ABSTRACT

The Earth's atmosphere was produced by exhaustive degassing of the upper mantle during the first 200My, but the lower mantle has retained an appreciable fraction of its initial inventory of primordial ³He. The lower mantle has retained most of its initial inventory of the heavy noble gases, and it is presently accumulating radiogenic ⁴He and ⁴⁰Ar. Most of the radiogenic ⁴⁰Ar in air was produced in the crust during the first 2.5 Gy. Extinct radionuclides have augmented the atmospheric inventory of ¹³⁶Xe by less than 1 % and that of ¹²⁹Xe by about 5%. Terrestrial Ar, Kr, and Xe are type-Y, but the He and Ne are of solar wind origin. Terrestrial Xe may not be isotopically unique in the solar system; its composition can be related to that in meteorites by consideration of nucleogenetic heterogeneities and mass dependent fractionation. The atmospheres of the terrestrial planets were produced by exhaustive degassing of specific regions. Observed similarities in the abundance patterns of noble gases in meteorites and in the terrestrial planets rule out elemental fractionation in the evolution of their atmospheres. The degassed portions of Mars, Earth, and Venus are estimated to be 1-2%, 17% and 100%, respectively. The iron cores of these bodies were produced by heterogeneous accretion.