The Campanian Ignimbrite and its precursor eruptions - implications for tephrochronology

The Campanian Ignimbrite (CI, Campi Flegrei, Naples, Italy) has been the focus of research for many decades and is well constrained in terms of eruptive volume (>200 km3), chemistry (phonolitic-trachytic to tephriphonolite-trachyandesite), 40Ar/39Ar age (39 ka) and distribution (plinian fall deposits cover >1500 km2.; proximal deposits are correlated with Y5, a widespread ash horizon found in cores from the eastern Mediterranean). Given its fall footprint across Europe it is a pivotal marker horizon in marine and lacustrine cores and archaeological sites as far afield as Russia (Pyle et al., 2006). However, its use in tephrochonology is only possible if “diagnostic chemistry” can be determined in proximal units where the age is well-constrained. Defining a diagnostic chemistry for the CI is complicated by the compositional variability of it’s erupted products, and by the existence of several proximal fall units erupted during the period 59 to 39 ka. Juvenile clasts (glass, phenocrysts, & melt inclusions) from fall deposits in the Campanian and several of the largest pre-Campanian units (TLa, TLc, TLf from Trefola Quarry, Naples) were analysed by LA-ICP-MS in order to: (a) define the proximal variation of juvenile clasts; (b) investigate the processes that lead to compositional variation among the eruptive products from Campi Flegrei, in order to determine which signatures provide the most reliable geochemical fingerprint; and (c) to decide upon the CI fingerprint for use in tephrochronology in distal archaeological sites and marine/lacustrine cores. The pre-CI juvenile products are highly evolved, undersaturated magmas (dominantly phonolites) and are less chemically heterogeneous than the CI. Geochemical plots support fractionation of magnetite and feldspar, Consistent ratios of elements with similar degrees of incompatibility indicate that the pre-CI and the first erupted CI products may share a common source at depth before source and/or crustal contamination, as also evidences by Sr and Nd isotope compositions (Arienzo et al., 2009).