Abstract: | 3He/4He ratios in lavas erupted during the last 360 years at Mt. Vesuvius are between 2.2 and 2.7 RA (RA = atmospheric ratio of 1.39 × 10−6), and are among the lowest values measured in young volcanic rocks. They are also identical to values measured in summit crater fumaroles sampled during 1987–1991. This agreement indicates that the 3He/4He ratio in the crater fumaroles faithfully tracks the magmatic value. The relatively low and uniform 3He/4He ratio in the lavas reflects either a mantle source enriched in (U + Th)/3He, or a mixture of magmatic and crustal components. |