Eiji Izawa

Bio-deposition of Amorphous Silica by an Extremely Thermophilic Bacterium, Thermus spp.

The bio-deposition of amorphous silica, which occurred in vitro by exposure to the extremely thermophilic bacterium Thermus spp. began from the latter part of the exponential phase of growth of the bacteria. The concentration with which the deposition occurred exceeded the solubility of amorphous silica of neutral pH at the temperature 60~85°C. Our observations suggest that Thermus spp. promotes the formation of siliceous minerals in a geothermal environment.

Kinetic Characteristic of Bubble Nucleation in Superheated Water using Fluid Inclusions

Water in fluid inclusions achieved a superheated condition by isochoric cooling method and the kinetic characteristic of bubble nucleation in water was investigated. The conventional study did not succeed in drawing a picture of bubble nucleation in water. The present study demonstrates that the homogeneous nucleation theory can describe the kinetic property of bubble nucleation in water, and that density fluctuation is a random event which brings about bubble nucleation in superheated water, and the lifetime of superheated water can be predicted with respect to the depth of penetration into the metastable region by the theory.

Magmatic contributions to hydrothermal systems

Although there is agreement that many hydrothermal systems in the upper crust derive their thermal energy from magmas, debate continues over the extent to which magmas contribute water, metals, and sulfur to hydrothermal systems. A multidisciplinary seminar was held November 10–16, 1991, in Ebino and Kagoshima, Japan, to establish current understanding about this topic and to explore the major unanswered questions and the most promising research directions. The thirty-eight participants were from Japan (eighteen), the U.S. (thirteen), Canada and New Zealand (two each), and England, the Philippines, and Russia (one each). Disciplines represented were volcanology, geochemistry (volcanic-gas, water, isotopes, experimental, and modeling), igneous petrology, geothermal geology, economic geology, fluid-inclusion study, geophysics, and physical modeling.

Timing of Au-Ag mineralization and related volcanism at Otoge, Yamagata Prefecture, Northeast Japan.

K-Ar ages of Au-Ag mineralization and hydrothermal alteration at the Otoge deposit were determined to be 3.93±0.15, 3.99±0.10, 4.10±0.10 and 4.15±0.10 Ma for four sericite separates, proving that the Au-Ag bearing quartz veins at deep level and the sericite zone of the clay deposit at the surface of present level were formed contemporaneously. The ages are also close to those of Au-Ag-Pb-Zn mineralization at the Yatani deposit, 3 km northeast of the Otoge deposit, suggesting that mineralization in both areas occurred within a series of hydrothermal systems. The eruption ages for welded tuff of the upper part of the Otoge Formation were determined to be 4.85±0.13 Ma (biotite, K-Ar age) and 4.60±0.30 Ma (zircon, fission track age), predating the mineralization.