Intro to Rb-Sr Age Dating -- Page 5 of 17

Dating Rocks with the Rb-Sr "Isochron" Method

Page 5 of 17

As you know, there are numerous radioactive isotopes that can be used for numeric dating. All of the dating methods rely on the fundamental principles of radioactive decay, but the specific materials that can be dated and the exact procedures for calculating a date are very different from one method to the next. The rest of this activity is about using the Rb-Sr method.

Rubidium occurs in nature as two isotopes: radioactive Rb-87 and stable Rb-85. Rb-87 decays with a half-life of 48.8 billion years to Sr-87. This half-life is so long that the Rb-Sr method is normally only used to date rocks that are older than about 100 million years.

Which minerals and rocks can be dated with the Rb-Sr method?

The minerals must contain Rb, which is a rather rare element. Fortunately, Rb behaves chemically very much like the more common potassium (K), so that most K-bearing minerals contain a small amount of Rb. Examples include the mica family (biotite and muscovite) and the feldspar family (plagioclase and orthoclase). These minerals are abundant in granite (an igneous rock) and gneiss (a metamorphic rock).

What steps are involved in Rb-Sr dating? (If you are in a hurry or have seen this before, click on step #4 to continue.)

1.	Select a fresh, unweathered rock sample.
2.	Crush the rock and separate the Rb-bearing minerals.
3.	Analyze the isotopic compositions of the whole rock and mineral separates on a mass spectrometer.
4.	Prepare an isochron diagram to calculate a date. Understanding the isochron diagram is the key to determining the age of a rock using the Rb-Sr method.