The clock of time in the minerals of Pompei

Berkeley and Padua, in their study published in Science Advances, refined the argon-argon technique by analysing samples of sanidine, a mineral extremely rich in potassium, using state-of-the-art mass spectrometers and new irradiation techniques. The samples were collected by Professor Andrea Marzoli from the Department of Land, Environment, Agriculture and Forestry of the University of Padua at Villa di Poppea in Oplontis, a suburban area of nearby Pompei corresponding to present-day Torre Annunziata.

These are rock and mineral samples deposited during the initial phases of the eruption most famous in the world, namely that of AD 79, which destroyed Pompei and Ercolano. Described by Pliny the Younger, the eruption now offers geology a magnifying glass of incomparable precision with which to travel back in time and date precisely even events from thousands of years ago.

Knowing that the eruption had taken place in AD 79, the researchers attempted to confirm that date by using the Oplontis samples and subjecting them to analysis using the argon-argon method.

The chemical mechanism of the argon-argon method is based on the natural decay of the potassium-40 isotope present in volcanic minerals, which over time transforms into argon-40. Through neutron bombardment in the laboratory, potassium-39 is converted into argon-39: the age of the mineral is derived from the ratio between the two gaseous isotopes.

The age of the minerals obtained with this method is 1939 ± 13 years, which corresponds perfectly to the age of AD 79 derived from the writings of Pliny the Younger. The level of accuracy achieved, with an analytical error of 13 years, less than 1% of the value, had never before been reached for isotopic dating of such young materials. This level of accuracy will make it possible to calibrate other dating methods important for archaeological studies, such as the radiocarbon method. It also makes it possible to refine knowledge of the decay constant of potassium-40, that is, the rate at which this isotope is transformed into argon isotope 40, and therefore to increase the accuracy with which rocks are dated throughout the entire history of the Earth, from its very origin.

«The work demonstrates that 40Ar/39Ar dating can achieve levels of precision and accuracy previously thought unattainable for events from the historical period, strengthening its role as one of the main tools for reconstructing the history of the Earth,» says Paul Renne, director of the Berkeley Geochronology Center. «The results will help improve the chronological reconstruction of volcanic eruptions, climate change, mass extinction events and other geological processes, as well as providing an important benchmark for the calibration of other dating methods.»