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Abstract

Naturally occurring hydrogen gas (H2) represents a potentially major source of clean energy. It has been relatively overlooked so far but has gained more attention recently. The most promising mechanism for large-scale generation of such natural H2 is the serpentinization of mantle material as it reacts with water while it is brought into the “serpentinization window” (i.e., T = 200-350˚C) during mantle exhumation. We study such serpentinization-related natural H2 generation during rifting and subsequent rift inversion by means of numerical geodynamic models. In these numerical models we trace how, when, and where mantle material enters the serpentinization window, as well as when active, large-scale faults penetrate exhumed mantle bodies allowing for water circulation and serpentinization to occur. Although serpentinization-related natural H2 generation is a phenomenon best known from magma-poor rifted margins and slow spreading ridges, we find that volumes of natural H2 generated during inversion may be up to 20 times higher than during rifting, due to the colder thermal regime in rift-inversion orogenic environments. Moreover, suitable reservoirs and seals required for natural H2 accumulation are readily available in rift-inversion orogens, whereas they may not be present when serpentinization occurs in rift or drift settings. Our model results thus provide a first-order motivation to turn to rift-inversion orogens, rather than rifts and rifted margins, for natural H2 exploration. These model-derived insights are supported by indications of natural H2 generation in rift-inversion orogens such as the Western Alps, Pyrenees, and Caucasus.