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Olivine and pyroxene-bearing rocks in the oceanic crust react with hydrothermal fluids producing changes in the physical characteristics and behaviors of the altered rocks. Notably, these reactions tend to increase solid volume, reducing pore volume, permeability, and available reactive surface area, yet entirely hydrated and/or carbonated rocks are commonly observed in the field. We investigate the evolution of porosity and permeability of fractured dunites reacted with CO2-rich solutions in laboratory experiments. The alteration of crack surfaces changes the mechanical and transport properties of the bulk samples. Analysis of three-dimensional microstructural data shows that although precipitation of secondary minerals causes the total porosity of the sample to decrease, an interconnected network of porosity is maintained through channelized dissolution and coupled carbonate precipitation. The observed microstructure appears to be the result of chemo-mechanical coupling, which may provide a mechanism of porosity maintenance without the need to invoke reaction-driven cracking.

Plain Language Summary When carbon dioxide reacts with certain minerals, it can be transformed into solid carbonate minerals. These carbonate minerals can be deposited at the site of the interaction between the fluid and the solid. It is often assumed that the armoring of surfaces by the products of such a reaction will shut down the reaction in a self-limiting fashion. However, rocks that are the result of the complete conversion of reactive minerals to carbonate and other minerals via interaction with carbon dioxide are observed in nature. This paper presents the results of experiments designed to constrain the mechanism allowing such reactions to proceed to completion. We find that a coupled dissolution-precipitation process results in the formation of a system of channels in the rock that allow continuous fluid access to solid surfaces. This result is essential for the complete understanding of fluid-rock interactions in natural systems and may be important to consider when modeling geological carbon storage by mineral carbonation.