radioactive dating
Numerical and Relative Ages for Rocks
Geological time...
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.
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
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
Dating Rocks with the Rb-Sr "Isochron" Method
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
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.