Low temperature alteration of serpentinized ultramafic rock and implications for microbial life

Alteration (serpentinization) of ultramafic rocks is known to generate hydrogen through oxidation of ferrous iron in olivine or pyroxene and reduction of water. Hydrogen oxidation can act as a primary source for chemolithotrophic microbial communities and it is suggested that hydrogen availability is essential for the existence of a deep subsurface biosphere. In this study we focus on ongoing low-temperature water–rock processes in partially serpentinized dunite from the Leka Ophiolite Complex, mid-Norway. We aim to determine whether these reactions release reduced compounds and nutrients that can be utilized by the associated microbial communities. The study is based on the chemical and physical alteration features of a 51 m deep rock core as well as the chemical composition of groundwater from the core's borehole. The borehole intersects three main fracture zones. Observations during drilling indicated that water infiltration largely takes place along the deepest zone. The rock in the fracture zones is nearly completely altered to serpentine, brucite, magnetite and minor calcite. An increase in pH and decrease in Mg, Si and alkalinity of the groundwater along the borehole from the deepest to the outermost fracture zone reflect ongoing water–rock reactions where dissolution of brucite, serpentine and calcite and precipitation of chrysotile, hydromagnesite and iron-hydroxide are most likely to occur. Detection of both H2 and O2 in the groundwater influenced by the outermost fracture zone suggests that the local development of highly reducing conditions in the narrow fractures is due to low water/rock ratios and to dissolution of Fe(II)-containing brucite and serpentine resulting in the formation and successive diffusion of H2 into the oxic borehole. Oxidation of H2, reduced Fe and Mn, NH4+ and CH4 are all potential energy-yielding reactions that could be utilized by chemolithotrophic microorganisms using dissolved organic or inorganic carbon as possible carbon sources. Methane could result from microbial oxidation of H2 and reduction of CO2, or from the degradation of organic matter. The results demonstrate that low-temperature water–rock reactions in serpentinized ultramafic rock lead to production of reduced chemical species that could support primary producers in diverse microbial communities.

► In this study we examine ongoing water–rock reactions in serpentinized dunite. ► Dissolution and precipitation of brucite, serpentine, calcite/hydromagnesite dominate. ► H2 is produced through reactions with Fe(II)-containing brucite and serpentine. ► Local redox conditions provide a range of habitats for chemolithotrophic organisms.