ABUNDANCE OF THE LANTHANOIDS IN WATERS AND LACUSTRINE DEPOSITS OF YUGAMA, THE CRATER LAKE OF MTKUSATSU SHIRANE

The abundances of the lanthanoids in waters and lacustrine deposits of Yugama, the crater lake of Mt. Kusatsu-shirane, Gunma, and in a volca nic block erupted by its volcanic activity were determined by the neut ron activation analysis. The lanthanoids were detected at ppb levels i n water samples and at ppm levels in solid samples, and no exception t o the Oddo-Harkins rule was observed. The lanthanoid patterns in the s olid samples normalized to the Leedey chondrite were similar to each o ther and were typical ones observed in natural rocks, having negative slopes which were especially clear in the light lanthanoids. The patte rns in the water samples had more moderate negative slopes than those of the solid samples and were very close to the horizontal one. The pl ot of the partition coefficient, defined as the ratio of the concentra tion of an element in the water phase to that in the solid phase, agai nst the ionic radius of trivalent metals was found to have a peak at a bout 90-95 pm, i.e., trivalent metal ions with ionic radii of 90-95 pm were most preferentially fractionated into the water phase in acidic circumstances.