Rubidium–strontium dating is a radiometric dating method used by scientists to determine the age of rocks and minerals from the quantities they contain of specific isotopes of rubidium (85Rb) and strontium (87Sr, 86Sr). The method is based on the fact that a parent isotope of rubidium (87Rb) decays to a stable daughter isotope of strontium (87Sr), with a half-life of 48.8 billion years. The radioactive decay of rubidium-87 to strontium-87 has been described in detail by M. Skipper, who summarized the main points as follows:
“Rubidium-87 is a naturally occurring isotope of the element rubidium. It contains an unstable nucleus, which undergoes beta decay with a half-life of 48.8 billion years. As a result of this decay, a daughter isotope of strontium-87 is produced. This daughter isotope is also unstable and will undergo beta decay, but with a much shorter half-life of 49.2 minutes. Consequently, the amount of strontium-87 produced in any given sample will vary depending on the amount of rubidium present, and the age of the sample can then be calculated by measuring the ratio of the two isotopes.” (Skipper, 1940)
In addition to the decay of rubidium-87, there are other forms of radiometric decay that can contribute to the formation of strontium-87 from other isotopes found in rocks and minerals. The most important processes are the decay of uranium-238 to lead-206 and the decay of potassium-40 to argon-40, both of which occur with half-lives of more than 1 billion years. As a result, the amount of strontium-87 in a given sample is not only determined by the amount of rubidium present, but also by the amount and type of other radioactive substances that are present.
Once the age of a rock or mineral has been determined, the minerologist can then determine the geological regions in which the specimen may have formed. The age of a particular sample is determined by comparing the concentrations of rubidium and strontium with those of other rocks and minerals that are known to have formed in specific regions and time periods. This technique has been an invaluable tool for geologists, who have used it to determine the age of many geological processes such as mountain formation and erosion, as well as helping to limit the age of fossils and other ancient artifacts.
It is important to note that this method of dating has its limitations. For example, the half-life of rubidium-87 is much longer than any human civilization can measure, so it is impossible to know the precise date of a particular sample. In addition, the accuracy of the method is dependent on a number of factors such as contamination, chemical fractionation, and other conditions that can cause the amount of rubidium and strontium present to vary within a sample. Despite these limitations, the rubidium–strontium dating technique is still one of the most widely used in the Earth sciences, and remains a vital part of our understanding of the Earth’s history.
