³⁹Ar-⁴⁰Ar and Rb-Sr age determinations on quaternary volcanic rocks from the roman volcanic province

Abstract

We report ³⁹Ar-⁴⁰Ar and Rb-Sr analyses performed on mineral separates from very young (~350,000 yr) tuffs from the potassic Roman volcanic province to establish the feasibility of age measurements on potassic minerals with the high precision necessary for good stratigraphic definition. Several outcrops of the Villa Senni tuff in the Albano volcanic complex were sampled. The tuff contains fresh leucite and biotite crystals 0.1 - 1 cm in size and minor amounts of amphibole. Leucite and biotite separates were irradiated for the ³⁹Ar-⁴⁰Ar analyses with a fast neutron fluence of ~7 x 10¹⁴ n cm² in the TRIGA reactor facility at the University of California, Berkeley. The Be 4 M standard muscovite (Steiger, 1964; Lanphere and Dalrymple, 1965) was used as the irradiation monitor . Neutron fluence inhomogeneities were monitored by measuring the ⁵⁸Co γ activity induced in Ni wires that were placed in known positions. The monitor ⁴⁰Ar/K ratio is reported with an uncertainty of ~4 percent, which affects the absolute ages of our samples. The errors in the relative ages from the ma ss spectrometric analyses and the fluence normalizations amount to approximately 8000 years (2σ). We have analyzed the samples for Ar using the stepwise heating technique. Typically we used eight steps plus a high-temperature (~1650°C) re-extraction step which was never noticeably above the blank level. The first observation to be made is that the bulk ⁴⁰Ar/³⁶Ar ratios of leucites are much higher than those of biotites (leucites ~5,000; biotites ~360).

Additional details