Carbonate mineralization in percolated olivine aggregates: Linking effects of crystallographic orientation and fluid flow

Abstract

International audienceIn situ mineralization of CO2 in ultramafic rock-hosted aquifers is one of the promising solutions for decreasing CO2 concentrations in the atmosphere. Naturally altered ultramafic rocks suggest that carbonation processes are controlled by local heterogeneities in the structure of the rock and fluid transport at the water-rock interfaces. We studied the role of rock crystallographic anisotropy relative to the global fluid flow direction on the mineralization of CO2 by means of electron microscope analyses from the macro- to the micrometer scale (EBSD-FIB). The sample used for the measurements was a hot pressed olivine core percolated by water enriched in CO2 (pCO2 = 10 MPa) at 180 °C. During the percolation experiment, olivine was dissolved and two types of carbonates, dolomite, and magnesite, were precipitated on olivine surfaces. The results showed that the dissolution of olivine is controlled by its crystallographic properties as shown by the development of etch-pits only on the (010)ol planes and with elongated shapes parallel to the [010]ol axes. In contrast, the precipitation of carbonates is governed by hydrodynamic properties. Carbonates are heterogeneously distributed in the percolated rock. They are mainly located along the moderate (for dolomite) and the minor (for magnesite) flow paths, both oriented parallel to the principal fluid flow direction, which allow carbonates to be supplied with divalent cations (e.g., Ca2+, Mg2+, and Fe2+). In these flow paths, carbonate growth is systematically oriented normal to the flow that facilitates the development of chemical gradients with cationic supersaturation conditions for carbonate precipitation near the walls.In natural systems, the (010)ol planes are parallel to the Moho and the (100)ol planes are vertical; our study suggests that flow of CO2-rich fluids will induce precipitation of carbonates localized along, and preferentially clogging, vertical flow paths while favoring olivine dissolution along horizontal fluid pathways. This dual control of structure and fluid flow on carbonation mechanisms could be an important parameter allowing sustainable CO2 storage in peridotites, while limiting the risks of leakage toward the surface

Intracrystalline Reaction-Induced Cracking in Olivine Evidenced by Hydration and Carbonation Experiments

“…Our experimental data have provided ample evidence for the formation of new intracrystalline cracks during serpentinization. These results are in good agreement with those reported in natural serpentinites by other authors [9,21], who proposed that the nucleation and propagation of new cracks is driven by the crystallization stress accumulated at the tips of dissolution notches during progressive serpentinization [43,44]. We therefore support the idea that the large increase in volume associated with phase transformation does not necessarily impede the extent of serpentinization reactions, but rather triggers the formation of a hierarchical fracturation network allowing for serpentinization to continue during hydration.…”

“…This is a nontrivial issue, especially in a context where the increase in volume associated with phase transformations tends to form veins, obstructing circulation pathways, which may in turn impede fluid flow and inhibit the reaction [8,9]. However, even if serpentinization is in theory self-limiting in essence, the large bodies of fully serpentinized rocks outcropping in various geological settings around the world [9,10] indicate that situations of pervasive and sustained fluid flow may persist for long periods of time. For that to happen, pre-existing fluid circulation pathways have to remain open, and/or new ones have to be created in the course of the reaction [10].…”

“…For that to happen, pre-existing fluid circulation pathways have to remain open, and/or new ones have to be created in the course of the reaction [10]. In the former case, significant mass transfer is required between dissolution and precipitation sites [7,11,12], while the latter is achieved through rock fracturing [9,10] or the build-up of interconnected porosity [10,13]. Even if those features are well established and corroborated by various field observations e.g., [14,15] and in laboratory experiments e.g., [16,17], the mechanisms at play and the conditions under which they occur are still not well constrained.…”

Textural and compositional complexities resulting from coupled dissolution–reprecipitation reactions in geomaterials

“…For example, saw-tooth like replacement fronts have been observed in feldspar replacements (e.g., Niedermeier et al 2009;Norberg et al 2011), which has been suggested to be related to the weakness of the Al-O-Si bond relative to Si-O-Si bond within the tetrahedral framework of alkali feldspar (Arvidson et al 2004;Schott and Oelkers 1995;Schott et al 2009;Xiao and Lasaga 1994). Similarly, a number of investigations have illustrated the anisotropic dissolution of olivine, which is an important process in the fracture generation associated with serpentinization (King et al 2010Peuble et al 2015).…”

Mineralization of Carbon Dioxide: A Literature Review

“…The study of Peuble et al [48] on naturally altered ultramafic rocks suggested that carbonation processes are controlled by local heterogeneities in the structure of rock and fluid transport at the water-rock interfaces. In natural systems, flow of CO 2 -rich fluids will induce precipitation of carbonates localized along, and preferentially clogging, vertical flow paths while favoring olivine dissolution along horizontal fluid pathways.…”