Incomplete Crack Sealing Causes Localization of Fracturing in Hydrothermal Quartz Veins

• Published in: Geophysical research letters 2022-08, Vol.49 (15), p.n/a
• Main Authors: Späth, Michael, Urai, Janos L., Nestler, Britta
• Format: Article
• Language: English
• Subjects: Crack propagation; Cracking (corrosion); Cracking (fracturing); Crystal growth; Crystal structure; Crystals; Deformation; Earth crust; Epitaxial growth; Fluids; Fracture mechanics; Hydrothermal systems; Localization; Mechanical properties; Mechanics; Minerals; Permeability; Porosity; Quartz; Rock; Rocks; Sealing; Simulation; Transport; Veins (geology)
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL098643

Cyclic microfracturing and epitaxial crystal growth have long been recognized in crack‐seal veins, but an understanding of a single crack‐seal cycle is still missing. Here we present a phase‐field model that includes both fracture mechanics of crack propagation, and epitaxial crystal growth on the fracture walls, repeating this cycle multiple times in a polycrystalline, microporous quartz rock. Our simulations have two end members: If a vein completely seals, it is stronger than the host rock, cracking is delocalized, forming many single‐seal microveins. Incomplete sealing makes the vein weaker than the host rock and localizes the new fracture inside the vein, leading to multi‐crack‐seal. We suggest that the sealing degree is a key parameter in hydrothermal systems and multi‐crack‐seal veins are long‐lived, microporous sites of mechanical weakness. We generalize the phase‐field approach to conduct probabilistic simulations in between these two types, and show how systems of microveins and multi‐crack‐seal veins emerge. Plain Language Summary Fluids in the Earth's crust can alter permeability and porosity, precipitate and dissolve minerals, transport material and interact with deformation. This affects the transport and mechanical properties of the rock system and in turn has consequences for example, in subsurface engineering applications. In this work we simulate the processes of fracturing and crystal growth on grain scale in a microporous rock structure and show how different crystal structures form. The basic steps of a crack‐seal process and how fracturing and sealing interact are explored. Our results show that if a fracture completely seals a new crack will form in the host rock and many thin microveins form. In contrast, an incomplete sealing makes the vein weaker than the host rock and leads to a new cracking inside the vein, which enlarges the existing structure with each cycle. This implies that the degree of sealing is the cause of this division, where crack‐seal veins are microporous sites of mechanical weakness. Additionally, we perform probabilistic simulations which show how many single‐seal microveins form side‐by‐side with a few multi‐crack‐seal veins. Our studies provide valuable insight in structure‐property linkages and enable a better prediction of fracture‐sealing. Key Points Systematic phase‐field study captures elementary steps of the crack‐seal process at grain scale Incomplete sealing makes a vein weaker than the host rock and lo