ABSTRACT

In order to reveal elemental mobility during weathering rind formation, a basalt cobble buried in a fluvial terrace conglomerate layer is examined based on the elemental concentration profiles across the rind to fresh core for major and trace elements including rare earth elements. The weathering rind showed extreme depletion of SiO₂, FeO, MgO, CaO, Na₂O, Co, Zn, Sr, Ni, Y and LREEs and slight depletion of Al₂O₃ compared to fresh core. Dissolution of SiO₂ together with Al₂O₃ and Zn can be explained by basification of interstitial water by dissolution of alkali and alkaline earth elements under the groundwater table. While REEs and Y are immobile under the basic condition because of low solubility, dissolution of these elements may have occurred under the oxidative and acidic condition above groundwater table after subsequent uplifting the layer. Thus the dissolution of elements have occurred at two stages during weathering rind formation.