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

Abstract

AbstractTo 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.