H2-rich natural gas seepages on Earth: where do they come from?
We present the results of studies (geological context and geochemical characteristics) concerning H 2 -rich gas seepages in different areas of the Earth, comparing offshore and onshore geological contexts, and focusing notably in exhumed terrestrial mantle rocks (ophiolitic rocks) and in intraplate domains (including at depth old continental crust). These gas seepages are in all cases made up of mixtures of 3 main components which are H 2 , CH 4 and N 2 in various proportions, with locally relatively high contents in helium in onshore context. Furthermore, in hydrothermal volcanic systems, the gas mixtures are associated with the presence of CO 2 . The relative contents of the dominant gases show distinct characteristic types which are interpreted as reflecting different zones of gas generation. In all cases, it appears that H 2 generation is mainly favored by the oxidation of Fe II -rich rocks responsible for water reduction (hydrolysis). In absence of CO 2 , H 2 -bearing gases are either associated to ultra-basic springs (pH 10-12) or they seep directly within the fracture systems of the rocks. In volcanic contexts, pH decreases drastically correlated with the presence of acid gases. CH 4 is produced either directly by reaction of dissolved CO 2 with basic-ultrabasic rocks or, in a second step, by H 2 -CO 2 interaction by abiotic or microbial processes. H 2 is present in the gas when no more carbon is available to generate CH 4 (carbon restriction conditions) or when the H 2 -CO 2 association corresponds to an active and dynamic unstable system (case of volcanic gases along mid-oceanic ridges, hot-spot areas and arc systems). N 2 is issued either from water in equilibrium with atmosphere, or, in case of important enrichment in N 2 and He, it is interpreted as issued mainly from the continental crust, respectively by ammonium destabilization within metamorphic clay and radiogenic decay. Experiments have shown that the presence of ammonium favors H 2 generation. This type of study shows that, despite of a common prejudice, H 2 is not so rare in natural environments and this native H 2 can be seen as a potential resource of clean energy commonly associated with helium for which the demand is constantly increasing.