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