UNDERSTANDING THE NATURE OF WATER-ROCK INTERACTIONS IN A SERPENTINIZING SYSTEM: IMPLICATIONS FOR PLANETARY EXPLORATION AND SUBSURFACE HABITABILITY

Orbital detections of serpentine on Mars are thought to be important in assessing subsurface habitability, especially with the discovery of the Lost City hydrothermal system on Earth where gases generated by serpentinization (H₂, CH­₄) sustain communities of microbes. H₂ is produced from oxidation of Fe²⁺ to Fe³⁺ which goes into serpentine (first octahedral, then tetrahedral sites [Marcaillou et al., 2011, EPSL]) and magnetite, and CH­₄ forms biotically or abiotically from H₂. The goals of this work are to determine the nature of water-rock interactions, fluid chemistries, and past H₂ production in an Early Ordovician serpentinizing system using spectroscopic and isotopic analyses.

Serpentinites were studied during the Canadian Space Agency’s 2012 Mars Methane Analogue Mission in Norbestos, Quebec. Rock walls were imaged with a visible hyperspectral imager (420-720 nm), and samples were imaged with the same imager and a near infrared imager (650-1100 nm). Stable isotope analyses were done on carbonate coatings on serpentinites. Other laboratory analyses include ICP-AES, C content, XRD, and Mössbauer spectroscopy.

Hyperspectral imaging allows mapping of variations in Fe oxidation state and coordination environments within serpentinite samples and outcrops. Regions enriched in tetrahedral Fe³⁺ were used to infer where serpentinization proceeded furthest and associated volumes of H₂ were greatest. Carbonates likely were precipitated once fluids became supersaturated with respect to Ca²⁺ derived from a harzburgite protolith. Stable isotope analyses of C and O in carbonates suggest two phases of carbonate formation. Samples south of a shear zone formed in a closed system at elevated temperatures with fluids from a deep groundwater source. These samples are enriched in ¹³C likely due to production of CH₄ (low δ¹³C) in a system closed to C addition where CH₄could escape. In contrast, carbonates within the shear zone probably formed more recently at lower temperatures, and isotopes follow a trend typical of kinetic fractionation.

The degree of serpentinization and H₂ production has been inferred, and carbonates show signatures of ancient serpentinization events and production of CH₄. These results suggest that similar recorders of habitable conditions could be preserved in serpentine-bearing rocks on Mars.