HELIUM ISOTOPIC RATIO IN VULCANO ISLAND FUMAROLES - TEMPORAL VARIATIONS IN SHALLOW LEVEL MIXING AND DEEP MAGMATIC SUPPLY

Intensive gas emanations occur throughout the island of Vulcano, Italy. Sharp fluctuations recorded in the crater gas composition suggest the presence of two separate volcanic reservoirs and continuous mixing with another source, ''crustal'' waters. This mixing differs between the beach and crater fumaroles. Gas samples from three crater fumaroles with temperatures ranging from 200 to 550 degrees C were sampled repeatedly over a one year period. During the same interval of time, six samples from submarine and subaerial beach fumaroles and water well gases were also sampled. Gases from one crater fumarole (F5) showed variations of (He-3/He-4)(fumarole) to (He-3/He-4)(air) between 5 and 6 correlated with variations of several chemical species. High He-3/He-4 ratios for the beach fluids, similar to those of crater fluids, suggest the existence of a unique large magmatic reservoir at depth feeding both the crater and beach intermediate reservoirs. However, temporal changes clearly indicate variable degrees of fluids mixing, and the geographic distribution of the He-3/He-4 ratios as well as the chemical composition of the fluids suggest the existence between the magma reservoir and the surface of two intermediate different reservoirs, independently related to crater and to beach fumaroles.