Hydrogen generation during hydrothermal alteration of peralkaline granite

It is well known that oxidation of ferrous to ferric iron by water can generate molecular hydrogen (H2), with the most widely recognized natural manifestation being serpentinization olivine and pyroxene in ultramafic rocks. A less yet extremely important source H2 are peralkaline igneous intrusions, where spectacular enrichments documented from fluid inclusions as free gas migrating through fractured rocks soils. Of these occurrences, best studied those at Strange Lake Canada, Lovozero Khibiny Russia, Ilímaussaq Greenland. Based on petrographic observations analysis, it has been proposed hydrothermal alteration arfvedsonite, an FeII-bearing amphibole, this context, although be unequivocally demonstrated. To investigate generation during granites, we performed experiments pure arfvedsonite grains arfvedsonite-bearing granite (10 wt% arfvedsonite) pluton (Canada). These materials, presence aqueous solutions, were sealed inside gold capsules or placed within titanium autoclaves, which allowed monitoring function temperature (280–400 °C), chlorinity (0 3 m NaCl), pH, starting mineral assemblage. Blank conducted quantify background amounts generated Au/Ti oxidation, diffusing reaction cells, release otherwise occluded minerals. Solids characterized XRD, SEM, TEM FIB foils, STXM-XANES. Outcomes study demonstrate production agpaitic arfvedsonite. The rate production, normalized specific surface area increases 1100 2200 pmol cm−2 day−1 between 280 400 °C, respectively. Chlorinity tends have a negative impact rate, while circumneutral alkaline conditions clearly promote generation. Altering whole samples instead only also enhances rate. aluminum, released microcline albite dissolution, may increase both solubility dissolution promoting precipitation phyllosilicates. At least two different types phyllosilicates observed, chlorite smectite. Magnetite secondary zircon identified reacted (Zr content = 1200 ppm). rates reported here 280–400 °C comparable, even faster than for harzburgite. major feature plutons formation aegirine replacing mineral. However, was never observed solids our experiments, under condition tested (up bar). This effect pressure, oxygen fugacity, progress, parameters remain investigated better constrain mechanism alteration. Agpaitic intrusions thus represent fertile geological setting deep microbial subsurface ecosystems, abiotic synthesis organic molecules, exploration alternative energy.