LANTHANOID ABUNDANCES OF ACIDIC HOT-SPRING AND CRATER LAKE WATERS IN THE KUSATSU-SHIRANE VOLCANO REGION, JAPAN

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.