Abstract
AbstractVein 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.
Highlights
Fractures provide important pathways for fluid migration in Earth’s crust (Newhouse, 1942; Cox et al, 1987; Nelson, 2001)
We focused on microveins filled by laterally wide, blocky crystals whose formation mechanism remains unexplained (Figs. 1C and 1D)
Models show that larger initial fracture apertures result in wider crystals, but there is a maximum width that vein crystals can reach that is dependent on the proportion of “fast-growing” grains in comparison to “slow-growing” and “inert” grains on fracture surfaces
Summary
Fractures provide important pathways for fluid migration in Earth’s crust (Newhouse, 1942; Cox et al, 1987; Nelson, 2001). This study fills this gap by applying the phasefield method to simulate enigmatic wide-blocky microstructures in natural calcite veins from Somerset, UK. We were able to replicate the natural microstructures quantitatively and provide new insights on vein formation mechanisms, showing how variations in transgranular and intergranular segments on fracture surfaces lead to heterogenous crystal growth, producing microstructures that are not predicted by previous models.
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