Reassessment of WD-XRF method for obsidian provenance shareable databases

The X-ray fluorescence technique using peak intensity ratios of trace elements is one of the faster, less expensive and less invasive methods for obsidian sourcing. Moreover, XRF peak intensity ratios are particularly useful when the artifact is slightly altered, particularly affecting the alkali cations, or when its surfaces are covered with a fine film of carbonate deposits formed while the sample was buried. The limitation of this technique results from the dimensions of the obsidian samples: very small obsidian samples, nowadays about 1 cm in size, are difficult to measure given the very few X-rays emitted by their surface. An aluminium holder closed with a very thin Mylar© polyester film was prepared and the specimen, positioned at the centre of the surface of the Mylar film, was placed in the XRF spectrometer. The use of recent technology XRF apparatus equipped with a 4 kW Rh anode X-ray tube coupled with an accurate measurement of refined X-ray characteristic lines, that exclude the X-ray background contribution, of five trace elements (Rb, Sr, Y, Zr, Nb), made it possible to determine the source geological outcrop of archaeological obsidian samples as little as 7 mm and as light as 40 mg. A database of peak intensity ratios, background and interference free, of five trace elements was produced for obsidian samples from Monte Arci, Palmarola, Lipari, Pantelleria, Gyali and Melos. This database may be used by other laboratories provided the X-ray intensity of the trace elements Rb, Sr, Y, Zr, Nb are measured as net intensities, that is purified from the contribution of the background and from interelemental interferences. A comparison between data obtained with two spectrometers, of different generation and differently calibrated, confirmed the possibility of obtaining a database that can be shared with other laboratories. The proposed XRF method was tested on Neolithic obsidians from four Apulian sites (Ripa Tetta at Lucera, Palestra ex G.I.L. at Foggia, Pulo di Molfetta and Grotta di Santa Barbara at Polignano a Mare), whose source outcrops had been also determined by SEM-EDS.