Mantle helium in the Red Sea brines

Abstract

HELIUM isotope studies on terrestrial samples have revealed the existence of two helium components which are clearly distinct from atmospheric helium. The first of these, which we term ‘crustal helium’, was identified in 1946 in natural gas wells¹. This crustal component is produced by radioactive decay of U and Th to ⁴He, with ³He production by (n, α) reactions on Li; the resulting helium is characterised by ³He/⁴He ≃ 10⁻⁷, one-tenth of the atmospheric ratio². The second component, ‘mantle helium’, was discovered as ‘excess ³He’ in deep ocean water, attributed to a flux of primordial helium from the mantle³. Studies of the ³He/⁴He ratio in deep water on the East Pacific Rise⁴ and in helium trapped in submarine basalt glasses^5,6 have shown that this mantle component is characterised by ³He/⁴He ≃ 10⁻⁵, about 10 times the atmospheric ratio and 100 times the ratio in crustal helium. Basalt glasses from the Western Pacific Lau Basin, the East Pacific Rise, and the Mid-Atlantic Ridge contain trapped helium with similar ³He/⁴He ratios, indicating that mantle helium in at least three areas in which new lithosphere is being formed has a unique and uniform isotopic signature.