40Ar- 39Ar and Rb-Sr age determinations on Quaternary volcanic rocks

Abstract

40Ar- 39Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides 40K and 87Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk 40/36 ratios (above 1000) and in which the 40Ar and the 39Ar were very well correlated, yielding well-defined ages averaging 3.38±0.08×10 5 years. Two leucites contained Ar with lower bulk 40/36 ratios (∼400), and in at least two release steps from these leucites the 40Ar/ 36Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic 40Ar from trapped 40Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators. Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in 87Sr/ 86Sr as large as 16 parts in 10 4 and 87Rb/ 86Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from 3.8±0.2×10 5to3.3±0.2×10 5 years and are in good agreement with the 40Ar- 39Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial 87Sr/ 86Sr. However, different tuffs show small differences in initial 87Sr/ 86Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the 40Ar- 39Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.