Experimental mineralogical replacement to investigate the role of texture on interface morphology, reaction kinetics and pore-scale transport during metasomatism

Abstract

Dolomitization is a key diagenetic process that reorganizes porosity and permeability in carbonate rocks, yet the coupling between interface kinetics and transport heterogeneity remains poorly constrained. We performed time-resolved hydrothermal dolomitization (metasomatism) experiments on ooid grainstone and chalk. We integrate to our results new datasets from published experiment on marble. Hence, we investigated how carbonate rocks and crystalline metamorphic textures impact the dynamics of replacement fronts. Pseudo 4D micro-CT, along with SEM, EBSD and microprobe analyses reveal that the mechanism of dolomitization depends on the texture of the host-rock and the chemistry of the diagenetic fluid. The three investigated rocks have different grain size, porosity and permeability. In both grainstone and chalk, the resulted dolomite rims propagate rapidly and produce wavy reaction fronts, invoking a progressive replacement inward within the host-rock. Applied mass-balance calculations across the observed replacement interface highlight a positive volume change between the grainstone and marble, and a negative volume change regime in the chalk. Texture, i.e. the combined effect of grain and pore size distribution, crystallographic properties and chemical content, controls the regime of the metasomatism. Roughness characterization of the reaction front shows that the front propagates randomly (H = 0.5) under the swelling regime observed in the marble and grainstone, while it shows a persistent behaviour in the shrinking regime of the chalk (H = 0.8). Beyond offering a framework in which established replacement processes are linked together as depending on texture, this work paves the way for using roughness analysis of replacement fronts as a different diagnostic tool to understand the mechanisms at work during metasomatism.