Lead-lead Dating - The Formation of The Geochron

The Formation of The Geochron

The development of the Geochron was mainly attributed to Clair Cameron Patterson’s application of Pb-Pb dating on meteorites in 1956. The Pb ratios of three stony and two iron meteorites were measured. 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 planetesimals. Assuming the process of isotopic differentiation is identical on Earth as it is on other planets, the core of these planetesimals would be depleted of uranium and thorium, while the crust and mantle would contain higher U/Pb ratios. As planetesimals 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 planetesimals.

Samples of iron meteorite 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

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. The stony meteorites however, exhibited very high 207Pb/204Pb versus 206Pb/204Pb ratios, indicating that these samples came from the crust or mantle of the planetesimal. These samples define an isochron in figure 1, whose slope gives the age of meteorites 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.

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