Liassic limestones at the Somerset coast (UK) contain dense arrays of calcite microveins with a common but poorly understood microstructure, characterized by laterally wide crystals that form bridges across the vein. This paper investigates the formation mechanisms and evolution of these wide-blocky vein microstructures by a combination of high-resolution analytical methods (ViP microscopy, optical CL and SEM techniques (EDS, BSE, CL and EBSD)), laboratory experiments and phase-field modelling.
Results indicate that the studied veins formed in open, fluid-filled fractures, each in a single opening and sealing episode. As shown by optical and EBSD images, vein crystals grow epitaxially on wall-rock grains and we hypothesize that their growth rates differ depending on whether crystals are substrated on wall-rock grains that are fractured intergranularly (slow) or transgranularly (fast). Phase-field models support this hypothesis, showing that wide-blocky crystals only form in cases with significant growth rate differences that are dependent on the type of seed grain.
This provides strong evidence for “extreme growth competition”, a process, which we propose controls vein filling in many micritic carbonate reservoirs, as well as demonstrating that the characteristics of the fracture wall can affect filling processes in syntaxial veins.