Hydrogen Produced by Reduction of H2O in Rock Reaction: Peridotite vs Basalt

摘要

The extraction of abiogenic H2 from the earth as an energy resource[1] is a possibility raised by the recognition that ultramafic rocks yield hydrogen in the course of serpentinization [2–5], through which water is reduced to H2 as ferrous iron from peridotite minerals oxidizes. Given the straightforward oxidation of iron silicate to magnetite to make H2, it would appear that other mafic rocks such as basalt should also yield H2, however, H2 production from basalt-hosted springs and geothermal fluids is small, e.g. Icelandic waters, as compiled by Arnorsson, et al. [6]. To examine whether the apparent difference in H2 production between peridotite and basalt is rooted in thermodynamically constrained mineral-water equilibria, we compare computed reactions of water with basalt and peridotite. We find that the serpentinization of peridotite yields abundant H2 as ferrous iron silicates oxidize to form magnetite, but that little H2 forms in basalt reaction because ferrous silicates alter to ferrous iron chlorite instead of magnetite or other ferric minerals. A principal cause of the difference is that chlorite requires alumina to form, but alumina concentration is minimal in peridotite although substantial in basalt.