Rubidium-strontium Dating - Sources of Error

Sources of Error

Rb-Sr dating relies on correctly measuring the Rb-Sr ratio of a mineral or whole rock sample, plus deriving an accurate 87Sr/86Sr ratio for the mineral or whole rock 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.

• The system must have remained closed to Rb and Sr diffusion from the time at which the rock formed or fell below the closure temperature (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 metasomatism 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 hydrothermal fluids. 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. 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.

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.