Reliability of volcano-sedimentary biotite ages across the Eocene-Oligocene boundary (Apennines, Italy)

This paper presents and discusses volcano-sedimentary biotite radioisotopic ages, obtained from the Paleogene sequence of the northeastern Apennines using RbSr and KAr dating methods. In particular, we discuss criteria which enable us to select the most reliable ages. K contents of < 6.4%, or traces of vermiculite indicate dubious reliability. Ion microprobe analyses of individual biotite flakes from each dated sample allowed us to determine whether these volcano-sedimentary biotites are composed of one or more populations of minerals. Chemically heterogeneous samples suspected of yielding spurious radioisotopic ages were tested for geochronological homogeneity using ⁴⁰Ar³⁹Ar laser fusion probe dating on several groups of grains. ⁴⁰Ar³⁹Ar step-heating dating or biotite from two stratigraphic levels from one locality yielded age spectra with good plateaux. Two other samples representing the same stratigraphic levels in another locality yielded somewhat disturbed spectra. We selected K-rich biotites showing good geochemical homogeneity, and/or displaying a wide plateau. and/or giving reasonably consistent KAr and RbSr ages from at least five stratigraphically distinct layers; they draw a picture consistent with the sequence. The selected radioisotopic ages permit accurate calibration of well-known bio-, magneto- and chemostratigraphic events from 36.4 to 28.1 Ma including a tightly interpolated age value of 33.7 ± 0.4 Ma (2σ) for the much debated Eocene-Oligocene boundary.