Th-Pb isotopic system

Silver, L. T. U-Th-Pb Isotope systems in Apollo 11 and 12 regolithic materials and a possible age for the Copernicus impact event. Paper presented at Amer. Geo-phys. Union 52nd Ann. Meeting. Washington, D. C. 1971. 

The U-Th-Pb isotope systems have half-lives ranging from 0.7 to 4.6 Ga, and can therefore be used to investigate variations in time-integrated U/Pb (pi) and Th/U (ki) ratios, and to constrain models for the evolution of the mantle and crust from early Archaean time to the present (Sinha Tilton 1973 Stacey Kramers 1977 Kramers Tolstikhin 1997). The Pb isotope compilation and model of Kramers Tolstikhin (1997) highlight the range in the initial Pb/ Pb ratios (and thus in time-integrated pi) in suites of Archaean to early Proterozoic rocks. Inferred differences between high-pi lower crust and... 

The transport balance model of Kramers and Tolstikhin (1997) for the U-Th-Pb isotope system is particularly sensitive to the extent to which crustal material can be recycled into the mantle and offers some constraints on the mechanism of crustal recycling. They concluded that the rate of recycling of crustal material back into the Earth s mantle has increased with time, particularly since 2.0 Ga. We know from Pb-isotope studies that some sediment recycling took place during the Archaean (Halla, 2005), but it would seem as though the proportion was small. 

The use of an MC instrument has the obvious advantage of the simultaneous measurement of the signal intensities for all isotopes of interest, thereby providing a better precision on the measured ratios. The main disadvantage is that only a limited number of isotopes can be measured, so the Th-Pb isotopic system cannot be used simultaneously with the U-Pb system on all MC instruments [48,49]. Also, normalization of U and Pb intensities to Zr and Si to account for... 

Data for Sm-Nd, Rb-Sr, and Th-U-Pb isotopic systems are readily affected by contamination from the enclosing lavas and by the secondary enrichment events. Only the Re-Os system seems capable of giving reliable ages for the... 

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... 

Lancelot, J. R. Allegre, C. J. (1974). Origin of carbonatitic magmas in the light of Pb-U-Th isotope system. Earth Planet. Sci. Letters, 22, 233-8. 

This method has been most widely used in (i) the °K- °Ar system, (ii) the U-Th-He system, (iii) the U-Pb dating of zircon, and (iv) the U-Th-Pb dating of monazite using electron microprobe measurements. The last is a developing method with large errors because of the low analytical precision and because isotopes are not measured. 

The common-lead method looks at the isotopic evolution of lead in systems with U/Pb and Th/Pb ratios similar to or less than the ratios in bulk solar system materials. The original formulation, by Holmes and Houtermans, is a single stage model that accounts for the isotopic composition of any sample of common lead in terms of primordial lead plus radiogenic lead produced in the source up to the time that lead was separated from uranium and thorium. Multistage models that more accurately describe the evolution of natural systems have been developed. The common-lead method is used in cosmochemistry primarily to study the time of differentiation and reservoir evolution in differentiated bodies... 

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. 

LU Hf but differ markedly from Rb-Sr, U-Pb and Th-Pb. Sm-Nd are immobile under hydrothermal condidons and so their isotopic composition reflects the actual proportions of rock or magma involved in specific petrological processes. The Sm- Nd system, however, has the disadvantage that small amounts of recycled crust mixed with a large proportion of a mantle component become isotopicaliy in Hisibte. 

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. 

Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

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