Terrestrial Noble Gases

The same fractionation relationship between the terrestrial noble gases or Q and the solar noble gases suggests that the fractionation occurred in the early solar nebula, which seems to be the only likely locale for the observed severe and large-scale fractionation. This relation, in turn, suggests that the terrestrial noble gases were already fractionated before Earth formation. The latter inference is in perfect accordance with the fractionated solar primordial noble gases in the Earth, the conclusion discussed in Section 7.2. 

Primordial He in the upper mantle. The first clear evidence for the degassing of primordial volatiles from the solid Earth came from He isotopes. The isotopic composition of He in the atmosphere provides a reference, with a He/ He ratio of Ra = 

75 X 10 atoms %e/g in that volume. However, this does not consider losses to space, nor sources that added volatiles directly to the atmosphere. 

Primordial He in the deep Earth. The He/ He ratios measured in ocean island 

along with an initial value of He/ He = 120-330 Ra and a present value of 38 Ra, then the reservoir has (6.1-7.9) x 10 ° atoms He/g. If losses of He had occurred, the initial He concentration of this reservoir would have been greater. This provides an important constraint on the amount of He that must have been trapped within the mantle. It should be emphasized that this calculation does not make any assumptions regarding the volume or location of this reservoir in the mantle, issues that are presently contentious (see Porcelli and Eallentine 2002). Nevertheless, the presence of such a reservoir anywhere in the mantle requires that such concentrations were trapped in at least some portion of the mantle. 

Mantle compositions have greater °Ne/ Ne ratios, due to the presence of unfractionated solar Ne within the Earth, and greater Ne/ Ne ratios due to addition of nucleogenic Ne. 

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In summary, we may reasonably conclude that except for Ne, and apart from radio-and nucleogenic components, the isotopic composition of atmospheric noble gases well represent the bulk Earth and can be assumed to approximate a primordial terrestrial noble gas component. Thus defined, primordial terrestrial noble gases are... 

Let us discuss the first issue, that is, how terrestrial noble gases, in extremely small but not negligible amounts, were captured by the Earth. Adsorption of noble gases by Earth-accreted dust grains had once been favored as a noble gas capture... 

It is generally assumed that except for He, which is not gravitationally bound to the Earth, the majority of terrestrial noble gases presently reside in two major reservoirs, namely, the atmosphere and the mantle. Noble gases in other reservoirs such as the hydrosphere or the crust are likely to be insignificant in comparison with these other two reservoirs. The latter conclusion is based on a number of observations on noble gas contents in a variety of samples from these reservoirs. However, a considerable... 

Amari, S., Ozima, M. (1988) Extra-terrestrial noble gases in deep sea sediments. Geochim. Cosmochim. Acta, 52, 1087-95. 

Tolstikhin, I. N. (1978) A review Some recent advances in isotope geochemistry of light noble gases. In Terrestrial Noble Gases, E. C. Alexander, Jr. M. Ozima, Eds., pp. 33-62. Tokyo Japan Scientific Societies Press. 

Ozima M., Wieler R., Marty B., and Podosek F. A. (1998) Comparative studies of solar, Q-gases and terrestrial noble gases, and implications on the evolution of the solar nebula. Geochim. Cosmochim. Acta 62, 301-314. 

PorceUi D. and Ballentine C. J. (2002) Models for the distribution of terrestrial noble gases and evolution of the atmosphere. In Noble Gases in Geochemistry and Cosmo-chemistry. Rev. Mineral. Geochem. 47 (eds. D. Porcelh, C. J. Ballentine, and R. Wieler). Mineralogical Society of America, Washington, DC. pp. 411 -480. 

Porcelli D. and Ballentine C. J. (2002) Models for the distribution of terrestrial noble gases and the evolution of the atmosphere. Rev. Mineral. Geochem. 47, 412-480. 

Models for the evolution of terrestrial noble gases must necessarily consider appropriate starting conditions. The initial incorporation of noble gases and the establishment of terrestrial... 

Ozima M. and Igarashi G. (1989) Terrestrial noble gases constraints and implications on atmospheric evolution. In Origin and Evolution of Planetary and Satellite Atmospheres (eds. S. K. Atreya, J. B. Pollack, and M. S. Matthews). University of Arizona Press, Tucson, pp. 306—327. 

Igarashi, G. 1986. Components of xenon in carbonaceous chondrites and fission component in the terrestrial atmosphere. Japan-US Seminar on Terrestrial Noble Gases, Abstracts, 20-23. 

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