Devolatilization‐induced pressure build‐up: Implications for reaction front movement and breccia pipe formation

Abstract

Generation of fluids during metamorphism can significantly influence the fluid overpressure, and thus the fluid flow in metamorphic terrains. There is currently a large focus on developing numerical reactive transport models, and with it follows the need for analytical solutions to ensure correct numerical implementation. In this study, we derive both analytical and numerical solutions to reaction‐induced fluid overpressure, coupled to temperature and fluid flow out of the reacting front. All equations are derived from basic principles of conservation of mass, energy and momentum. We focus on contact metamorphism, where devolatilization reactions are particularly important owing to high thermal fluxes allowing large volumes of fluids to be rapidly generated. The analytical solutions reveal three key factors involved in the pressure build‐up: (i) The efficiency of the devolatilizing reaction front (pressure build‐up) relative to fluid flow (pressure relaxation), (ii) the reaction temperature relative to the available heat in the system and (iii) the feedback of overpressure on the reaction temperature as a function of the Clapeyron slope. Finally, we apply the model to two geological case scenarios. In the first case, we investigate the influence of fluid overpressure on the movement of the reaction front and show that it can slow down significantly and may even be terminated owing to increased effective reaction temperature. In the second case, the model is applied to constrain the conditions for fracturing and inferred breccia pipe formation in organic‐rich shales owing to methane generation in the contact aureole.

Coupling changes in densities and porosity to fluid pressure variations in reactive porous fluid flow: Local thermodynamic equilibrium

“…Fluid pressure variations induced by volume changes of reaction can influence the extent of reaction [e.g., Aarnes et al, 2012]. Some models have already included these effects [Connolly, 1997] whereas others have used simplications such as using lithostatic pressure to compute stable phase assemblage in each local equilibrium domain [Duesterhoeft et al, 2014;Tirone et al, 2009].…”

“…Provided that an interconnected network of pores exists that results in sufficiently permeable rock, fluid will flow due to gradients in fluid pressure. The generation of fluid pressure gradients and porosity variations may result from rock deformation (such as compaction), it may be a result of volume change during chemical reactions or volume changes due to thermal processes for example in contact aureoles [e.g., Connolly, 1997;Aarnes et al, 2012;Putnis and Putnis, 2007].…”

Effect of grain‐scale pressure variations on garnet growth: a numerical approach

“…Such a competition between devolatilization generated fluid pressure and relaxation by flow has been studied thoroughly by Aarnes et al . (). Moreover, several recent studies pointed out that a metamorphic reaction may occur over shorter duration than previously anticipated, from the order of tens of years to a few thousand years (Camacho et al ., ; Ague & Baxter, ; John et al ., ).…”

The basalt pipes of the Tunguska Basin (Siberia, Russia): High temperature processes and volatile degassing into the end-Permian atmosphere

“…Pressure build-up mechanisms included metamorphism of sediments, where carbonates, shale, and halite-anhydrite rocks generate CH 4 , CO 2 , and SO 2 , which lead to hydrofracturing (cf. Aarnes et al, 2012;Jamtveit et al, 2004a). Release of magmatic volatiles (CO 2 , HCl, SO 2 ) was likely minor compared to the volumes of metamorphic gases, although the carbonate in some of the breccia samples probably has a magmatic origin.…”