The slope of the line dictates the age of the sample.
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.
This article summarizes the principles of radiometric dating which are relevant to potassium–argon and argon–argon dating, and then considers these two forms of dating in more detail focusing on their strengths and weaknesses as dating methods.
The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined.
This is well known for the Cenozoic time-scale but, due to poorer preservation of carbonate sequences in the Mesozoic and earlier, it is not completely understood for older sequences.
Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism.
Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock (generally during potassic alteration or calcic (albitisation) alteration.
The important concept in isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same Sr as the mineral.
Although this is a potential source of error for terrestrial rocks, it is irrelevant for lunar rocks and meteorites, as there are no chemical weathering reactions in those environments.
The utility of the rubidium–strontium isotope system results from the fact that Sr with a half-life of 48.8 billion years.
In addition, Rb is a highly incompatible element that, during partial melting of the mantle, prefers to join the magmatic melt rather than remain in mantle minerals. The radiogenic daughter, Sr, is produced in this decay process and was produced in rounds of stellar nucleosynthesis predating the creation of the Solar System.
The uncertainty analysis is applied to a set of important parent–daughter pairs and the need for more precise half-life data is examined.