Hydrothermal Origin of Smectite in Volcanic Ash

Abstraet--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 (%o), which are comparable to the values of smectite (~lsO = +6.5 and +9.4%0) from currently active geothermal fields. Evaluation of the oxygen isotope data from smectite (~180 = +6.9 and +8.4%,~) and quartz (~lsO = +6.6 to +11.2%o) in volcanic ash erupted from Shinmoe-dake in 1959 indicates a hydrothermal origin at elevated temperatures (150 to 270 ~ from waters that have calculated ~sO values (~180 = 3 to +5%0) that are enriched relative to local meteoric water (51sO = 7 to -8%0). 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 Ixm, ~80 = + 12.2%o) 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.