Wide-blocky veins explained by dependency of crystal growth rate on fracture surface type: Insights from phase-field modeling

Abstract

Vein microstructures contain a wealth of information on coupled chemical and mechanical processes of fracturing, fluid transport, and crystal growth. Numerical simulations have been used for exploring the factors controlling the development of vein microstructures; however, they have not been quantitatively validated against natural veins. Here we combined phase-field modeling with microtextural analysis of previously unexplained wide-blocky calcite veins in natural limestone and of the fresh fracture surface in this limestone. Results show that the wide-blocky vein textures can only be reproduced if ~10%–20% of crystals grow faster than the rest. This fraction corresponds to the amount of transgranularly broken grains that were observed on the experimental fracture surfaces, which are dominantly intergranular. We hypothesize that transgranular fractures allow faster growth of vein minerals due to the lack of clay coatings and other nucleation discontinuities that are common along intergranular cracks. Our simulation results show remarkable similarity to the natural veins and reproduce the nonlinear relationship between vein crystal width and vein aperture. This allows accurate simulations of crystal growth processes and related permeability evolution in fractured rocks.

Kinematics of Crystal Growth in Single‐Seal Syntaxial Veins in Limestone ‐ A Phase‐Field Study

“…In this study, we systematically evaluated vein formation in a diverse range of limestones. Our numerical results show many similarities with natural vein microstructures and builds on the previous numerical works of Nollet et al (2005); Lander and Laubach (2015); Prajapati et al (2018); Spruženiece et al (2020Spruženiece et al ( , 2021, where controlling factors in vein formation are discussed.…”

“…In the present work, we utilize the same vectors for surface energy (Equation 4) and kinetic anisotropy (Equation 7) and applied the same factor E  (in Equation 7) as in the previous studies of Spruženiece et al (2020Spruženiece et al ( , 2021, which gives good agreement to microscopic observations. When a complete data set from crystal growth experiments for the growth velocities of the crystal facets and the growth velocity drop from rough to euhedral surfaces is available, both the anisotropy vectors and the factor E  (in Equation 7) can be calibrated (as in Prajapati, Abad Gonzalez, et al, 2020;Wendler et al, 2016 for quartz) and quantitative results about the cementation kinetics can be computed.…”

Progressive veining during peridotite carbonation: insights from listvenites in Hole BT1B, Samail ophiolite (Oman)

“…Owing to chemical-mineralogical replacements that occur during carbonation, however, the mechanism is more complex during listvenite formation. 470 Current models of vein formation treat the host rock as a non-reactive substrate with vein formation due to precipitation from aqueous solution in fluid-filled fractures (Ankit et al, 2015;Hilgers et al, 2001;Hubert et al, 2009;Spruženiece et al, 2021a;Spruženiece et al, 2021b). In the case of carbonate veining during listvenite formation, however, mechanical opening was accompanied by replacement of the host serpentinite (Fig.…”

Spacing and Strain During Multiphase Boudinage in 3D

“…However, to preserve the high grain angularity seen in the amphibolite, considerable dilatation would have been necessary, which would require a fluid pressure that should manifest in much more significant mineralisation (Stel, 1981). The absence of layer-bound fractures in very thin amphibolite is possibly an effect of intergranular fracturing (Abe & Urai, 2012, Spruženiece et al, 2021 saturating the layer before cracks can coalesce into localised fractures. This is not surprising considering that the layer is less than 10 -20 grains across, well below the expected shear band thickness (e.g.…”