Distributión of trace elements in altered pyroclastites from the Monte Vulture volcano (Southern Italy)

Resumen

Three pyroclastic deposits from Monte Vulture volcanic arca (Potenza, southern Italy) looking like paleosols in the field were investigated in a previous study for mineralogy and major elements to estimate the stage of the weathering. Here is dealt with the behaviour of some trace elements (Ce, La, Ba, Ni, Cr, V, Rb, Sr, Y, Zr and Nb) in the same deposits to give a comprehensive geochemical picture. The distribution of the chemical elements within the whole rock and after its attack with Na-pyrosulfate (residue + solute) has been considered. Ba and Sr, as well as their distribution, appear to be controlled by the residual crystals in each of the deposits; La, Ce, Y and Nb are more concentrated in the solute that once was representad by vitric component, allophane, and Fe-Si-Al gels, biotite, carbonates and analcite; V, Cr, and Ni show similar trends in whole rock and in solute. In particular La, Ce, Y, V, Cr and Ni in the lowermost unit increase with the depth, as well as the contents of gels and allophane. Probably this behaviour was superimposed by the fluctuation of the water tables, as documented by the occurrence of a carbonate level upon the unit lies. lt is concluded that the earliest stage of weathering did not affect the trace element distribution and that interpretations about chemical changes in deeply altered pyroclastic rocks should be always the outcome of careful accurate analyses