25 



century or so that they have also proved amenable to scientific 

 inquiry. Here, too, progress has been impressive — in recent years the 

 known fossil record has been extended further and further into the 

 remote past. The oldest known fossils are about 3.5 b.y. old, an age 

 approaching, but markedly less than, the oldest rocks (approximately 

 3.8 b.y. old) on Earth, only one billion or so years younger than the 

 age of the planet itself. 



THE AGE OF THE EARTH 



Knowledge of the age of the Earth is of fundamental impor- 

 tance to our understanding of the time when life originated. To fix 

 the age of the Earth, one must determine the isotopic abundances of 

 certain elements. 



The isotopic composition of lead (Pb) at the time of formation 

 of the solar system can be accurately determined by measurements 

 of Pb in certain undifferentiated meteorites rich in water, carbon 

 dioxide, and other volatile materials. Assuming the mantle source of 

 terrestrial lead has always been a well-mixed reservoir containing 

 uranium (U) and thorium (Th), decay of 238 U, 235 U, and 232 Th will 

 enrich this primordial lead in the radiogenic daughter isotopes to the 

 extent observed in modern samples of lead from Earth's mantle in a 

 time of 4.43 b.y. Measurement of lead samples of various ages, 

 extending back to 3.8 b.y. ago, shows that the assumption of a 

 well-mixed reservoir is a good, but not perfect, approximation. 

 Correction of the calculated age of the Earth for the observed devia- 

 tion from a uniform source leads to an age about 100 m.y. older; i.e., 

 4.53 b.y. 



Accurate measurements by U-Pb, and other isotopic clocks in 

 meteorites, yield 4.55 ±0.02 b.y. for the time of formation (or differ- 

 entiation) of their parent bodies. Measurement of the decay products 

 of extinct radioactivities; e.g. aluminum ( 26 A1) and iodine ( 129 I), 

 shows that those bodies in the solar system from which the meteor- 

 ites were formed are within about 1-100 m.y. of the time at which 

 the forming solar system itself was produced. 



Formation of the Sun and planets post-dates this time. Thus, 

 the age of the Earth is firmly bracketed between 4.65 b.y. and 



