Aqueous Conditions and Habitability Associated with Formation of a Serpentinite: Using Analyses of Ferric Iron and Stable Carbon Isotopes to Reconstruct Hydrogen Production

Abstract:

Serpentine deposits on Mars have generated significant interest because byproducts of serpentinization, H₂ and CH₄, can be important energy sources for subsurface microbial communities. H₂ is produced through Fe²⁺ oxidation to form magnetite and Fe³⁺-bearing serpentine. In serpentine, Fe³⁺ goes into octahedral sites first, then tetrahedral sites [Marcaillou et al., 2011, EPSL]. We use Fe oxidation state and coordination in an Early Ordovician serpentinite in Norbestos, Quebec, as proxies for H₂ production and stable isotopes of carbonates to understand past aqueous conditions at the Canadian Space Agency’s 2012 Mars Methane Analogue Mission site.

Rock outcrops were imaged with a visible hyperspectral imager (420-720 nm), and samples were imaged in the laboratory with the same imager and a near infrared imager (650-1100 nm). Other analyses determined major element chemistry (ICP-AES and C analyses), mineralogy (XRD), Fe phases (Mössbauer spectroscopy), and stable isotopes of carbonates.

Fe oxidation state and coordination (tetrahedral vs octahedral) were mapped in samples and outcrops using imaging data. We focused on locations with tetrahedral Fe³⁺ in serpentine as these are the most serpentinized sites with maximum H₂ production. Carbonate samples from ~100-200 m south of a shear zone are enriched in ¹³C (δ¹³C up to +16.12‰ vs VPDB) resulting from production of CH₄ depleted in ¹³C in a system closed to C addition but open to CH₄ escape. This alteration occurred at elevated temperatures and low water/rock ratios. In the shear zone, lower δ¹³C values (most < +2‰) positively correlated with δ¹⁸O likely result from kinetic fractionation under recent low temperature conditions.

Spectroscopy suggests that much of this deposit underwent advanced serpentinization to produce significant H₂. Isotopic signatures of carbonates precipitated during serpentinization outside the shear zone illuminate the temperatures (elevated) and chemistries of fluids (high Ca²⁺, low CO₂, alkaline) and gases (H₂, CH₄) in an ancient habitable environment. These results suggest that serpentinites identified on Mars, where there has been limited recent aqueous activity, could preserve evidence of fluid composition and levels of hydrogen production, providing a promising context to search for biosignatures.