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Unlabelled: The oxidation of molecular hydrogen (H2) is thought to be a major source of metabolic energy for life in the deep subsurface on Earth, and it could likewise support any extant biosphere on Mars, where stable habitable environments are probably limited to the subsurface. Faulting and fracturing may stimulate the supply of H2 from several sources. We report the H2 content of fluids present in terrestrial rocks formed by brittle fracturing on fault planes (pseudotachylites and cataclasites), along with protolith control samples. The fluids are dominated by water and include H2 at abundances sufficient to support hydrogenotrophic microorganisms, with strong H2 enrichments in the pseudotachylites compared to the controls. Weaker and less consistent H2 enrichments are observed in the cataclasites, which represent less intense seismic friction than the pseudotachylites. The enrichments agree quantitatively with previous experimental measurements of frictionally driven H2 formation during rock fracturing. We find that conservative estimates of current martian global seismicity predict episodic H2 generation by Marsquakes in quantities useful to hydrogenotrophs over a range of scales and recurrence times. On both Earth and Mars, secondary release of H2 may also accompany the breakdown of ancient fault rocks, which are particularly abundant in the pervasively fractured martian crust. This study strengthens the case for the astrobiological investigation of ancient martian fracture systems., Key Words: Deep biosphere-Faults-Fault rocks-Seismic activity-Hydrogen-Mars. Astrobiology 16, 690-702.

Stable isotope compositions of ancient halite fluid inclusions have been recognized to be valuable tools for reconstructing past environments. Nevertheless, in order to better understand the genesis of halite deposits, it could be of great interest to combine both δ2H and δ18O measurements of the water trapped as inclusions in the defects of the mineral lattice. We developed a method combining off‐axis integrated cavity output spectroscopy (OA‐ICOS) connected on line with a modified elemental analyzer (EA‐OA‐ICOS) to perform those measurements. The first step was to test the method with synthetic halite crystals precipitated in the laboratory from isotopically calibrated waters. Water isotopic signatures have been measured with conventional techniques, equilibration for δ18O and chromium reduction for δ2H. Then, we modified and optimized a conventional EA to connect it online with an OA‐ICOS instrument for H2O measurements. The technique is first evaluated for calibrated free water samples. The technique is also evaluated for salt matrix effect, accuracy, and linearity for both isotopic signatures. Then, the technique is used to measure simultaneously δ2H and δ18O values of halite water inclusions precipitated from the evaporation experiments. Data generated with this new technique appeared to be comparable with those inferred from prior off‐line technique studies. The advantages offered by the OA‐ICOS technique are the simultaneous acquisition of both isotopic ratios and the substantial reduction of data acquisition time and sample aliquot size. Natural halite samples have been analyzed with this method. Natural halite samples as old as Precambrian have also been analyzed with this method. [ABSTRACT FROM AUTHOR]

We demonstrate an improved method based on continuous-flow elemental analyser pyrolysis isotopic ratio mass spectrometry (CF-EA-PY-IRMS) to measure the ²H/¹H ratios of water trapped in halite crystals. Two challenges to overcome are the low hydrogen concentration of samples (10-50 μmol H2·g-1) and the high chloride concentration released when reacting halite in an elemental analyser. We describe an optimization procedure for determining the ²H/¹H ratio of this trapped water with an acceptable accuracy. This technique involves the use of a high-temperature Cr reactor to quantitatively convert H2O into H2. The initial step was performed on halite crystals precipitated from a water reservoir where ²H/¹H ratios were monitored from its initial stage until the end of evaporation. The ²H/¹H isotopic analyses were automated online in continuous-flow mode. Precision of the method was determined for those "synthetic" samples with hydrogen concentrations ranging from 0.2 to 0.5 wt%. ²H/¹H isotopic ratios of evaporating waters bracket the compositions of water inclusions. The formation of fluid inclusions is not instantaneous and records the isotopic signature of the residual waters across a time range during which the isotopic values of the water still evolve. This property explains why the δ²HVSMOW standard deviation of ±5‰ (2σ) observed for 10-mg aliquots of halite exceeds the instrumental error (about ±1.5‰ 2σ) determined on the basis of IAEA-CH7, NBS 30, and NBS 22 references along with calibrated waters with and without added halite crystals. We also applied this method to Mesoproterozoic (1.4 Ga) and Neoproterozoic (0.8 Ga) halite samples with relatively low hydrogen concentrations (300-1500 ppm). The measured δ²HVSMOW values for Precambrian waters range from -89‰to -54‰. We propose that this technique offers a new perspective and great potential for palaeoenvironmental reconstructions based on the ²H/¹H analyses of water trapped in halite. [ABSTRACT FROM AUTHOR]

A suite of Permian sylvite samples from Boulby potash mine, Yorkshire, UK, consistently yield traces of hydrogen upon analysis by a cold crush technique for liberating volatiles from entrapped fluid inclusions. In contrast, accompanying halite samples do not yield hydrogen. These data suggest the formation of hydrogen by radiolysis of water due to irradiation from potassium in the sylvite. The data indicate radiolysis as a mechanism for subsurface hydrogen generation, where it is available as an electron donor for a deep biosphere. [ABSTRACT FROM AUTHOR]