Generation of Hydrogen along the Mid-Atlantic Ridge: Onshore and Offshore

Since the 1980s, oceanic ridges have been proven to be sites at which diagenetic processes (such as serpentinization) result in the generation of natural hydrogen, which escapes through oceanic vents. The water depths in this setting and the location of ocean ridges far offshore would seem to preclude exploitation of this resource, but similar geological contexts are found onshore. Iceland is located along the axis of the Mid-Atlantic Ridge (MAR) and is also a hot spot. As a result, the emerging ridge allows for the study of hydrogen generation within this specific oceanic extensional context. Geothermal energy is well developed in Iceland; accordingly, the presence of natural hydrogen is known based on data from numerous geothermal wells which allowed us to constrain the hydrogen occurrences and compare them with MAR emissions. The results show that H 2 contents are high only in the neo-volcanic zone and very low outside the immediate vicinity of this active axis. Values reaching 198 mmol H 2 /kg fluid have been recorded in Landmannalaugar. Farther north, the gas mixture in the Námafjall area reaches up to 57 vol% hydrogen. These well data are in the same range as those along the MAR. The oxidation of ferrous minerals, combined with the reduction of water, allows for the formation of hydrogen. In Iceland, H 2 concentrations in steam seem to be enhanced by both the low concentrations of NaCl in hydrothermal fluids and the strong fracturing of the upper crust, which provides a rapid and constant supply of meteoric fluids for oxidation reactions.