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Dynamics and Evolution of the Solid Earth

Table 1

Table 1. Click to enlarge.

The isotopic makeup of noble gases and their inter-relationships are central to understanding the origin of the Earth, its atmosphere and hydrosphere, and chemical heterogeneity of the Earth's mantle. Changes in noble gas isotope composition are closely related to processes controlling the distribution of K, U and Th, the major heat producing nuclides in the Earth. The isotopic makeup of every rare gas is modified by the radioactive decay of one or more of these heat producing nuclides (Table 1). The case of radon is unusual because it has no stable isotopes, and its geochemical distribution is completely controlled by radioactive decay of U and Th. The geochemical distribution of helium is directly related to alpha-particle (4He) production by U and Th. The neon isotope composition in terrestrial systems is modified by nucleogenic processes, in which 21Ne is produced when neutrons or alpha-particles collide with Mg and O target atoms. The radioactive decay of 40K controls the Ar isotope composition. Radiogenic production of 131,132,134,136Xe has occurred over geological time by the spontaneous fission of 238U and extinct 244Pu (half-life t1/2=82 my), while 129Xe production occurred by radioactive decay of extinct 129I (t1/2=17 my).