U/Pb ratios

Isochrons can be used for coeval samples with a range of U-Pb ratios to determine the age and initial Pb ratio in a similar manner to that used in standard U-Th techniques where initial Th is significant. Ideally, sub-samples of pristine calcite from the same growth layer and exhibiting a considerable range of U/Pb ratios can be used to define... 

Equation 5-61 involves only Pb isotopic ratios and is called the Pb-Pb isochron. There are a few advantages of this Pb-Pb isochron compared to the individual U-Pb isochrons. One is that Pb isotopic ratios can be measured more accurately than U/Pb ratios. Hence, the above equation has the advantage of Ar-Ar method compared to the K-Ar method. Secondly, if there was recent (such as yesterday or in the last million years) U loss or gain or Pb loss, the loss or gain would not affect the Pb isotopic composition and hence would not affect Pb-Pb dating (but it would affect U-Pb isochrons). Pb gain would affect the Pb-Pb isochron. 

The Pb-Pb system can be applied in two ways. If the sample is sufficiently large and diverse that mineral separates with different U/Pb ratios can be produced, an internal isochron can be generated. The equation for the 207pb -206Pb isochron can be obtained by combining Equations (8.30) and (8.31) ... 

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. 

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,... 

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. 

SHRIMP 238U/206Pb ages as old as 7.2 Ga, much older than the Pb/ Pb Concordia age of 2.68 Ga. Isotope dilution thermal mass spectrometry analysis of the same zircons demonstrated that the ion microprobe U/Pb ratios were too low. This was attributed to the presence of a labile Pb component within amorphous microdomains that had an anomalously low U/Pb ion yield, which invalidated the SHRIMP U-Pb ion yield calibration. 

The above explanation does not account for the elevated ° Pb/ °" Pb ratios of continental rocks and their sedimentary derivatives relative to mantle-derived basalts (Figures 5(a) and 21). This special feature can be explained by a more complex evolution of continents subsequent to their formation. New continental crust formed during Archean time by subduction and accretion processes must have initially possessed a U/Pb ratio slightly higher than that of the mantle. At that time, the terrestrial ratio was signifi-... 

Figure 1 Helium isotope data from various mantle-derived volcanics. The upper axis is the He/ He ratio R) normalized to the atmospheric ratio Rj f. As indicated by the data for selected segments of the MORB away from ocean islands falls almost entirely within the range of (7-9)Ra- While there are hotspot basalts that are characterized by high U/Pb ratios and low He/ He ratios (HIMU), many major oceanic hotspots, as well as continental hotspots, have high He/" He ratios (source Porcelli and Ballentine, 2002).

The Hi parameter is the time-averaged U/Pb ratio between the age of the Earth T and the formation of the rock at time t. Thus it reflects the trace element characteristics of the source of each sample, assuming that the source had the same U/Pb ratio from T = 4.57 Ga to the time of formation of the sample. This is clearly an oversimplification geologically, but pj values remain a powerful tool for accessing time-averaged source characteristics, particularly for old rocks, because U has a relatively short half-life compared with other long-lived decay schemes. 

Thus, U/Pb ratios are not generally fractionated significantly by the crust formation processes widely invoked in the generation of Archaean greenstone belts and the TTG suites... 

Sinha, A. K. 1971. Primary differences in U/Pb ratios in two primitive erustal rocks. Carnegie Institution of Washington Yearbook, 69, 405—408. 

Th-U-Pb systematics and Sm-Nd systematics address in part different aspects of Earth differentiation. First, Th/Pb and U/Pb ratios are affected by core formation, whereas the Sm/Nd ratio is not. Second, in the silicate Earth Th/Pb... 

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