Mantle serpentinization and H2 production during the opening of North Atlantic

The opening of the North Atlantic exhumed a large amount of subcontinental mantle to the seafloor. The exhumed subcontinental mantle could be wider than 100 kilometers (Whitmarsh et al., 2001) and have developed significant low-temperature hydrothermal activity (Klein et al., 2015), leading to serpentine and hydrogen production, in marked contrast to oceanic mantle (Albers et al., 2021). However, how the broad subcontinental mantle was exhumed to the seafloor and how much of the hydrogen associated with subcontinental mantle serpentinization was produced is still poorly understood. In this presentation, we use 2D elasto-visco-plastic thermomechanical modelling and parameterized hydrothermal cooling to obtain the temperature and deformation field during continental rifting. The temperature field and the water-rock ratio assumed to be related to the deformation field are then used by thermodynamic modelling to estimate serpentine and hydrogen production. Our results indicate that the gravitational driven depleted lower lithospheric mantle flow controls the width of exhumed subcontinental mantle and thus the amount of mantle hydration during rifting. Modelling results show a 45 km thick lower lithospheric depleted mantle with a negative density anomaly of 10kg/m3 can produce an approximately 100 km wide exhumed subcontinental mantle. We test two different end-member fluid flow scenarios, the first assumes that the circulation of seawater occurs mainly in the high strain rate zone (analog to fault), so that cooling and serpentinization are mainly localized near the fault; The second scenarios assumes that the convection of seawater can occur throughout the shallow part of lithosphere, so that serpentine distribution is more diffuse. These two scenarios provide lower and upper limits for the amount serpentinization and its associated hydrogen production, respectively. The modelling results are calibrated using the geological and geophysical observations of the well-studied West Iberian margin and applied to the other less studied North Atlantic magma-poor margins to give a possible range of serpentinization and associated hydrogen production during the opening of the North Atlantic.