Lead-lead dating

Lead-lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite. For most dating requirements it has been superseded by uranium-lead dating (U-Pb dating), but in certain specialized situations (such as dating meteorites and the age of the earth) it is more important than U-Pb dating.

Decay equations for common Pb-Pb Dating

There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb. ²⁰⁴Pb is the only non-radiogenic lead isotope, therefore is not one of the daughter isotopes. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from ²³⁸U,²³⁵U and ²³²Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. This shifts the ratio of radiogenic Pb verses non-radiogenic ²⁰⁴Pb (²⁰⁷Pb/²⁰⁴Pb or ²⁰⁶Pb/²⁰⁴Pb) in favor of radiogenic ²⁰⁷Pb or ²⁰⁶Pb. This can be expressed by the following decay equations:

::$\{left(frac\{^\{207\}Pb\}\{^\{204\}Pb\}\; ight)\_\{P\; =\; \{left(frac\{^\{207\}Pb\}\{^\{204\}Pb\}\; ight)\_\{I\; +\; \{left(frac\{^\{235\}U\}\{^\{204\}Pb\}\; ight)\}\; \{left(\{e^\{lambda235t\}-1\}\; ight)\}$

::$\{left(frac\{^\{206\}Pb\}\{^\{204\}Pb\}\; ight)\_\{P\; =\; \{left(frac\{^\{206\}Pb\}\{^\{204\}Pb\}\; ight)\_\{I\; +\; \{left(frac\{^\{238\}U\}\{^\{204\}Pb\}\; ight)\}\; \{left(\{e^\{lambda238t\}-1\}\; ight)\}$

where the subscripts P and I refer to present-day and initial Pb isotope ratios, λ235 and λ238 are decay constants for ²³⁵U and ²³⁸U, and t is the age.

The concept of common Pb-Pb dating (also referred to as whole rock lead isotope dating) was deduced through mathematical manipulation of the above equations. [(Nier, A.O., Thompson, R.W. and Murphy, B.F.(1941). The isotopic constitution of lead and the measurement of geological time 3. Phys. Rev. 60, 112-17. )] It was established by dividing the first equation above by the second, under the assumption that the U/Pb system was undisturbed. This rearranged equation formed:

::$left\; [frac\{left(frac\{^\{207\}Pb\}\{^\{204\}Pb\}\; ight)\_\{P\}-left(frac\{^\{207\}Pb\}\{^\{204\}Pb\}\; ight)\_\{I\{left(frac\{^\{206\}Pb\}\{^\{204\}Pb\}\; ight)\_\{P\}-left(frac\{^\{206\}Pb\}\{^\{204\}Pb\}\; ight)\_\{I\; ight]\; =\; \{left(frac\{1\}\{137.88\}\; ight)\}\{left(frac\{e^\{lambda235t\}-1\}\{e^\{lambda238t\}-1\}\; ight)\}$

where the factor of 137.88 is the present-day ratio ²³⁸U/²³⁵U ratio. As evident by the equation, initial Pb isotope ratios, as well as the age of the system are the two factors which determine the present day Pb isotope compositions. If the sample behaved as a closed system then graphing the difference between the present and initial ratios of ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb should produce a straight line. The distance the point moves along this line is dependent on the U/Pb ratio, whereas the slope of the line depends on the time since Earth’s formation. This was first established by Nier et al, 1941. [( Nier, A.O., Thompson, R.W. and Murphy, B.F.(1941). The isotopic constitution of lead and the measurement of geological time 3. Phys. Rev. 60, 112-17. )]

The Formation of the Geochron

The development of the Geochron was mainly attributed to Patterson’s application of Pb-Pb dating on meteorites in 1956. The Pb ratios of three stony and two iron meteorites, where measured. [(Patterson, C.C. (1956). Age of meteorites and the Earth. Geochim. Cosmochim. Acta 10, 230-7)] The dating of meteorites would then help Patterson in determining not only the age of these meteorites but also the age of Earth’s formation. By dating meteorites Patterson was directly dating the age of various planetismals. The process of isotopic differentiation is identical on Earth as it is on other planets, therefore the core of these planetismals would be depleted of Uranium and Thorium, while the crust and mantle would contain higher U/Pb ratios. As planetismals collided, various fragments were scattered and produced meteorites. Iron meteorites were identified as pieces of the core, while stony meteorites were segments of the mantle and crustal units of these various planetismals.

Samples of iron meteroite from Canyon Diablo (Meteor Crater)Arizona were found to have the least radiogenic composition of any material in the solar system. The U/Pb ratio was so low that no radiogenic decay was detected in the isotopic composition [(Dickin, A.P, 2005.Radiogenic Isotope Geology. United Kingdom: University Press, Cambridge. p 117)]

As illustrated in figure 1, this point defines the lower (left) end of the isochron. Therefore troilite found in Canyon Diablo represents the primeval lead isotope composition of the solar system, dating back to 4.55 +- 0.07 Byr. [(Dickin, A.P, 2005.Radiogenic Isotope Geology. United Kingdom: University Press, Cambridge. pp117)] The stony meteorites however, exhibited very high ²⁰⁷Pb/²⁰⁴Pb verses ²⁰⁶Pb/²⁰⁴Pb ratios, indicating that these samples came from the crust or mantle of the planetismal. These samples define an isochron in figure 1, whose slope gives the age of meterites as 4.55 Byr. Patterson also analyzed terrestrial sediment collected from the ocean floor. This was representative of the Bulk Earth composition, and was plotted on the isochron. The isotope composition of this sample was found to lie on the meteorite isochron, therefore giving good evidence that earth had the same origin as the meteorites, as well as the same age, therefore solving the age of the Earth and giving rise to the name geochron.

Lead isotope isochron diagram used by C. C. Patterson to determine the age of the Earth in 1956. Animation shows progressive growth over 4550 million years (Myr) of the lead isotope ratios for two stony meteorites (Nuevo Laredo and Forest City) from initial lead isotope ratios matching those of the Canyon Diablo iron meteorite.

References