Tectonics, Serpentinization and Natural Hydrogen Generation at the Continent-Ocean Transition in Magma-poor Rifted Margins

Serpentinization of ultramafic rocks results from relatively low-temperature metamorphic reactions and is associated with the generation of natural hydrogen. Ocean opening at magma-poor rifted margins occurs under relatively cold conditions and provides a window of opportunity for the emplacement of shallow peridotites, firstly beneath thinned continental crust in the hyperextended distal margin, and secondly in the subsequent oceanward exhumed mantle at the continent-ocean transition [COT] before oceanic spreading is established. The domain covering both the distal margin and the COT is substantial, and interpretation of seismic data suggests that a variable width band of about 100 km of exhumed mantle persists along both conjugate margins of the southern North Atlantic. This raises questions about how much hydrogen has been produced in the past, how much is currently accumulated under sedimentary successions and, finally, the current hydration state of the shallow mantle in order to assess its potential as an energy source to produce additional hydrogen via stimulation. The occurrence of serpentinization and hydrogen generation in the lherzolitic rocks at magma-poor margins depends on the presence of suitable thermodynamic conditions in terms of pressure and temperature, as well as access to water. The latter requires the embrittlement of the rifted margin crust, which also depends on the rheological properties of the mantle and continental rocks, the thermal regime, and the tectonic stresses. The sequence of mantle upwelling in the distal margin and the COT, and how it is related to the deposition of sedimentary layers is also of key importance, as sediment provides a blanket of low-permeability that may promote trapping and profoundly affects the patterns of hydrothermal circulation and so the temperature field. Temperature is a key factor as it modulates the thermodynamics of serpentinization, as well as the 120ºC isotherm, which is relevant to the limits of life that can lead to biological hydrogen consumption. Thus, extension rates in rifted margins strongly control the onset of magmatism and the serpentinization of the shallow mantle beneath the hyperextended thinned continental crust and the COT. By coupling a geodynamic model with thermodynamic calculations, we discuss the above effects, focusing on the sensitivity of serpentinization and molecular hydrogen generation in the distal margin and the COT to spreading rates as control factor. Simulations indicate that at full spreading rates of 15–20 mm.yr^-1 past hydrogen generation is likely to reach its optimal conditions, whereas spreading at 30 mm.yr^-1 has less opportunity for the presence of shallow serpentinized mantle and hydrogen production, marking the transition to faster spreading styles.