metasomatism or atmospheric weathering, and isotopic trends are consistent with kinetic fractionation. The extensive presence of tetrahedral Fe3+ in serpentine shows the system liberally produced H2 while the isotope systematics have implications for preservation of indicators of the aqueous conditions that formed serpentinites on Mars and their habitability.

Reference
Greenberger RN, Mustard JF, Cloutis EA, Pratt LM, Sauer PE, Mann P, Turner K, Dyar MD, Bish DL (2015) Serpentinization, iron oxidation, and aqueous conditions in an ophiolite: Implications for hydrogen production and habitability on Mars. Earth and Planetary Science Letters 416, 21–34
Link to Article [doi:10.1016/j.epsl.2015.02.002]

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