Primordial radionuclides

In the marine environment, the numerous radionuclides can be classified into three broad categories based on their production or origin (1) those derived from the weathering of continental rocks, the primordial radionuclides, (2) those formed from cosmic radiation, the cosmogenic radionuclides, and (3) those artificially introduced into nature, the anthropogenic or transient radionuclides and tracers. The primordial radionuclides (e.g. Th, and U) were... 

Table 1 Physical constants for some primordial radionuclides...

Primordial or U-Th series radionuclides can be discharged into the ocean through several possible pathways, illustrated schematically in Figure 3 ... 

Neder H, Heusser G, Laubenstein M (2000) Low-level y-ray germanium-spectrometer to measure veiy low primordial radionuclide concentrations. ApplRadiat Isot 53 191-195 Palacz ZA, Freedman PA, Walder AJ (1992) Thorium isotope ratio measurements at high abundance sensitivity using a VG 54-30, an energy-filtered thermal ionization mass spectrometer. Chem Geol 101 157-165... 

Alpha (a) particles Helium nuclei (2 He). A common by-product of the radioactive decay of primordial radionuclides. 

Daughters The radionuclides produced by the decay of primordial radionuchdes. 

Primordial radionuclides Long-lived radionuclides that were present at Earth s formation. The most... 

X 10 yr) and ends with stable ° Pb, after emission of eight alpha (a) and six beta (jS) particles. The thorium decay series begins with Th (ti/2 = 1.41 X 10 °yr) and ends with stable ° Pb, after emission of six alpha and four beta particles. Two isotopes of radium and Th are important tracer isotopes in the thorium decay chain. The actinium decay series begins with (ti/2 = 7.04 X 10 yr) and ends with stable Pb after emission of seven alpha and four beta particles. The actinium decay series includes important isotopes of actinium and protactinium. These primordial radionuclides, as products of continental weathering, enter the ocean primarily by the discharge of rivers. However, as we shall see, there are notable exceptions to this generality. 

We discussed two different types of radiochronometers. Those based on long-lived radionuclides for which a portion of the primordial abundance is still present provide absolute ages relative to the present time on suitable samples. Examples of how these chronometers are used to date individual objects (chondrules, CAIs, achondrites) and fractionation events (planetary differentiation, magma generation) were discussed. 

Cosmogenic and Primordial Radionuclide Measurements in Apollo 11 Lunar Samples by Nondestructive Techniques, Proc. Apollo 11 Lunar Sci. Conf. (1970) 2,1455-1469. 

Several radionuclides are continuously produced by the impact of cosmic radiation, and the main representatives of this group are C, Be, Be and H. Radionuclides present in nature in extremely low concentrations, such as Pu and its decay products or products of spontaneous fission of U and Th, are not considered in this list. Radionuclides existing from the beginning, i.e. since the genesis of the elements, are called primordial radionuclides. They comprise the radionuclides of group (2) and Th and Pu. 

In any case, the primordial radioactivity on the earth was appreciably higher than at present. The ratios of the aetivities at the time of the birth of the earth to those at present are listed in Table 15.4 for some long-lived radionuclides that represent the main radioactive inventory on the earth. The relatively high activity of about 2 10 y ago is the reason for the operation of the natural nuclear reactors at Oklo at that time (seetion 11.8). 

A number of short-lived radionuchdes also existed at the time that the Sun and the rocky bits of the solar system were forming (Table 1). These nuclides are sufficiently long-lived that they could exist in appreciable quantities in the earhest solar system rocks, but their mean fives are short enough that they are now completely decayed from their primordial abundances. In this sense they are referred to as extinct nuchdes. Although less familiar than the still-extant radionuclides, these short-lived isotopes potentially play similar roles their relative abundances can, in principle, form the basis of various chronometers that constrain the timing of early chemical fractionations, and the more abundant radioisotopes can possibly provide sufficient heat to drive differentiation (i.e., melting) of early accreted planetesimals. The very rapid rate of decay of the short-lived isotopes, however, means that inferred isotopic differences translate... 

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