Carbon and oxygen isotopic composition of carbonates in limestone blocks and related geodes from the “black pozzolans” formation of the Alban Hills

A determination was made of the carbon and oxygen isotopic composition in the carbonates of ejected limestone blocks and related fluorine and boron bearing geodes found in the Corcolle “black pozzolans” from the Alban Hills. In all samples considered, the 18 O 16 O and 13 C 12 C ratios increase from the outside in, the latter more markedly than the former. In particular, the isotopic composition of oxygen always falls within the range of sedimentary and metamorphic limestones, while unusually low δ 13C values are recorded for carbon. The data obtained are interpreted as a result of a complex metamorphic process governed by temperature and by the reactions with volcanic gases and hydrothermal solutions.