ABSTRACT

Compositional and mineralogical (primary silicate minerals, volcanic glass, secondary minerals) properties of weathered basaltic ash soil ("Andisol") derived from Mt. Fuji, central Japan, with an age of ca. 6,000 y.B.P. to 240 y.B.P. were studied. X-ray fluorescence analysis indicates that the order of relative elemental mobilities during the weathering is Na, Ca > K > Mg > P > Si > Ti, Fe > Al > Mn. The trends of soil water chemistry (H₄SiO₄ concentration) with depth were calculated based on a dissolution (volcanic glass)-precipitation (allophane) kinetics-fluid flow coupling model. In order to calculate the trends, the data on present-day annual rainfall, solubility of basalt glass, porosity and specific gravity of the soil, deposition rate of volcanic ash and grain size of volcanic glass were used. The calculated results were compared with analytical trends of soil water chemistry. From this comparison, the dissolution rate constant of basalt glass was estimated to be 10^-10.2±0.5 (mole Si m^-2s^-1). This value is consistent with previously determined experimental long-term dissolution rate constant of basalt glass reported in the literature (Berger et al., 1994).