Hydrothermal Origin Of Smectite In Volcanic Ash

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Journal Article: Hydrothermal Origin Of Smectite In Volcanic Ash

Smectite and fine-grained quartz were separated from 6 volcanic ash samples collected in Japan from Shinmoe-dake Volcano, southern Kyushu, and Mt. Usu Volcano, southern Hokkaido. Oxygen isotope ratios of smectite in the volcanic ash range from +6.9 to +12.9 per mill (parts per thousand), which are comparable to the values of smectite (delta(18)O = +6.5 and +9.4 parts per thousand) from currently active geothermal fields. Evaluation of the oxygen isotope data from smectite (delta(18)O = +6.9 and +8.4 parts per thousand) and quartz (delta(18)O = +6.6 to +11.2 parts per thousand) in volcanic ash erupted from Shinmoe-dake in 1959 indicates a hydrothermal origin at elevated temperatures (150 to 270 degrees C) from waters that have calculated delta(18)O values (delta(18)O = -3 to +5 parts per thousand) that are enriched relative to local meteoric water (delta(18)O = -7 to +8 parts per thousand). This precludes an authigenic formation of the smectites under ambient temperatures in crater lakes and/or somma-atrios which had been previously proposed as a plausible mechanism. A peculiar clay flow was extruded on the somma-atrio of Mt. Usu Volcano. The isotopic composition of the clay (<0.5 mu m, delta(18)O = + 12.2 parts per thousand) and evidence from geophysical exploration over volcanic vents of Shinmoe-dake support the concept that extensive hydrothermal alteration is taking place within volcanic vents.

C. Mizota and K. Faure

Published Journal 
Clays and Clay Minerals, 1998

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C. Mizota,K. Faure. 1998. Hydrothermal Origin Of Smectite In Volcanic Ash. Clays and Clay Minerals. (!) .