U/Th-Pb decay systems

Pb from h. The Pb systems are discussed last because of the complexities of the U-Th-Pb system in several aspects. One is that each of the nuclides and h undergoes a long chain of decay to the final stable nuclide decays... 

The major application of TIMS is to the geochronology and tracer studies using terrestrial radiometric systems, for example, U-Th-Pb, Rb-Sr, Sm-Nd, and Lu-Hf. Geochronology exploits the radioactive decay in closed systems to obtain the date of a specific geological event. Tracer studies use the growth of daughter nuclides from radioactive decay to evaluate the interaction between geochemical systems and/or reservoirs. The application of TIMS in cosmochemical analysis is limited however, it is used to measure the isotopic compositions as tracers of nucleosynthesis processes. This includes the measurement of radionuclides observed mainly in meteorites, for example, Mn-Cr, Al-Mg, Fe-Ni, and Ca-K system in addition to the above-mentioned systems. 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

There are a multitude of papers concerning the age dating of rocks by mass spectrometry on the basis of different geochronological systems with well defined constants of radioactive decay. They include such systems as Rb-Sr, K-Ar, K-Ca, Re-Os, Nd-Sm, U-Pb, Th-Pb, Pb-Pb or Lu-Hf, which are discussed in detail in the literature.26 Therefore only a few relevant examples were briefly sketched here. 

The U (uranium)-Th (thorium)-Pb (lead) isotopic system represents three independent decay schemes and is a powerful but complex tool with which to unravel the history of the Earth s mantle (Text box 3.2). During planetary accretion U and Th are refractory, lithophile elements and will reside in the mantle. Pb on the other hand is a volatile and chalcophile/ siderophile element and may in part, be stored in the core. Initial U and Th concentrations are derived from chondritic meteorites, and initial Pb isotope compositions are taken from the iron-sulfide troilite phase in the Canyon Diablo meteorite. The initial bulk Earth U/Th ratio was 4.0 0.2 (Rocholl Jochum, 1993). 

The actinides, Th and U, also decay by a decay en route to the stable Pb isotopes. For each actinide nucleus, several a particles (" e nuclei) are produced. However, the same problems that make it difficult to use U,Th-He dating in terrestrial studies (Farley 2002, this volume), the recoil upon creation and the extremely low temperature at which it is lost, make it difficult to apply to meteorites as well. So the K-Ar system, particularly the " Ar- Ar version, is used far more often than is the U-Th-He system, and hence will be mentioned far more often in applications. 

The decay products of the long-lived radioactive systems are important tools for tracing geological time and Earth processes. The main parent-daughter pairs used for studies in the Earth and Planetary sciences are Rb-Sr, Th-U-Pb, Sm-Nd, Lu-Hf, and Re-Os. Traditionally most practitioners have not focused their careers... 

Thorium is about three times as abundant as U and about as abundant as Pb or Mo. Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% Th02 along with rare-earth minerals. Thorium has several isotopes whose atomic masses range from 212 to 236. All of the isotopes are unstable. Th occurs naturally, has a half-life of 1.4 x 10 yr, and decays through emission of an a particle. Six a and four p decay steps occur before Th becomes stable Pb. Hydroxides of thorium(IV) are the dominant species in soil and aquatic systems, although carbonate complexes also form. 

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