U-Pb isotopic system

One drawback of the Pb-Pb system is that two-stage or three-stage evolution of the U-Pb isotopic system can produce linear arrays on a Pb-Pb isochron plot (Gale and Mussett, 1973). If these arrays are interpreted as single-stage isochrons, incorrect dates may be obtained. In some cases, measurements of the U/Pb ratio can help identify multistage samples. But if several events occurred sufficiently early in solar system history, even a concordia approach may not be able to identify a multistage system (Tera and Carlson,... 

The differentiation histories of Earth, Mars, and the Moon are constrained by short-lived isotope systematics and the U-Pb isotope system. The 182Hf-182W... 

Because of the numerous secondary effects that modify the whole-rock elemental and isotopic compositions, our discussion below will focus solely on mineral-separate analyses for the Rb-Sr, Sm-Nd, Lu-Hf, and U-Pb isotope systems. In comparison, the Re-Os system is much more robust to the effects of... 

Extensive studies of peridotite xenoliths using the Rb-Sr, Sm-Nd and U-Pb isotope systems have... 

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... 

Teufel S and Heinrich W (1997) Partial resetting of the U-Pb isotope system in monazite through hydrothermal experiments An SEM and U-Pb isotope study. Chem Geol 137 273-281 Trukhin AN, Boatner LA, (1997) Electronic structure of SCPO4 single ciystals optical and photoelectric properties. Proc 13th Inti Conf Defects in Insulating Ciystals. Mater Sci Forum, Transactions Technical Publications, p 239-241... 

Williams, I.S. (2001) Response of detrital zircon and monazite, and their U—Pb isotopic systems, to regional metamorphism and host-rock partial melting, Cooma Complex, southeastern Australia. Aust. J. Earth Sci., 48 (4), 557-580. 

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. 

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. 

Antonio Lanzirotti received his PhD in Geochemistry from the State University of New York at Stony Brook for studies of U-Pb isotope systematics and trace element behavior in metamorphic systems, in particular towards the development of high precision U-Pb chronometers for low U/Pb metamorphic minerals. He is currently a beamline scientist with the Consortium for Advanced Radiation Sources (CARS) at the University of Chicago. He has been involved in synchrotron radiation research since 1992, is the beamline local contact for the X-ray Microprobe beam line (X26A) at the National Synchrotron Light Source. 

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... 

The coupled Pb and U- Pb decay systems can provide age information based on Pb/ ° Pb and Pb/ Pb only. This is advantageous as Pb isotopic compositions can be determined with higher precision than the elemental parent/daughter ratio that must be applied in conventional chronological studies. This technique of Pb-Pb dating can provide the most precise absolute age data for solids formed early in the solar system and is the lynchpin for the calibration of extinct radioactive decay systems. Analytical improvements that have been achieved... 

The dating of terrestrial materials (geochronology) does not provide any significant information on cosmochronology, but as early as 1953 an estimate for the age of formation of the Solar System was obtained from U-Pb isotopic analyses on meteoritic lead. Subsequently meteorites were dated by the U-Pb technique to give an age of 4.551 0.004 x 10 y, and the fine-scale separation of events that took place during planetary formation can be resolved down to a few million years. 

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... 

In summary, when using an isotopic system to determine the age, the meaning of the age is the closure age, as defined in Figure 1-20. The temperature at the time of closure is referred to as the closure temperature (Tc), which varies from one mineral to another. Tc decreases as diffusivity in the mineral increases, or activation energy decreases, or grain size decreases. Tc of some isotopic systems (such as U-Pb in zircon) is high and the isotopic systems hence record an older age that... 

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. 

Another coupled system is the Sm-Nd system, with two Sm isotopes ( Sm and Sm) undergoing ot-decay to become two Nd isotopes ( Nd and Nd). The half-life of Sm is 106 billion years and that of is 103 million years. In principle, the concepts for the U-Pb system (such as concordia and discordia, Nd-Nd isochron) can also be applied to the Sm-Nd system. However, the Sm-Nd coupled system has not found many applications. One reason is that the half-life of " Sm is so short that it is an extinct nuclide. Secondly, the half-lives of Sm and " Sm are very different, by a factor of 1000 (in contrast, the half-lives of and 235 differ only by a factor of 6.3). Hence, the coupled system has found only limited applications to very old rocks, such as meteorites and very old terrestrial rocks. 

If an internal isochron cannot be generated, a model age can be determined from the measured 207pb /206Pb of the sample and the assumed initial lead isotopic ratios. For studies of the early solar system, this initial lead composition is assumed to be that measured in troilite (FeS) from the Canyon Diablo meteorite. Troilite is a uranium-free mineral and its host meteorite formed very early in the history of the solar system. Because the U/Pb ratio of the solar system is low, the lead incorporated into the troilite should not have evolved significantly from the initial composition in the solar system. 

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... 

Equation (8.47), with t = 0 and the composition of lead from meteoritic troilite used for the initial isotopic ratio of lead, was used by Clair Patterson (1955,1956) to determine the age of the Earth. In the 1950s, the largest uncertainty in determining the age of the Earth was the composition of primordial lead. In 1953, Patterson solved this problem by using state-of-the-art analytical techniques to measure the composition of lead from troilite (FeS) in iron meteorites. Troilite has an extremely low U/Pb ratio because uranium was separated from the lead in troilite at near the time of solar-system formation. Patterson (1955) then measured the composition of lead from stony meteorites. In 1956, he demonstrated that the data from stony meteorites, iron meteorites, and terrestrial oceanic sediments all fell on the same isochron (Fig. 8.20). He interpreted the isochron age (4.55+0.07 Ga) as the age of the Earth and of the meteorites. The value for the age of the Earth has remained essentially unchanged since Patterson s determination, although the age of the solar system has been pushed back by —20 Myr. 

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