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[SCG58-P18] Evolution of 3-D pore structures in feldspar replacement: Insights from hydrothermal experiments and repeated X-ray tomography
Keywords:feldpar, hydrothermal, pore, 3D, time variation
In Earth’s crusts, fluid pathways have been generally thought to be cracks or grain boundaries in crystalline rocks. Recently, the presence of abundant pores in altered minerals such as feldspars have been discovered, and such reaction-induced porosity has been attracted much attention1. In particular, it is known that hydrothermal alteration of feldspars commonly produces abundant porosities, and a number of experimental studies have been conducted on a variety of replacement reactions including albite (Ab) by K-feldspar (Kfs), and Kfs by Albite, plagioclase by Ab. However, as the porosity was observed only after the experiments, and the observations were conducted by SEM, the three-dimensional (3D) shape and connectivity of the pores, and their temporal evolution are still poorly understood due to the destructive analysis. In this study, we conducted the hydrothermal alteration of two reactions with contrasting volume change: (1) Kfs by Ab (ΔV = +8.85%) and (2) Ab by Kfs (ΔV = -8.14%), respectively. Then, we obtained the data of 3D-pore structures by the high-resolution non-destructive 3D X-ray computed tomography (X-CT). Based on the repeated experiments and observations for a single sample, we clarify the temporal evolution of pores.
The hydrothermal experiments were conducted by the cold seal vessel. The Ab or Kfs sample 1 mm x 1 mm x 2 mm was enclosed into the gold tube with KCl aq (2 mol/L) or NaCl aq (2 mol/L) solutions, respectively, then the gold tube was welded. The experiment was carried out at 600°C and 150 MPa for 48 hours. After the experiments, the 3D-structures of the altered feldspars were measured by a high-resolution 3D X-CT (voxel size: ca. 1.7 μm) at High Energy Accelerator Research Organization (KEK). After the 3D data aquation for the first run, the second run (48 h) and X-ray CT measurements were conducted
The 3D images reconstructed from X- CT data of the two types of feldspar alteration revealed the progress of the reaction zone from the surface or fractures. The width of the front of Ab by Kfs replacement increased from 50 μm at 48 h to 75μm at 96 h, and that of Kfs by Ab from 20μm to 50μm. Microscale pores (5-30 μm in size) were abundantly always formed preferentially at the reaction front, meaning that the pores moved with the progress of the reaction front. The geometry of the pores is contrasting between the Ab and Kfs alterations. In the case of Ab alteration by Kfs, the platy pores are aligned almost parallel to the main fracture plate, and all pores look isolated. In contrast, in the Kfs by Ab alteration, the pores show a 3D tree-like shape with many branches, and some were partially connected to the main fracture. The fractal dimensions measured for both types of pores ranges from 1.85 to 2.2, but the pores of the Kfs by Ab reaction were characterized by larger specific surface area than that of the pores of the Ab by Kfs reactions.
In both reactions, the pores move with the migration of the reaction zone via dissolution at the front and precipitation at the back probably in response to the gradient of the ionic concentrations (K+ and Na+). It is important to note that in both cases pores are completely isolated in 3D, in spite that fluid-mediated reaction proceeds. This means that nano-scale porosity, which may be smaller than the voxel size of 1.6 µm, could play important roles in the progress of the reaction front. In addition, the difference in the shape of the pores between Ab by Kfs and Kfs by Ab can be attributed to the volume change; the latter with volume reduction could produce porosity more easily, resulting in the new tree-shaped pores.
Plümper et al., 2017, Nature Geoscience, vol. 10, pp.685-691
Nurdiana et al., 2021, Science Direct , vol. 18, pp.388-389
The hydrothermal experiments were conducted by the cold seal vessel. The Ab or Kfs sample 1 mm x 1 mm x 2 mm was enclosed into the gold tube with KCl aq (2 mol/L) or NaCl aq (2 mol/L) solutions, respectively, then the gold tube was welded. The experiment was carried out at 600°C and 150 MPa for 48 hours. After the experiments, the 3D-structures of the altered feldspars were measured by a high-resolution 3D X-CT (voxel size: ca. 1.7 μm) at High Energy Accelerator Research Organization (KEK). After the 3D data aquation for the first run, the second run (48 h) and X-ray CT measurements were conducted
The 3D images reconstructed from X- CT data of the two types of feldspar alteration revealed the progress of the reaction zone from the surface or fractures. The width of the front of Ab by Kfs replacement increased from 50 μm at 48 h to 75μm at 96 h, and that of Kfs by Ab from 20μm to 50μm. Microscale pores (5-30 μm in size) were abundantly always formed preferentially at the reaction front, meaning that the pores moved with the progress of the reaction front. The geometry of the pores is contrasting between the Ab and Kfs alterations. In the case of Ab alteration by Kfs, the platy pores are aligned almost parallel to the main fracture plate, and all pores look isolated. In contrast, in the Kfs by Ab alteration, the pores show a 3D tree-like shape with many branches, and some were partially connected to the main fracture. The fractal dimensions measured for both types of pores ranges from 1.85 to 2.2, but the pores of the Kfs by Ab reaction were characterized by larger specific surface area than that of the pores of the Ab by Kfs reactions.
In both reactions, the pores move with the migration of the reaction zone via dissolution at the front and precipitation at the back probably in response to the gradient of the ionic concentrations (K+ and Na+). It is important to note that in both cases pores are completely isolated in 3D, in spite that fluid-mediated reaction proceeds. This means that nano-scale porosity, which may be smaller than the voxel size of 1.6 µm, could play important roles in the progress of the reaction front. In addition, the difference in the shape of the pores between Ab by Kfs and Kfs by Ab can be attributed to the volume change; the latter with volume reduction could produce porosity more easily, resulting in the new tree-shaped pores.
Plümper et al., 2017, Nature Geoscience, vol. 10, pp.685-691
Nurdiana et al., 2021, Science Direct , vol. 18, pp.388-389