Separation and analysis of Theran volcanic glass by INAA, XRF and EPMA

Pumice from the "Minoan eruption" on Thera (Cyclades, Greece) was investiga ted to reveal the differences between the composition of the bulk material, which contains a certain percentage of crystallites, and the pure glass ph ase. This is important for the identification of vitric tephra layers found in the Eastern Mediterranean region in archaeological context and in deep sea drilling cores. Eruption products, deposited at some distance, have usu ally lost their crystalline fraction due to gravity separation and consist only of glass shards. Only major element distributions in such layers and i n pumiceous glass phases have been published up to now, but these data are not sufficient for a reliable identification of the volcanic source, as sev eral other eruptions are known to have produced chemically very similar lay ers in this region (Milos, Nisyros, Yali, Kos). Therefore, a technique has been developed to separate the glass phase from the primary pumice to revea l differences in the trace element distributions obtained by instrumental n eutron activation analysis (INAA). X-ray diffractometry and microscopical t echniques were applied to check the purity of this fraction. The concentrat ions of the major constituents, in particular Al, Ca, Fe, K, ME, Mn, Na, Si , and Ti were determined by X-ray fluorescence (XRF) and electron probe mic roanalysis (EPMA), those of Al, Ba, Ca, Ce, Co, Cr, Cs, Dy, Eu, Fe, Hf, It, La, Lu, Mn, Na, Nd, Rb, Sb, Sc, Sm, Ta, Ti, Th, U, Yb and Zr by instrument al neutron activation analysis and partly also by X-ray fluorescence. Subtl e differences between the compositions of the glass phase and the bulk mate rial are explained by differentiation during partial crystallization and th eir applicability to the classification of tephra layers is demonstrated.