Noble-gas state in the earth's interior — Some constraints on the present state

Abstract

The noble-gas state in the present Earth's interior is discussed on the basis of selected data on recent samples. For this purpose, careful sample selection, experimental procedures and the usage of suitable isotope systems are discussed in detail. Based on the 3He/ 4He- 40Ar/ 36Ar diagram, we can infer at least four typical sources for noble-gas isotopes: MORB type (M-type), plume type (P-type), atmosphere type (A-type) and crust type (C-type). Among them, only M- and P-type sources are directly related to the mantle materials. The M-type source is assigned to the depleted mantle, whereas the P-type source to the fertile mantle which is supposed to be located beneath the former. The P-type source materials are characterized by higher 3 He 4 He and lower 40 Ar 36 Ar ratios compared to the MORB's values. All observed data for terrestrial samples seem to be explained by the mixing of these components including those derived from the arc areas and from the hot-spot areas. The apparent lesser heterogeneity of noble-gas isotopes in the Earth's deep interior probably reflect the easier mobility of noble gases at high temperatures compared with those of solid elements. Based on higher 3 He 4 He and lower 40 Ar 36 Ar ratios in the P-type source materials compared with MORB's values, we infer that noble gases (and probably volatiles) are still retained in the Earth's deep interior with significant amounts, which might play an important role to form mantle plumes and some other processes. The 40 Ar 36 Ar ratio for the total Earth is estimated to be ∼350 or less at present. Compared with the values for the other planets, it is conjectured that the K 36 Ar ratio for each planet might have been already determined during the accretion process.