Estimation of Illitization Rate of Smectite from the Thermal History of Murakami Deposit, Japan

Abstract

ABSTRACTThe research on illitization of smectite in the natural environment affords information on the long-term durability of bentonite which is the candidate for buffer material.Murakaml bentonite deposit in central Japan, where the bentonite and rhyolitic intrusive rock were distributed, was surveyed and the lateral variation of smectite to illite in the aureole of the rhyolite was studied.The radiometric ages of some minerals from the intrusive rock and the clay deposit were determined. Comparison of the mineral ages ( obtained by K-Ar, Rb-Sr and fission-track methods ) with closure temperature estimated for the various isotopic systems allowed the thermal history of the area. The age of the intrusion was 7.1± 0.5 Ma(; Mega d'annees), and the cooling rate of the intrusive rock was estimated to be approximately 45 °C/Ma.Sedimentation ages of the clay bed were mostly within the range from 18 to 14 Ma. However, the fission-track age of zircon in the clay containing illite/smectite mixed layers was 6.4±0.4 Ma, which was close to that of the intrusion. The latter value could be explained as the result of annealing of fission-tracks in zircon. The presence of annealing phenomena and the estimated cooling rate concluded that illitization had occured in the period of 3.4 Ma at least under the temperature range from above 240±50 to 105 °C. Illite-smectite mixed layers occured from smectite in the process. The proportion of iliite was about 40 %. Approximately, 29 kcal/mol as a value of activation energy was calculated to the illitization.The hydrogen isotopic ratio ( D/H ) of constitution water of the illite was determined. The values that were calculated for the water, which was related to the illitization, fell within the range of hydrogen isotopic ratios of seawater.