We investigated helium isotopes on gas extracted by crushing from melt and fluid inclusions in minerals from Plinian and inter-Plinian tephra and lavas of Vesuvius, Italy. Erupted products of different ages were considered, from Avellino eruption (1995 BCE) to the last eruption of 1944, with special focus on the 79 AD Plinian eruption. (Formula presented.) ratios between 1.5 and 2.7 (Formula presented.) were measured, with the highest values associated with rocks representative of the roof and the walls of the magma chambers (cumulates). Lowest values occurred in sanidines representative of magma-skarn interfaces. Noteworthy, the highest measured values of the 79 AD pumices were comparable with both lavas and tephra emitted from flank vents and under open-conduit conditions during the Medieval Period and Present Period of Vesuvius activity, and present-day fumarolic discharges. (Formula presented.) values are buffered within an extended, deep-seated reservoir at about 10 km filled with magma rising from the mantle. A fact that might potentially limit the accuracy of future eruption forecasting through monitoring of (Formula presented.) changes in Vesuvius fumaroles. Ageing and interaction with crustal rocks emerged as possible mechanisms that lowered the (Formula presented.) ratio of the melt during its intra-crustal magma chambers stay, with highest values associated with more dynamic conditions. Copyright © 2022 Gherardi, Barsanti, Principe and Magro.

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