Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (<200°C vs. >200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg + Fe) < 0.8, and shows distinct compositional trends toward a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios are 0.6–0.92 and 0.58–0.8, respectively; our data show that 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the sample’s initial serpentinization produced ∼120 to >300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow-spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to >300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of <200°C. At slow-spreading MORs, in contrast, serpentinization may produce 200–350 mmol H2, most of which is related to magnetite formation at >200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought

Lateral coexistence of ductile and brittle deformation shapes magma-poor distal margins: An example from the West Iberia-Newfoundland margins

A long-standing problem in solid Earth science is to understand how low-angle normal faults form, their role in the development of tectonic asymmetry of conjugate margins, and how they relate to mantle hydration during continental breakup. The latter requires water to reach the mantle through active brittle faults, but low angle slip on faults is mechanically difficult. Here, we incorporate observations from high-resolution multichannel seismic data along the West Iberia-Newfoundland margins into a 2D forward thermo-mechanical model to understand the relationship between evolving rift asymmetry, detachment tectonics, and mantle hydration. We show that, during extreme extension, slip on active faults bifurcates at depth into brittle and ductile deformation branches, as a result of the cooling of the faults' footwall and heating of their hangingwall. The brittle deformation penetrates the Moho and leads to mantle hydration, while ductile deformation occurs in localized shear zones and leads to the formation of detachment-like structures in the distal margin sections. Such structures, as for example ‘S’ in the West Iberia-Newfoundland margins, are thus composed of several shear zones, active at low-angles, ∼25°-20°, and merging with the Moho at depth. The final sub-horizontal geometry of these structures is the result of subsequent back-rotation of these shear zones by new oceanward faults. Our results reproduce remarkably well the final sedimentary, fault, crustal architecture, and serpentinisation pattern observed at the West Iberia-Newfoundland margins. However, they challenge widely accepted ideas that such detachment-like structures formed by brittle processes, separate crust from mantle and caused conjugate margin asymmetry. Our model provides a quantitative framework to study hydrothermal systems related to serpentinization during extreme extension, their associated hydrogen, methane production, and the chemosynthetic life they sustain

A new free-surface stabilization algorithm for geodynamical modelling:Theory and numerical tests

The surface of the solid Earth is effectively stress free in its subaerial portions, and hydrostatic beneath the oceans. Unfortunately, this type of boundary condition is difficult to treat computationally, and for computational convenience, numerical models have often used simpler approximations that do not involve a normal stress-loaded, shear-stress free top surface that is free to move. Viscous flow models with a computational free surface typically confront stability problems when the time step is bigger than the viscous relaxation time. The small time step required for stability (<2. Kyr) makes this type of model computationally intensive, so there remains a need to develop strategies that mitigate the stability problem by making larger (at least ~10 Kyr) time steps stable and accurate. Here we present a new free-surface stabilization algorithm for finite element codes which solves the stability problem by adding to the Stokes formulation an intrinsic penalization term equivalent to a portion of the future load at the surface nodes. Our algorithm is straightforward to implement and can be used with both Eulerian or Lagrangian grids. It includes α and β parameters to respectively control both the vertical and the horizontal slope-dependent penalization terms, and uses Uzawa-like iterations to solve the resulting system at a cost comparable to a non-stress free surface formulation. Four tests were carried out in order to study the accuracy and the stability of the algorithm: (1) a decaying first-order sinusoidal topography test, (2) a decaying high-order sinusoidal topography test, (3) a Rayleigh-Taylor instability test, and (4) a steep-slope test. For these tests, we investigate which α and β parameters give the best results in terms of both accuracy and stability. We also compare the accuracy and the stability of our algorithm with a similar implicit approach recently developed by Kaus et al. (2010). We find that our algorithm is slightly more accurate and stable for steep slopes, and also conclude that, for longer time steps, the optimal α controlling factor for both approaches is ~2/3, instead of the 1/2 Crank-Nicolson parameter inferred from a linearized accuracy analysis. This more-implicit value coincides with the velocity factor for a Galerkin time discretization applied to our penalization term using linear shape functions in time

The Role of Crustal Strength in Controlling Magmatism and Melt Chemistry During Rifting and Breakup

International audienceThe strength of the crust has a strong impact on the evolution of continental extension and breakup. Strong crust may promote focused narrow rifting, while wide rifting might be due to a weaker crustal architecture. The strength of the crust also influences deeper processes within the asthenosphere. To quantitatively test the implications of crustal strength on the evolution of continental rift zones, we developed a 2-D numerical model of lithosphere extension that can predict the rare Earth element (REE) chemistry of erupted lava. We find that a difference in crustal strength leads to a different rate of depletion in light elements relative to heavy elements. By comparing the model predictions to rock samples from the Basin and Range, USA, we can demonstrate that slow extension of a weak continental crust can explain the observed depletion in melt chemistry. The same comparison for the Main Ethiopian Rift suggests that magmatism within this narrow rift zone can be explained by the localization of strain caused by a strong lower crust. We demonstrate that the slow extension of a strong lower crust above a mantle of potential temperature of 1,350 degrees C can fit the observed REE trends and the upper mantle seismic velocity for the Main Ethiopian Rift. The thermo-mechanical model implies that melt composition could provide quantitative information on the style of breakup and the initial strength of the continental crust.Plain Language Summary There are various regions within the continents today that appear to be extending and breaking apart, known as rifts. There are examples of regions where the extension is distributed over hundreds of kilometres, such as the Basin and Range in North America. There are also regions of narrow focused extension such as the Main Ethiopian Rift. Both these example regions also have a history of magmatism, with basaltic rocks outcropping at the surface. The width of a rift can be related to the strength of the continental crust, weak crust gives a wide rift and vica-versa, yet such relations are subjective. In this study we explore if the chemical composition of the erupted basaltic rocks is a function of the crustal strength. We developed a numerical model that approximates the extension of the upper mantle, including the continental crust. Our model finds that the magmatism in the wide Basin and Range can be explained by the break-up of a weak continental crust. The same model would suggest that the magmatism observed in Ethiopia is due to extension of a strong continental crust. Such a result is contentious, as previous studies have suggested that both regions have weak continental crust

