木川田 喜一

Concentrations of arsenic and bromine dissolved in hot spring waters have been determined by neutron activation analysis using 0.5 cm(3) of sample waters without any chemical pretreatment. The samples prepared for neutron irradiation were simply pieces of filter papers which were infiltrated with samples. With the results of satisfactorily high accuracy and precision, this analytical method was found to be very convenient for the determinations of arsenic and bromine dissolved in water at ppm to sub-ppm levels.

Concentration of lanthanides (Lns) in alunite group mineral samples from the Kusatsu-Shirane volcano area, Gunma, Japan, were determined by neutron activation analysis. Their Ln abundance patterns showed enrichment of light Lns relative to their original rocks and GSJ geochemical reference samples of feldspars. It was found that the concentration of light Lns increased with increasing concentration of K and P. The positive correlation between the light Lns and P concentration suggested the formation of florencite, whereas the positive correlation between the concentration of light Lns and K may reflect the difference between the solubility of double salts of K and Ln sulfates of light Lns and heavy Lns, respectively.

Long-term changes in temperature, pH and major element chemistry during 1969 and 1999 were investigated for the three hot springs, Manza-Yubatake, Okumanza and Manza-Karabuki, in the Manza hot spring area located in the western foot of Kusatsu-Shirane volcano, Gunma, Japan. At Manza-Yubatake and Okumanza, water temperatures have been decreasing for at least these 60 years except occasionally small irregular changes probably accompanied by the change of volcanic activity of Kusatsu-Shirane volcano. The concentrations of dissolved components of Manza-Karabuki water have widely fluctuated obviously related with volcanic activities, while the water temperature has rarely been fluctuated. The waters of the three hot springs show similar ratios among the concentration of the analyzed dissolved components except calcium and sulfate ions, which are possibly controlled by the solubility of gypsum. The similarity of chemical composition and the highest concentration of dissolved species in the Manza-Karabuki waters indicate that Manza-Yubatake and Okumanza waters are formed by the mixing between local meteoric water and the Manza-Karabuki water, which has the closest chemical composition to the primitive hydrothermal fluids related to the volcanic activity in the Manza area.

The mobility of lanthanides (Lns) accompanying interactions of powdered andesitic rocks with acidic hot spring water and acidic hot spring water vapor/steam was studied in the experiments set up in the field. At the early stage of rock alteration, light Lns were slightly easier to be leached out by acidic hot spring waters from andesitic rocks than heavy Lns. This result qualitatively agreed with the result obtained in the previous experiments carried out in the laboratory. It was also found that the Ln abundance patterns (Ln patterns) obtained by normalization with Ln contents of the original rock showed slightly positive Eu anomalies at the early stage of rock alteration and turned to show negative Eu anomalies and the extent of negative anomaly monotonously increased as rock alteration proceeded. This trend in the direction of Eu anomaly was correlated to the degrees of decompositions of major rock-forming minerals and interstitial materials in the rock. At the earliest stage of rock alteration in acidic environments, the interstitial materials with relatively low Eu contents were decomposed, which resulted in the positive Eu anomaly. In the following stage, feldspar that had a relatively high Eu content was decomposed, which led to the negative Eu anomaly. (C) 2001 Elsevier Science B.V. All rights reserved.

Contents of lanthanoids (Ln's) of rock salts have been measured by neutron activation analysis. Original salt samples were treated in advance of neutron irradiation so that Ln's were enriched and amounts of interfering nuclides were reduced. The contents of Ln's were at ppt - sub ppb levels and were comparable with or slightly lower than those of salar salts. The Ln abundance patterns of the salts were those with relative depletion in the heavy Ln's, thus having negative slopes. It was indicated that, when salt deposit was formed, Ln's were taken up by anhydrite more preferentially than by halite.

A brine sample and two deposit samples of a salt lake in China were analyzed for their contents of lanthanoids (Ln's), thorium and uranium by neutron activation analysis. Five Ln's were determined at sub ppb levels. Th and U contents were at about the same levels as those of Ln's. The lanthanoid abundance patterns (Ln pattern) of the three samples are similar to each other, each having a negative slope in the light Ln region. There seems no substantial difference in distribution between the solution (brine) and solid (deposit) phases among Ln's.

