THE PRIMORDIAL NOBLE GASES IN THE EARTH: A KEY CONSTRAINT ON EARTH EVOLUTION MODELS

Abstract

We made an extensive compilation of noble gas data obtained from MORB, oceanic island basalts (OIB) and diamonds. On the basis of this data base, we discuss the primordial noble gases in the Earth and its implications on mantle evolution. We conclude that in the contrary to a common assumption, the primordial Ne, Ar, Kr, and Xe in the Earth are distinctly different from the solar noble gases; the primordial noble gases were derived from the solar noble gas through Rayleigh distillation-like, mass-dependent fractionation. The primordial Ar, Kr, Xe are identical with the atmospheric components, but Ne observed in the mantle-derived materials differs significantly from the atmospheric Ne. We suggest that Ne was further fractionated in the atmosphere. While the abundances of Ar, Kr, and Xe in the mantle-derived materials are well correlated to each other, He (and Ne in MORB) is totally uncorrelated with the heavier noble gases. This indicates that He (and Ne in MORB) is decoupled from the heavier noble gases in the mantle source. He–Ne–Ar systematics indicate that He was open to the MORB and OIB mantle sources (and Ne in the MORB mantle source); while He is excess in the MORB mantle source, it is deficient in the OIB mantle source relative to the closed system values. However, it is not known whether this apparent complementary characteristic corresponds to some fundamental mantle dynamics or accidental coincidence.