radioactive dating

Radioactive Isotopes - the "Clocks in Rocks"
       Numerical and Relative Ages for Rocks
Geological time... 

Parent Decay and Daughter Growth Curves
Radiocarbon Dating
Dating Rocks with the Rb-Sr "Isochron" Method
Getting a Rock Sample Ready for the Mass Spectrometer
A Mass Spectrometer is used to Measure Isotopic Ratios

A numerical (or "absolute") age is a specific number of years, like 150 million years ago. A relative age simply states whether one rock formation is older or younger than another formation. The Geologic Time Scale was originally laid out using relative dating principles.

relative timescale

Numerical dating, the focus of this exercise, takes advantage of the "clocks in rocks" - radioactive isotopes ("parents") that spontaneously decay to form new isotopes ("daughters") while releasing energy. For example, decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus). ("87" is the atomic mass number = protons + neutrons.
Numerical ages have been added to the Geologic Time Scale since the advent of radioactive age-dating techniques.
 Many minerals contain radioactive isotopes. In theory, the age of any of these minerals can be determined by:

 1) counting the number of daughter isotopes in the mineral, and
 2) using the known decay rate to calculate the length of time required to produce that number of daughters.


It illustrates how the amount of a radioactive parent isotope decreases with time. This amount is a percentage of the original parent amount. Time is expressed in half-lives. Experiment by dragging on the graph. For example when 42% of the parent still remains, 1.23 Half-Lives of time has passed.

Parent Decay and Daughter Growth Curves

The half-life of U-235 decaying to Pb-207 is 713 million years. Note that this half-life can be obtained from the graph at the point where the decay and growth curves cross. Determine the half-lives for the other three isotopes and enter your estimate into the text fields below each graph. Note the differences in scale between the various graphs

Radiocarbon Dating
carbon 14 cycle

The radiocarbon dating method was developed in the 1940's by Willard F. Libby and a team of scientists at the University of Chicago. It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artifacts and geologic events up to about 50,000 years in age. The radiocarbon method is applied in many different scientific fields, including archeology, geology, oceanography, hydrology, atmospheric science, and paleoclimatology. For his leadership, Libby received the Nobel Prize in  Chemistry in 1960. See.. our C14 dating calculator

carbon 14 decay curve

Dating Rocks with the Rb-Sr "Isochron" Method

Rubidium-strontium decay curve

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?
         1. Select a fresh, unweathered rock sample.
Sample Selection
A geologist collects a fresh, unweathered hand sample for age dating. Fresh is the key word here, and means that the chemistry of the sample has NOT been changed since the sample formed. Weathering alters the chemistry of rocks including their isotopic compositions. Therefore, a highly weathered rock may yield unreliable age information.
         2. Crush the rock and separate the Rb-bearing minerals.

Getting a Rock Sample Ready for the Mass Spectrometer

For reliable age determination, careful sample preparation is an important and often tedious process. The rock is mechanically crushed into small fragments. Fragments of the Rb-bearing minerals are then separated from the whole rock using a variety of methods, such as a magnetic separator. These materials are then used to prepare a "whole-rock" sample and several "mineral separate" samples. The whole rock sample will yield the weighted average isotopic composition of all the minerals in the rock. Each mineral separate will yield the composition of that particular mineral.

Other Steps
There are other steps that must be carried out to prepare a sample for analysis by a mass spectrometer, such as converting the sample to a solution by dissolving the mineral separates in selected acids, using techniques of column chemistry to increase the concentration of the small amounts of Rb and Sr in the solution and then precipitating the concentrated solution as a "salt" compound. It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer for analysis.

  3. Analyze the isotopic compositions of the whole rock and mineral separates on a mass spectrometer.

A Mass Spectrometer is used to Measure Isotopic Ratios
Mass Spectrograph

A Mass Spectrometer is a very powerful and sophisticated instrument. Many types exist. Below is a simplified diagram of the electro-mechanical mass spectrometer system and a picture of a modern instrument. Understanding how a mass spectrometer functions is beyond the level of this activity. But you should know that it measures the amounts of various isotopes present in
 specially prepared samples of rocks and minerals as well as other materials.

4.Prepare an isochron diagramthat shows the decay curve to calculate a date. Understanding the isochron diagram is the key to determining the age of a rock using the Rb-Sr method.

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