Geochemical fingerprints of volcanic materials: Identification of a pumice trade route from Pompeii to Rome
Geochemical investigations on samples of pumice and lava extracted from the mortars and concretes of the Forum of Caesar and the Forum and Markets of Trajan are pre- sented and integrated with previous analyses of lava samples from Rome and Pompeii, and of pumices from the Colosseum and the Dioclethian's Bath, with the aim to identify the lithological provenance of these materi- als. Discrimination diagrams based on ratios of selected trace elements (i.e., Zr/Y, Nb/Y, Th/Ta, Nb/Zr) allow us to recognize the geo- chemical signature of the different volcanic regions of central Italy. These diagrams in- dicate that in the mortars of the Forum of Caesar and Forum of Trajan, the Roman builders integrated the scanty pumices from deposits of the Monti Sabatini Volcanic Dis- trict near Rome with others coming from the Vesuvius and Phlegrean Fields. Some of the pumices employed in the vaulted ceil- ings of these monuments display a peculiar geochemistry that has a unique correspon- dence with a pyroclastic deposit recovered in a borehole drilled on the southern fl anks of Vesuvius. Similarly, lava fl ows from this borehole display a different trace-element signature with respect to that of other coeval lava fl ows of the northern fl anks of Vesuvius, and it has a closer correspondence to that of the archaeological lava samples. These observations, integrated with historical and archaeological data, indicate that the Roman builders had a profound empirical knowl- edge of the physical properties of the volcanic rocks from different regions, and suggest that a systematic cultivation of lightweight volca- nic material occurred in the area of Pompeii for exportation to Rome and, likely, to other regions of the Mediterranean.