Formation of wide-blocky calcite veins by extreme growth competition

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. Supplementary material:https://doi.org/10.6084/m9.figshare.c.5172371