Isotopic (Sr-Nd) and major element fingerprinting of distal tephras: an application to the Middle-Late Pleistocene markers from the Colli Albani volcano, central Italy

We describe the diagnostic lithological and chemical features of distal tephras from major Middle-Late Pleistocene (560-36 ka) explosive eruptions of the Colli Albani volcanic district, central Italy. In particular, we explore the time-dependent variability of the Sr and Nd isotope compositions as a tool for recognising and pinpointing individual Colli Albani tephra in distal settings. The distal tephras investigated are in lacustrine and fluvial sediments of central Apennine intermountain basins located 70 km-100 km east of Colli Albani. The recognition of the Colli Albani tephras is essentially based on the K-foiditic composition of their glass, which, within the Italian volcanological framework, is a distinctive character of the Colli Albani pyroclasts. In detail, these tephras are attributed to the following eruptive units: Tufo Pisolitico di Trigoria (561 +/- 2 ka); Tufo del Palatino-Tufo di Bagni Albule (530 +/- 2/527 +/- 2 ka), Tufo di Bagni Albule-Pozzolane Rosse air-fall sequence (517 +/- 1 to 500 +/- 3 ka), Pozzolane Rosse (457 +/- 4 ka), Villa Senni (365 +/- 4 ka), and Albano 5-7 (41 +/- 7 to 36 +/- 1 ka). These correlations are supported by Ar-40/Ar-39 dating of the distal tephras correlated to the Pozzolane Rosse (457.4 +/- 1.7 ka), Villa Senni (365 +/- 2 ka) and Albano 5-7 (41 +/- 9 ka) and by Sr-87/Sr-86 measured on clinopyroxene crystals and fresh glassy scoria from distal Colli Albani tephras that range from similar to 0.711 to similar to 0.709. These ratios are similar to those that characterise the individual proximal correlative units, and show the same decreasing trend over time. In contrast, the N-143/Nd-144 ratios for proximal and distal bulk samples and clinopyroxene increase from similar to 0.51212 to similar to 0.51215 from the oldest to the youngest tephra deposit. In summary, the study of Sr and Nd isotope compositions that is here applied on products from the Colli Albani volcanic district is a powerful, complementary tool to the more traditional tephrostratigraphic methods (e.g., componentry and electron microprobe analysis) for fingerprinting of distal tephras over a large region of the central Mediterranean, and over a large time interval, such as from 560 ka to 36 ka. (c) 2013 Elsevier Ltd. All rights reserved.