Progressive alkali ion exchange with stratigraphic position in a pyroclastic cooling-unit; Mt. Torro, Sabatini, Central Italy

Significant variation of K2O/Na2O ratios and total alkali contents, elevated Loss on Ignition (12–16 wt.%) values and elevated Fe3+/Fe2+ are shown in new data from a road cut profile of pyroclastic rocks in an area of young ultra-potassic rocks. Decreasing K2O/Na2O ratios and total alkali contents are systematically related to stratigraphic position, being most pronounced at the centre of the deposit, probably because of the slower cooling rate. Pumice bombs are less altered than ash but show the same variation pattern. Except for the alkalis, hydration and the oxidation state of iron, the geochemical compositions of the rocks are unchanged.