Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a rock.* The system must have remained closed to Rb and Sr diffusion from the time at which the rock formed or fell below the (generally considered to be 650 °C);* The minerals which are taken from a rock to construct an isochron must have formed in chemical equilibrium with one another or in the case of sediments, be deposited at the same time;* The rock must not have undergone any which could have disturbed the Rb-Sr system either thermally or chemically 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 d 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.
Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed.
During fractional crystallization, Sr tends to become concentrated in .
Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed.
Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.
Calculating the age The age of a sample is determined by analysing several minerals within the sample.
The Sr87/Sr86 ratio for each sample is plotted against its Rb87/Sr86 ratio on a graph called an isochron.
If these form a straight line then the samples are consistent, and the age probably reliable.The slope of the line dictates the age of the sample.ources of error Rb-Sr dating relies on correctly measuring the Rb-Sr ratio of a mineral or whole rock sample, plus accurately deriving an accurate Sr ratio for the mineral or whole rock sample.The ideal scenario according to ), 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.