Refinements

Since Holmes's initial work in 1911, many improvements have been made to the process of radiometric dating. Of major importance was the discovery in 1913 that the atoms of a chemical element can exist in two or more different forms, called 'isotopes', which are the same chemically but have different atomic masses. Some isotopes are stable, and it is only the unstable ones that undergo radioactive decay. Of crucial importance for radiometric dating purposes, different unstable isotopes of the same element often have very different half-lives. For example, most uranium consists of the isotope U238, with a half-life of 4.$ billion years, but about 0.7 per cent is the isotope U235, with a half-life of 713 million years. Lead comes in several stable isotopic forms - some that occur in minerals only through the decay of other, radioactive isotopes, and others that that were present in the minerals from when they first formed. It became apparent to Holmes and others that for better accuracy, it would be necessary to measure isotope ratios (for example U238 to its decay product, the lead isotope Pb2o6), rather than just uranium to lead.

This discovery of isotopes initially complicated the process of radiometric dating, but in time made it more precise. One initial effect was a reappraisal of Holmes's time scale - as a result of not compensating for these (then unknown) factors, his computed ages were too high. Nevertheless, researchers were beginning to realise that radiometric methods held promise for reassessing the Earth's age. In 1921, Henry Russell obtained 4 billion years as a rough approximation to the age of the Earth's crust, based on an average of its maximum age calculated from its total uranium and lead content, and a minimum age based on the oldest known (at that time) Precambrian minerals. Arthur Holmes later revised Russell's calculation, based on different estimates of the amount of uranium and lead in the crust, giving an age of between 1.6 and 3 billion years. Over the following years, several more different ages for the Earth's crust were computed and published. These included 3.4 billion years (Rutherford, 1929); and 4.6 billion years (Meyer, 1937). Meanwhile, older and older rocks were being found in different parts of the world.

By the 1940s, it became apparent that to calculate an accurate age for the Earth, one piece of data was still needed - the ratio of different isotopes of lead in the Earth's crust at the time of its formation. Holmes made isotope measurements on some ancient lead ore-bearing rocks from Invigtut in Greenland and by 1946, these gave an age of 3015 million years, which was the first really reliable estimate of a minimum age for the Earth. Holmes went on to estimate that the origin of the uranium, from which the lead was derived, must be around 4460 million years ago, but he thought that origin was within the gas cloud from which the Earth was formed, rather than being the date of the Earth's origin.

A decade later, following the end of the Second World War, American scientists Harrison Brown and Claire Patterson, who had worked on the Manhattan Project and the development of the atomic bomb, became interested in using meteorites to calculate the age of the Solar System. In 1953, Patterson managed to determine the lead isotope content of the Canyon Diablo meteorite, which blasted Meteor Crater in Arizona around 50,000 years ago. From this, he calculated an age of 4510 million years and compared it with an age of 4560 million years calculated for lead values from earth-bound granite and basalt rocks. He concluded that the similarity between the dates indicated that this was also the date at which the Earth first formed. By 1956 Patterson had made further measures from different meteorites and deep-sea sediments, which represented a generalised sample of Earth rocks. Again, the average worked out at 4.55 billion years, very close to Holmes's figure.

Claire Patterson, 1922-95, American chemist who worked with Harrison Brown on the Manhattan Project and after the Second World War, first at the University of Chicago then California Institute of Technology (from 1952), worked on the calculation of the Earth's age, finally settling on 4.55 billion years.

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