Mantle helium in hydrothermal plumes in the Galapagos Rift

Abstract

THE ³He/⁴He ratio in deep Pacific water is 20–30% higher than in atmospheric helium because of injection of primordial helium from the mantle^1,2. The largest ³He enrichments in the Pacific have been found in water on the crest of the East Pacific Rise where the isotopic ratios indicate² that the excess helium component has a ³He/⁴He ratio about ten times the atmospheric ratio, in agreement with the ratios measured in trapped helium in the glassy rims of oceanic tholeiites^3,4. Recent measurements in this laboratory⁵ have shown that the hot brines in the axial rift of the Red Sea are very highly enriched in mantle helium. ³He and ⁴He are respectively 3300 and 380 times supersaturated relative to atmospheric solubility equilibrium in seawater, with a ³He/⁴He ratio of 1.2×10⁻⁵, or 8.6 times the ratio in atmospheric helium. Comparison of the enrichments of various elements in the Red Sea brines and in brines associated with salt domes⁶ shows that helium is the only component in the Red Sea brines which unequivocally requires derivation from hydrothermal circulation of seawater in basalts. The helium isotopes are thus an extremely powerful and sensitive tracer for the detection and mapping of hydrothermal systems in oceanic spreading centres.