Researchers have dated material from the 79 CE eruption of Mount Vesuvius with substantially greater precision than previous argon-based measurements. The study, published in Science Advances, shows how improvements in mass spectrometry and sample preparation could strengthen the dating of relatively recent volcanic events.
The eruption witnessed by Pliny the Younger provides a rare opportunity to compare a laboratory measurement with a known historical date. Using argon-40/argon-39 dating, the researchers calculated that the eruption occurred 1,938 ± 13 years before the samples were measured in 2025. This corresponds to 87 ± 13 CE and is consistent with the accepted date of 79 CE.
The result achieved a relative precision of 0.7 percent, compared with uncertainties of approximately 100 years in earlier argon-40/argon-39 studies of the same eruption.
Researchers analyzed sanidine crystals from white pumice collected at Villa Poppea in Oplontis, about 1.8 kilometers from Pompeii.
Sanidine is a potassium-rich mineral that can be used for argon dating. Over time, radioactive potassium-40 decays to argon-40. By measuring this process, researchers can estimate how long ago a mineral cooled and trapped the argon produced within it.
The team separated sanidine from approximately 2 kilograms of pumice. The crystals were cleaned, sieved, and handpicked to remove visible inclusions or attached material. Scanning electron microscopy of 50 grains found no evidence of compositional zoning.
The samples were exposed to neutrons for six minutes at the Oregon State University TRIGA reactor. This converted some potassium-39 into argon-39, providing a reference for calculating the amount of radiogenic argon-40.
The researchers then heated the crystals in stages with a carbon dioxide laser, releasing argon for analysis. A multicollector mass spectrometer measured several argon isotopes simultaneously. Low-noise amplifiers improved the precision of measurements from small ion beams.
The final dataset included 153 heating steps from 48 groups of crystals. Material from two of the 10 irradiation wells was excluded after results indicated contamination with older mineral grains or dust. Those samples produced dates ranging from the expected eruption age to more than 20,000 years.
The researchers attributed the improved result to several analytical refinements. These included short neutron irradiation, cadmium shielding, close placement of reference standards, laser-based incremental heating, and improved correction for interfering isotopes produced during irradiation.
The analysis also detected a small amount of excess argon that was probably trapped in the magma. Identifying and correcting for this component helped prevent the age from being overestimated.
Because the Vesuvius eruption has a known historical date, the researchers were also able to refine the decay constant describing the conversion of potassium-40 to argon-40. Their estimate was almost twice as precise as a previous value determined through direct radioactive decay counting.
A more accurate decay constant could improve argon-based dates across geological time and make it easier to compare results with radiocarbon and uranium-lead dating. For more recent volcanic events, narrower uncertainty ranges could help researchers reconstruct eruption frequency, magma development, and long-term volcanic hazards.
The approach depends on the availability of suitable potassium-rich minerals and careful control of contamination and analytical interference. Its performance may therefore vary among volcanic deposits.
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