Chemical weathering of volcanic rocks, Pantelleria Island: information from soil profile and soil solution investigations

Concentrations of major, minor and trace elements were determined in soil profiles and soil solutions from the island of Pantelleria, Sicily Channel, to evaluate the weathering extent of soils evolved on trachytic and pantelleritic rocks and the aqueous transport of elements by their soil solutions. The chemical index of alteration (CIA) indicates a low-to-moderate degree of weathering; consistently, the mineralogical and geochemical imprints of the parent rocks are generally preserved. The chemical weathering appears to be incongruent, owing to primary minerals and glass dissolving to a variable degree while secondary minerals have formed. Based on the calculated saturation state of primary and secondary minerals with respect to the soil solutions and on mineralogical observations, olivine and pyroxene are unstable and dissolving whereas magnetite and hematite are stable. The main weathering products are amorphous phases, such as ferrihydrite, amorphous aluminium hydroxide and silica. Clay mineral formation appears to be incipient and mainly limited to kaolinite and smectite, although for some soil profiles a hydrothermal origin is suggested for these minerals. Chemical characteristics of soil solutions and elemental mobility are controlled by the element source and water-rock interaction.