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EMU Notes in Mineralogy - volume 16

Mineral reaction kinetics: Microstructures, textures, chemical and isotopic signatures
(W. Heinrich and R. Abart, editors)

Chapter 16.  Reaction-induced fracturing: Chemical-mechanical feedback
O. I. Ulven and A. Malthe-Sørenssen

Volume-changing mineralization reactions induced by fluids play an important role in the large-scale dynamics of the Earth’s crust, effectively determining e.g. the rates of chemical weathering on the surface of the Earth and the density of the oceanic crust through serpentinization reactions. In addition, permanent CO2 storage through carbonation of ultramafic rock also depends onmechano-chemical processes – processes where the chemical reactions are closely coupled to fluid transport and deformation. This coupling between transport, reaction, deformation and fracture may have a first-order impact on the effective reaction rate – the coupling may significantly accelerate the overall reaction.

This chapter addresses our understanding of the chemical-mechanical feedback during reaction-induced fracturing and introduces theoretical and computer-modelling methods used to address dynamics in such coupled systems. Basic geological scenarios where such a feedback is expected to be important are introduced, and the underlying chemical processes are described. Then we provide a brief review of relevant literature that discusses reaction-induced fracturing.

We introduce a simplified one-dimensional model that can be used to understand both volume-decreasing and volume-increasing reactions, and which also produces useful quantitative predictions for the motion of the reaction front. We further address volumechanging reactions for both a pure diffusion system, which is simple to understand and describe, and for a diffusion-reaction system. Then, we introduce an analytical model for the onset of fracturing in diffusion-reaction-fracturing models, and use this theory to validate a more general numerical model, which can also predict fracture propagation beyond fracture initiation.

We introduce a theoretical description of systems with and without feedback from fracturing, and models where the effects of fluid transport in fractures can be tuned. Relevant dimensional quantities are introduced and generalized results for mechanochemical processes are discussed in terms of the fundamental dimensionless numbers characterizing the system.

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