Modeling of Continental Normal Fault Earthquakes

Abstract The magnitude of earthquakes on continental normal faults rarely exceeds 7.0 Mw. However, because of their vicinity to large population centers they can be highly destructive. Long recurrence time, relatively small deformations, and limited observations hinder our understanding of the deformation patterns and mechanisms controlling the magnitude of events. Here, this problem is addressed with 2D thermomechanical modeling of normal fault seismic cycles. The 2020 Samos, Greece Mw7.0 earthquake is used as an example as it is one of the largest and most studied continental normal fault earthquakes. The modeling approach employs visco‐elasto‐plastic rheology, compressibility, free surface, and a rate‐and‐state friction law for the fault. Modeling of the Samos earthquake suggests the pore fluid pressure ratio on the fault ranges from 0 to 0.7. The model demonstrates that most of the deformation during interseismic and coseismic periods, besides on the fault, occurs in the hanging wall and footwall below the seismogenic part of the fault. The largest vertical surface displacement during the earthquake is the subsidence of the hanging wall in the vicinity of the fault, while the uplift of the footwall and remote part of the hanging wall is significantly smaller. Modeling of the seismic cycles on normal faults with different setups shows the dependency of the magnitude on the thermal profile and dipping angle of the fault; low heat flow and low dipping angle are favorable conditions for the largest events, while steep normal faults in the areas of high heat flow tend to have the smallest magnitudes

Multitaper spectral method to estimate the elastic thickness of South China: Implications for intracontinental deformation

The effective elastic thickness (Te) represents the thickness of the elastic layer or the flexural rigidity of the lithosphere, the equivalent of which can be calculated from the spectral analysis of gravity and topographic data. Studies of Te have profound influence on intracontinental deformation, and coupling of the tectonic blocks. In this paper, we use the multitaper spectral estimation method to calculate the coherence between Bouguer gravity and topography data, and to obtain the Te map of South China. Through the process of correction, we discuss the relationships of Te versus heat flow, and Te versus seismicity. The results show that Te distribution of South China is affected by three factors: the original age, which controls the basic feature; the Mesozoic evolution, which affects the Te distribution; and the neotectonic movement, which shaped the final distribution. The crust age has a positive correlation with the first-order Te distribution; thus the Yangtze Craton has a relatively higher Te (about 50 km) whereas the Te in Cathaysia block is only 10–20 km. By analysis and comparison among the tectonic models of South China, the Te distribution can be well explained using the flat-subduction model. As is typical with neotectonics, the region with a higher heat flow is related with a lower Te. The seismicity does not have a clear relationship with Te, but the strong seismicity could cause a low Te. Seismogenic layer (Ts) has a similar trend as Te in the craton, whereas in other areas the relationship is complex

Rifted margin architecture and crustal rheology: Reviewing Iberia-Newfoundland, Central South Atlantic, and South China Sea

© 2016 Elsevier Ltd Crustal rheology controls the style of rifting and ultimately the architecture of rifted margins. Here we review the formation of three magma-poor margin pairs, Iberia-Newfoundland, the central segment of the South Atlantic Rift, and the South China Sea by integrating observational data into a numerical forward modelling framework. We utilise a 2D version of the finite element code SLIM3D, which includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology and is able to reproduce a wide range of rift-related deformation processes such as flexure, lower crustal flow, and faulting. Extension in cold, strong, or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries. Hot, weak, or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system. This simple recipe provides the standard framework to understand initial rift geometry, however, it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture. Asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles, which we reproduce with weak and strong crustal rheologies, respectively. Exemplified by the Iberia-Newfoundland conjugates and the Central South Atlantic, we define three characteristic rift phases: an initial phase of simultaneous faulting, an intermediate phase of rift migration that involves sequential fault activity, and finally, the breakup phase. Crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins: we illustrate that weak rheologies generally prolong the phase of simultaneous faulting, while rift migration is enabled by initial fault asymmetry as well as relatively weak crust. Formation of the predominantly symmetric and wide margins of the South China Sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup. The weak crustal rheology of this region relates to the South China Sea\u27s pre-rift history where plate convergence lead to crustal thickening and magmatic additions in a back-arc regime shortly before the onset of rifting

Serpentinization-driven H2 production from continental break-up to mid-ocean ridge spreading: unexpected high rates at the West Iberia margin

Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe- rich, with XMg Mg/(Mg + Fe) 300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow- spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to >300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of 200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought.researc