One of the two naturally occurring isotopes of rubidium, 87Rb, decays to 87Sr with a half-life of 49.23 billion years.
The radiogenic daughter, 87Sr, produced in this decay process is the only one of the four naturally occurring strontium isotopes that was not produced exclusively by stellar nucleosynthesis predating the formation of the Solar System.
The ratio 87Sr/86Sr in a mineral sample can be accurately measured using a mass spectrometer.
If the amount of Sr and Rb isotopes in the sample when it formed can be determined, the age can be calculated from the increase in 87Sr/86Sr.
Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite.
Therefore, given sufficient time for significant production (ingrowth) of radiogenic 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order.
[2][3] Development of this process was aided by German chemists Otto Hahn and Fritz Strassmann, who later went on to discover nuclear fission in December 1938.
For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar, K-feldspar, hornblende, biotite, and muscovite.
Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.
The ideal scenario according to Bowen's reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende (i.e.; tonalite or diorite), which is low in K (and hence Rb) but high in Sr (as this substitutes for Ca), which proportionally enriches the melt in K and Rb.
This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate.
The resulting Rb–Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different.
This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
If these form a straight line then the subsamples are consistent, and the age probably reliable.
After measurements of Rubidum and Strontium concentration in the mineral we can easily determine the age, the t value, of the sample.
Several preconditions must be satisfied before a Rb–Sr date can be considered as representing the time of emplacement or formation of a rock.
One of the major drawbacks (and, conversely, the most important use) of utilizing Rb and Sr to derive a radiometric date is their relative mobility, especially in hydrothermal fluids.
Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement.
A Rb–Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock.
The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.
Initial 87Sr/86Sr ratios are a useful tool in archaeology, forensics and paleontology because the 87Sr/86Sr of a skeleton, sea shell or indeed a clay artefact is directly comparable to the source rocks upon which it was formed or upon which the organism lived.
Strontium isotope stratigraphy relies on recognised variations in the 87Sr/86Sr ratio of seawater over time.
In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers (for example U–Th) leads to greater uncertainties in the exact shape of the Sr isotope seawater curve.