Exchangeable cation composition of the smectite‐rich plate boundary fault at the Japan Trench

Abstract

To better understand physicochemical processes in smectite‐rich fault zones, we examined exchangeable cation composition of samples from the slip zone of the 2011 Tohoku‐oki earthquake (Mw9.0) recovered by the Integrated Ocean Drilling Program Expedition 343. Our chemical analyses revealed that the exchangeable Ca2+ and Mg2+ are enriched in the slip zone, while Na+ is depleted. K+ shows a complicated depth profile probably due to K fixation. Based on fluid chemistry data, we estimated apparent selectivity coefficients of exchange reactions in the ternary Ca2+‐Mg2+‐Na+ system. The results suggest that the Na+ to Mg2+ exchange reaction on smectite might have progressed in the slip zone. One explanation for this feature is local progress of the reaction triggered by coseismic thermogenesis during the earthquake. Considering that the frictional property of smectite gouge is dependent on the exchangeable cation composition, chemical processes as observed in this study are intimately linked to physical aspect of smectite‐bearing faults.

Surface Physicochemical Properties of Smectite‐Rich Fault Gouge: A Case Study of the Japan Trench Plate‐Boundary Fault

“…Our analysis of the surface properties of the fault samples yielded values that correspond closely to Na‐type smectite. This is explained by the very high contents of smectite in the fault (Kameda et al., 2015) and by the fact that the exchangeable cations of smectite are mainly Na (Kameda et al., 2016). In this section, we further examine the interparticle interactions in the in situ fault by adopting the parameters obtained from our analyses.…”

“…The zeta potential $$\zeta $$ of the samples was estimated by electrophoretic light scattering using a Zetasizer Nano instrument (Malvern). For electrophoresis, the samples were dispersed in three types of solutions—distilled water, 0.5 M NaCl solution, and simulated solutions—for the extracted pore fluid of the fault gouge during the drilling survey (Na + = 455.8 mM, Ca 2+ = 24.7 mM, K + = 7.7 mM, and Mg 2+ = 37.8 mM; Kameda et al., 2016) at a solid weight concentration of ∼0.1%. The dispersions were then sonicated and disaggregated for 10–15 min before measurement.…”

“…Mineralogical analyses have revealed that the fault zone is marked by extremely high smectite contents (60–80 wt.%) compared with surrounding sequences (<20 wt.%; Figure 1b; Kameda et al., 2015). Analysis by the Schollenberger method (extracting cations in ammonium acetate solution) has also shown that the exchangeable cations of smectite are enriched in Na (Kameda et al., 2016). In this study, a total of 10 fragments from various parts of the fault zone were used for laboratory measurements, as described below (Figure 1b).…”

Cohesional Slip on a Plate Subduction Boundary During a Large Earthquake

“…Detailed information on the standard samples is given by Kameda and Morisaki (2017). The solid mixtures were dispersed in NaCl solutions (0.6 M ) because the pore fluid of the natural fault rock has high ionic strength comparable with that of seawater (Kameda et al, 2016). A 50‐ml polypropylene conical tube was used to prepare the suspensions with different water contents.…”

Fault weakening caused by smectite swelling

“…The swelling pressure of the expanding rock has been previously estimated using Gouy-Chapman theory, which is based on the repulsive force that stems from an overlap of the diffuse double-layers of adjacent charged plates (i.e., diffuse double-layer theory; Bolt 1955;van Olphen 1963;Mitchell and Soga 2005). However, the salinity of fault zone pore fluid is as high as that of seawater (Kameda et al 2016), indicating that this theory cannot precisely predict reasonable pressure values (Komine et al 2009). Therefore, we used both pure water and artificial pore water (Na + = 455.8 mM; Ca 2+ = 24.7 mM; K + = 7.7 mM; Mg 2+ = 37.8 mM; Kameda et al 2016) as the reacting fluids at a fluid pressure of 0.2 MPa.…”