Geochemical study on Yugama, a crater lake at Kusatsu-Shirane volcano, has been conducted since 1966. Amounts of various cationic species in Yugama water started increasing around 1981, slightly before the phreatic eruptions in 1982-1983, and kept increasing until 1985. In 1986, they turned to decrease and at present restore their former levels before the eruptions. The concentration of sulfate ion showed a secular change similar to those of cationic species, but no such variation was observed for chloride ion even during the 1976 and 1982-1983 eruptions. However, it started increasing in 1989. A high correlation between the concentrations of chloride and hydrogen ions suggests an increasing influx of hydrogen chloride from the deep volcanic systems under the lake. A Cl^--SO₄^2- is an excellent monitor of the variation in volcanic activity at Kusatsu-Shirane volcano; all of the three past activities since 1966, i.e., the high-level subsurface activity in 1968, the eruption in 1976 and the eruptions in 1982-1983, showed a reverse secular change with time in the Cl^--SO₄^2- plot. This could be attributable to the function of Yugama water as a condenser of volatiles released underground.

Laboratory experiments on leaching of lanthanoids (Ln's) from andesitic rocks by acidic aqueous solutions were carried out in order to find some clue to better elucidation of Ln patterns of acidic hot spring waters in nature. Ln's are different from major elements but phosphorus (P) in behavior upon leaching: While leaching of the major elements continued even at the reaction time (t(R)) of 64 h, that of Ln's seemingly finished by t(R) = 2 or 4 h. The behavior of P upon leaching was dissimilar to that of other major elements but rather resembled that of Ln's. This indicates that a part of each Ln may exist as phosphates in rocks. Light Ln's were found to be leached out by acidic waters more easily than heavy Ln's. This could be due to the difference in ionic radii of Ln's and/or be attributed to the difference in resistance to dissolution by acidic waters among various Ln-containing minerals in the rocks used in the present experiments. The results obtained are successfully applied to the elucidation of the Ln patterns of the waters from the Kusatsu-Yubatake hot spring, the Kusatsu spa, Gunma and from the Obuki hot spring, the Tamagawa spa, Akita.

The lanthanoid (Ln) contents of nine hot spring and one crater lake waters in the Kusatsu-shirane volcano region, Japan were measured by neutron activation analysis. The waters examined were all acidic with pH ranging from 1.21 to 4.21. All fourteen naturally occurring Ln's were determined for six waters and at least seven Ln's were determined for the other waters. The Ln contents were as a whole at the ppb levels. The Ln patterns obtained by normalization with the Ln contents in Leedey chondrites differed from water to water but were classifiable into two groups; one with a relative enrichment of the light Ln's and thus showing negative slopes and the other having no or little slope (horizontal patterns). The horizontal patterns were found for the Kusatsu-yubatake and Bandaiko hot spring waters and the Yugama water. The Ln patterns of the waters in the Kusatsu area are explained by considering that the light Ln's are leached from rock by flowing acidic waters faster than the heavy Ln's, that in the Ln distribution between acidic water with high Ln contents and rock the heavy Ln's are relatively more preferentially distributed into the solution phase than the light Ln's are, and in addition that andesitic rocks in the Kusatsu area altered by flowing acidic waters have flatter Ln patterns than those of unaltered ones. It was also found that the fluctuation of the Ln pattern from the expected smooth line was in general larger for the heavy Ln's than for the light Ln's, which could be a manifestation of the tetrad effect.

Determination of the lanthanoids, thorium and uranium in silicate rocks has been investigated by neutron activation analysis (NAA). Seven or eight lanthanoids and thorium and uranium were determined by non-destructive instrumental NAA. The numbers of the lanthanoids determined were increased and errors on the final values were reduced by pre-irradiation treatments, which included a coprecipitation process with aluminium as collector to remove the alkali metals and halogens and a solvent extraction process to eliminate iron. The necessity of scandium removal was indicated.

Neutron activation analysis was successfully applied to the determination of Th and U at ppt levels in a neutral hot spring water. Blank test corrections were found to be essential to reach the final determined values. Normal NAA is a better method for the Th determination than epithermal NAA, while both NAAs are nearly equally effective for U determination.