5:15 PM - 6:30 PM
[SCG63-P12] Exchangeable cation composition of the smectite-rich plate boundary fault at the Japan Trench
Keywords:Tohoku-oki earthquake, cation exchange reaction, pelagic smectite
The exchangeable cation compositions (Na+, Ca2+, K+ and Mg2+) for the bulk samples were determined by two methods. Four samples including two slip zone samples were analyzed by extracting cations in ammonium acetate solution (Schollenberger method), and the concentrations of extracted cations in the solution were measured using an atomic absorption photometer (Z-2000; Hitachi). Other samples were analyzed by extracting cations with cobaltihexamine [Orsini and Remy, 1976], following standard NF X31-130 at the INRA (Institut National de Recherche Agronomique) soil analysis laboratory in Arras, France.
Our chemical analyses revealed that the fractional concentrations of exchangeable Ca2+ and Mg2+ are higher in the slip zone than in surrounding host rocks, while Na+ is depleted in the slip zone. K+ shows a complicated depth profile, and this is probably due to strong interaction of K+ with smectite interlayer such as K-fixation. Based on pore fluid chemistry data, we estimated apparent selectivity coefficient of exchange reactions in ternary system of Ca2+-Mg2+-Na+. Comparison of the coefficients suggests that Na+ to Mg2+ exchange reaction on smectite might have progressed markedly in the slip zone. One explanation for this feature is local progress of the reaction triggered by a recent thermogenic event, that was probably related with frictional coseimic slip during the earthquake. Considering that 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.