Abstract
Abstract. The olivine carbonation reaction, in which carbon dioxide is chemically incorporated to form carbonate, is central to the emerging carbon sequestration method using ultramafic rocks. The rate of this retrograde metamorphic reaction is controlled, in part, by the available reactive surface area: as the solid volume increases during carbonation, the feasibility of this method ultimately depends on the maintenance of porosity and the creation of new reactive surfaces. We conducted in situ dynamic X-ray microtomography and nanotomography experiments to image and quantify the porosity generation during olivine carbonation. We designed a sample setup that included a thick-walled cup (made of porous olivine aggregates with a mean grain size of either ∼ 5 or ∼ 80 µm) filled with loose olivine sands with grain sizes of 100–500 µm. The whole sample assembly was reacted with a NaHCO3 aqueous solution at 200 °C, under a constant confining pressure of 13 MPa and a pore pressure of 10 MPa. Using synchrotron-based X-ray microtomography, the three-dimensional (3-D) pore structure evolution of the carbonating olivine cup was documented until the olivine aggregates became disintegrated. The dynamic microtomography data show a volume reduction in olivine at the beginning of the reaction, indicating a vigorous dissolution process consistent with the disequilibrium reaction kinetics. In the olivine cup with a grain size of ∼ 80 µm (coarse-grained cup), dissolution planes developed within 30 h, before any precipitation was observed. In the experiment with the olivine cup of ∼ 5 µm mean grain size (fine-grained cup), idiomorphic magnesite crystals were observed on the surface of the olivine sands. The magnesite shows a near-constant growth throughout the experiment, suggesting that the reaction is self-sustained. Large fractures were generated as the reaction proceeded and eventually disintegrated the aggregate after 140 h. Detailed analysis show that these are expansion cracks caused by the volume mismatch in the cup walls, between the expanding interior and the near-surface which keeps a nearly constant volume. Nanotomography images of the reacted olivine cup reveal pervasive etch pits and wormholes in the olivine grains. We interpret this perforation of the solids to provide continuous fluid access, which is likely key to the complete carbonation observed in nature. Reactions proceeding through the formation of nano- to micron-scale dissolution channels provide a viable microscale mechanism in carbon sequestration practices. For the natural peridotite carbonation, a coupled mechanism of dissolution and reaction-induced fracturing should account for the observed self-sustainability of the reaction.
Highlights
Mantle peridotites are exposed widely on the Earth’s surface in tectonic settings such as mid-ocean ridges, subduction zones and ophiolites (Escartín et al, 1997; Fryer et al, 1995)
The sample assembly consists of a millimeter-sized synthesized San Carlos olivine cup, filled with loose San Carlos olivine sand and covered by a lid made of the same material as the cup
This coarse-grained olivine cup is referred to as large grain cup (LGC) in the following discussion
Summary
Mantle peridotites are exposed widely on the Earth’s surface in tectonic settings such as mid-ocean ridges, subduction zones and ophiolites (Escartín et al, 1997; Fryer et al, 1995). The transformation of olivine to serpentine and carbonates due to fluid–rock interaction is extensively observed in peridotite outcrops (e.g., Beinlich et al, 2012; Falk and Kelemen, 2015; Hansen et al, 2005). T. Xing et al.: Generating porosity during olivine carbonation peridotite can strongly affect the strength and tectonics of the oceanic lithosphere (Deschamps et al, 2013; Moore et al, 1996). Study of the olivine–fluid interaction is of great importance for understanding the alteration process of peridotite in a variety of tectonic regions. General peridotite alteration reactions can be formulated as follows (Hansen et al, 2005; Kelemen and Matter, 2008): Olivine + CO2 in fluid = Magnesite + Quartz, Mg2SiO4 + 2CO2 = 2MgCO3 + SiO2,
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