Radiogenic isotope dating

15-Jul-2019 11:04 by 2 Comments

Radiogenic isotope dating

Radioactivity and radioactive decay are spontaneous processes.Students often struggle with this concept; therefore, it should be stressed that it is impossible to know exactly when each of the radioactive elements in a rock will decay.

Here I want to concentrate on another source of error, namely, processes that take place within magma chambers.

The development and application of radiogenic isotopes to dating of geologic events, and to questions of growth, evolution, and recycling processes in the continental crust are mature areas of scientific inquiry.

By this we understand that many of the approaches used to date rocks and constrain the evolution of the continents are well established, even routine, and that the scope of data available on age and evolution of continents is very large.

Lava (properly called magma before it erupts) fills large underground chambers called magma chambers.

Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios.

Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.

Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.

Because design and building of instruments like radiation detectors or mass spectrometers requires a knowledge of physics, many of the early practitioners of rock dating were physicists, like Alfred Nier (cited above).

Since the 1970s, essentially all mass spectrometers have been constructed by specialized commercial firms, and the level of physics expertise among isotope geologists has been lower.

To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.

Radiometric dating is largely done on rock that has formed from solidified lava.

These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.