Rubidium-strontium dating

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The rubidium-strontium dating method is a radiometric dating technique that geologists use to determine the age of rocks.

Development of this process was aided by Fritz Strassmann, who later moved onto discovering nuclear fission with Otto Hahn and Lise Meitner.

The utility of the rubidium-strontium isotope system results from the fact that 87Rb (one of the isotopes of rubidium) decays to 87Sr with a halflife of 49 billion years. Different minerals in a given geologic setting can have a distinctly different ratio of strontium-87 to strontium-86 (87Sr/86Sr) as a consequence of different ages, original Rb/Sr values and the initial 87Sr/86Sr.

If these minerals crystallized from the same silicic melt, each mineral had the same initial 87Sr/86Sr as the parent melt. However, because Rb substitutes for K in minerals and these minerals have different K/Ca ratios, the minerals will have had different Rb/Sr ratios.

During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios (10 or higher) occur in pegmatites.

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.

Contents

Example

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. 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.

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