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.Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite.

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The age of a sample is determined by analysing several minerals within the sample. 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.

The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.

The important concept in isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same Sr as the mineral.

Strontium isotope stratigraphy relies on recognised variations in the Sr ratio of seawater over time.

The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined.

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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.

Rb-Sr dating relies on correctly measuring the Rb-Sr ratio of a mineral or whole rock sample, plus deriving an accurate 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.