Hydrothermal Origin of Smectite in Volcanic Ash

Chitoshi Mizota1 and Kevin Faure2
1 Faculty of Agriculture, Iwate University, Ueda 3-18-8, Morioka 020, Japan
2 Department of Geochemistry, Geological Survey of Japan, Higashi 1-1-3, Tsukuba, Ibaraki 309, Japan

Abstract: 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 (‰), which are comparable to the values of smectite (δ18O = +6.5 and +9.4‰) from currently active geothermal fields. Evaluation of the oxygen isotope data from smectite (δ18O = +6.9 and +8.4‰) and quartz (δ18O = +6.6 to +11.2‰) in volcanic ash erupted from Shinmoe-dake in 1959 indicates a hydrothermal origin at elevated temperatures (150 to 270 °C from waters that have calculated δ18O values (δ18O = −3 to +5‰) that are enriched relative to local meteoric water (δ18O = −7 to −8‰). 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 sommaatrio of Mt. Usu Volcano. The isotopic composition of the clay (<0.5 µm, δ18O = + 12.2‰) and evidence from geophysical exploration over volcanic vents of Shinmoe-dake support the concept that extensive hydrothermal alteration is taking place within volcanic vents.

Key Words: Hydrothermal Alteration • Oxygen Isotope Composition • Smectite • Volcanic Ash

Clays and Clay Minerals; April 1998 v. 46; no. 2; p. 178-182; DOI: 10.1346/CCMN.1998.0460208
© 1998, The Clay Minerals Society
Clay Minerals Society (www.clays.org)