Radiogenic radioactive decay

Lead occurs naturally as a mixture of four non-radioactive isotopes, and Pb, as well as the radioactive isotopes ° Pb and Pb. All but Pb arise by radioactive decay of uranium and thorium. Such decay products are known as radiogenic isotopes. 

Natural lead, a metallic element, is a mixture of the following four isotopes lead-204, lead-206, lead-207, and lead-208. Only lead-204 is a primordial isotope of nonradiogenic origin all the others are radiogenic, each isotope being the end product of one of the radioactive decay series of isotopes of thorium or uranium, namely, uranium-238, uranium-235, and thorium-232 the decay series of the uranium isotopes are listed in Figure 12 ... 

The concentration of the radioactive nuclide (reactant, such as Sm) decreases exponentially, which is referred to as radioactive decay. The concentration of the daughter nuclides (products, including Nd and He) grows, which is referred to as radiogenic growth. Note the difference between Equations l-47b and l-47c. In the former equation, the concentration of Nd at time t is expressed as a function of the initial Sm concentration. Hence, from the initial state, one can calculate how the Nd concentration would evolve. In the latter equation, the concentration of Nd at time t is expressed as a function of the Sm concentration also at time t. Let s now define time t as the present time. Then [ Nd] is related to the present amount of Sm, the age (time since Sm and Nd were fractionated), and the initial amount of Nd. Therefore, Equation l-47b represents forward calculation, and Equation l-47c represents an inverse problem to obtain either the age, or the initial concentration, or both. Equation l-47d assumes that there are no other ot-decay nuclides. However, U and Th are usually present in a rock or mineral, and their contribution to " He usually dominates and must be added to Equation l-47d. 

Although the radioactive decay constants are independent of temperature and pressure, the retention of the radiogenic daughter in a mineral depends strongly on temperature because at high temperatures diffusivity is high, resulting in... 

The other side of radioactive decay is radiogenic growth. As the parent decays away, the number of atoms of the daughter increases. Let D denote the number of daughter atoms (note that D is not diffusivity here), then D grows as... 

The first term on the right-hand side is the melting term the second term on the right-hand side is the radioactive decay of the nuclide and the last term on the right-hand side represents the radiogenic production by the parent of the nuclide. and Cj" are the concentrations of the nuclide... 

The production of Pb by radioactive decay of U may also affect the oxidation state of uraninite by a process called auto-oxidation (Frondcl 1958 Finch Murakami 1999). Radiogenic Pb4+ is a strong oxidizer and due to its instability in the presence of U4+, it is reduced to Pb2+ while simultaneously U4+ oxidizes to U6"1" leading to elevated U6+/U4+ ratios in uraninite. The U6+/U4+ ratios determined from XPS spectra of uraninite from Cigar Lake can be... 

Mother isotope Abundance of mother isotope a, % Half life 1/2/ a Radioactive decay Stable radiogene daughter nuclide Abundance of daughter isotope a, % Preferable application fields in geochronology... 

He and 3He 4He, not only in the crust but also in the Earth, is essentially radiogenic, and has been produced from radioactive decay of U, Th series elements. Only a significant source for nucleogenic 3He in the crust is a reaction 6Li(n, Cf) H( 7 , 2 = 12.3 a) —> 3He, where neutrons are derived from a spontaneous fission of 23SU and from reactions of light elements such as Na, Mg, Al, and Si with a particles emitted from U, Th decays. However, in a very shallow surface region (less than a few meters), the secondary cosmic ray neutrons would be more important. 

Cyclo-silicates, especially beryl and cordierite, are known to have an abnormally large amount of 4He and 40Ar, which cannot be accounted for the in situ production from the radioactive decay of U, Th, and 40Ar. Lord Rayleigh (Strutt, 1908) first discovered that beryls contained excess 4He, which cannot be accounted for by in situ radiogenic 4He from U and Th. Subsequently, excesses of both 4He and 40Ar in all... 

Figure 7.1 Hypothetical decay of a radionuclide (N) to a stable radiogenic daughter (D y, the successive loss of atoms of N from radioactive decay are followed by proportional increase in the daughter atoms. (Modified from Faure, 1986.)...

Rocks contain uranium and thorium in small concentrations, and their radioactive decay results in the production of radiogenic helium-4 (4He). The helium reaches the groundwater and is dissolved and stored. With time,... 

The radioactive decay of uranium and thorium results in the formation of a series of isotopes that are radiogenic by themselves and keep disintegrating into stable lead isotopes. These radioactive disintegrations are accompanied by the emissions of 4He atoms. Three such radioactive series exist ... 

The rubidium-strontium geochronometer used in the Rb-Sr geochronological method is based on the radioactive 3 -decay of Rb to Sr. The growth of radiogenic Sr in a Rb rich mineral can be described by the following Equation (9.6). In the rubidium-strontium age dating method, the radioactive Rb isotope with a natural isotope abundance of 27.85 % and a half-hfe of 4.88 X 10 ° years is fundamental to the 3 decay to the isobar Sr. The equation for the Rb-Sr method can be derived from the general equation of radioactive decay (Equation 8.8 in Section 8.8) ... 

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