ALBERT

Description: On its way to the surface, the Siberian Traps magma created a complex sub-volcanic plumbing system. This resulted in a large-scale sill emplacement within the Tunguska Basin and subsequent release of sediment-derived volatiles during contact metamorphism. The distribution of sills and the released sediment-stored gas volume is, however, poorly constrained. In this paper, results from a study of nearly 300 deep boreholes intersecting sills are presented. The results show that sills with thicknesses above 100 m are abundant throughout the upper part of the sedimentary succession. A high proportion of the sills was emplaced within the Cambrian evaporites with average thicknesses in the 115–130 m range and a maximum thickness of 428 m. Thermal modelling of the cooling of the sills shows that the contact metamorphic aureoles are capable of generating 52–80 tonnes of CO 2  m −2 with contributions from both marine and terrestrial carbon. When up-scaling these borehole results, an area of 12–19 000 km 2 is required to generate 1000 Gt CO 2 . This represents only 0.7–1.2% of the total area in the Tunguska Basin affected by sills, emphasizing the importance of metamorphic gas generation in the Siberian Traps. These results strengthen the hypothesis of a sub-volcanic trigger and driver for the environmental perturbations during the End-Permian crisis. This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.

Description: The subduction of partially serpentinized oceanic mantle may potentially be the key geologic process leading to the regassing of Earth’s mantle and also has important consequences for subduction zone processes such as element cycling, slab deformation, and intermediate-depth seismicity. Little is known about the quantity of water that is retained in the slab during mantle serpentinization. Recent studies using thermodynamical and/or experimental models of subduction zone processes have assumed that the mantle is uniformly serpentinized to a depth determined from the equilibrium stability of serpentine minerals in P-T space. This approach yields an incomplete picture of the pattern of serpentinization that may occur during bending-related faulting; an initial state that is essential for quantifying subsequent dehydration processes. In order to provide further constraints on the pattern of hydration and the amount of water trapped in the subducting mantle, we build a 2-D reactive-flow model incorporating the kinetic rate-dependence of serpentinization based on experimental results. After simulating hydration processes at the trench outer-rise, we find that the water content in serpentinized mantle strongly depends on the age of the subducting lithosphere and subduction rate, with values ranging between 1.8x105 and 4.0x106 kgm-2 reactive water uptake into the subducting mantle column. Serpentinization also results in a reduction in surface heat flux towards the trench caused by advective downflow of seawater into the reaction region. Observed heat flow reductions are larger than the reduction due to the minimum-water downflow needed for partial serpentinization, predicting that active hydrothermal vents and chemosynthetic communities should also be associated with bend-fault serpentinization. Model results agree with previous studies that the lower plane of double Benioff zones can be generated due to dehydration of serpentinized mantle at depth. The depth-dependent pattern of serpentinization including reaction kinetics predicts a separation between the two Benioff planes consistent with seismic observations.

Description: On its way to the surface, the Siberian Traps magma created a complex sub-volcanic plumbing system. This resulted in a large-scale sill emplacement within the Tunguska Basin and subsequent release of sediment-derived volatiles during contact metamorphism. The distribution of sills and the released sediment-stored gas volume is, however, poorly constrained. In this paper, results from a study of nearly 300 deep boreholes intersecting sills are presented. The results show that sills with thicknesses above 100 m are abundant throughout the upper part of the sedimentary succession. A high proportion of the sills was emplaced within the Cambrian evaporites with average thicknesses in the 115-130 m range and a maximum thickness of 428 m. Thermal modelling of the cooling of the sills shows that the contact metamorphic aureoles are capable of generating 52-80 tonnes of CO2 m(-2) with contributions from both marine and terrestrial carbon. When up-scaling these borehole results, an area of 12-19 000 km(2) is required to generate 1000 Gt CO2. This represents only 0.7-1.2% of the total area in the Tunguska Basin affected by sills, emphasizing the importance of metamorphic gas generation in the Siberian Traps. These results strengthen the hypothesis of a sub-volcanic trigger and driver for the environmental perturbations during the End-Permian crisis. This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.