HELIUM-ISOTOPES IN SOME HISTORICAL LAVAS FROM MOUNT VESUVIUS

He-3/He-4 ratios in lavas erupted during the last 360 years at Mt. Ves uvius are between 2.2 and 2.7 R(A) (R(A) = atmospheric ratio of 1.39 X 10(-6)), and are among the lowest values measured in young volcanic r ocks. They are also identical to values measured in summit crater fuma roles sampled during 1987-1991. This agreement indicates that the He-3 /He-4 ratio in the crater fumaroles faithfully tracks the magmatic val ue. The relatively low and uniform He-3/He-4 ratio in the lavas reflec ts either a mantle source enriched in (U + Th)/He-3, or a mixture of m agmatic and crustal components.