Page 242 - Laboratory Manual in Physical Geology

There are times in this laboratory that you may be About 350 different isotopes occur naturally. Some
referring to a rock unit and other times when you may be of these are stable isotopes , meaning that they are not
referring to a time unit, so you will need to use the cor- radioactive and do not decay through time. The others

rect kind of unit for rock or time. For example, notice in are radioactive isotopes that decay spontaneously, at
FIGURE 8.10 that Mucrospirifer (a brachiopod) is an index regular rates through time. When a mass of atoms of a
fossil for the Devonian Period of time. When writing about radioactive isotope is incorporated into the structure of
this, you would write that Mucrospirifer is found in the a newly formed crystal or seashell, it is referred to as a
Devonian System of rock, which represents the Devonian parent isotope . When atoms of the parent isotope decay
Period of time. to a stable form, they have become a daughter isotope .
A parent isotope and its corresponding daughter are
called a decay pair .
ACTIVITY Atoms of a parent isotope always decay to atoms of
their stable daughter isotope at an exponential rate that
8.4 Absolute Dating of Rocks does not change. The rate of decay can be expressed in
and Fossils terms of half-life —the time it takes for half of the parent
atoms in a sample to decay to stable daughter atoms.
| Radiometric Dating of Geologic
THINK How do geologists determine the
About It absolute age, in years, of Earth Materials
materials and events?
The decay parameters for all radioactive isotopes can be

OBJECTIVE Calculate absolute ages to date Earth represented graphically as in FIGURE 8.11 . Notice that the

materials and events. decay rate is exponential (not linear)—during the second
half-life interval, only half of the remaining half of parent
PROCEDURES atoms will decay. All radioactive isotopes decay in this way,
1. Before you begin , read Determining Absolute but each decay pair has its own value for half-life.
Ages by Radiometric Dating below. Also, this is Half-lives for some isotopes used for radiometric
what you will need : dating have been experimentally determined by physicists
____ calculator and chemists, as noted in the top chart of FIGURE 8.11 .
____ Activity 8.4 Worksheet (p. 222 ) and pencil For example, uranium-238 is a radioactive isotope
(parent) found in crystals of the mineral zircon. It decays
2. Then follow your instructor’s directions for to lead-206 (daughter) and has a half-life of about
completing the worksheets.
4500 million years (4.5 billion years).
To determine the age of an object, it must contain
atoms of a radioactive decay pair that originated when
the object formed. You must then measure the percent of
those atoms that is parent atoms ( P ) and the percent that is
Determining Absolute Ages by daughter atoms ( D ). This is generally done in a chemistry
laboratory with an instrument called a mass spectrometer .
Radiometric Dating Based on P and D and the chart at the bottom of
You measure the passage of time based on the rates and FIGURE 8.10 , find the number of half-lives that have elapsed

rhythms at which regular changes occur around you. For and the object’s corresponding age in number of half-lives.
example, you are aware of the rate at which hands move on a Finally, multiply that number of half-lives by the known
clock, the rhythm of day and night, and the regular sequence half-life for that decay pair (noted in the top chart of
of the four seasons. These regular changes allow you to mea- FIGURE 8.11 ).

sure the passage of minutes, hours, days, and years. For example, a sample of Precambrian granite
Another way to measure the passage of time is by the contains biotite mineral crystals, so it can be dated
regular rate of decay of radioactive isotopes. This technique using the potassium-40 to argon-40 decay pair. If
is called radiometric dating and is one way that geologists there are three argon-40 atoms in the sample for every
determine absolute ages of some geologic materials. one potassium-40 atom, then the sample is 25.0%
You may recall that isotopes of an element are atoms potassium-40 parent atoms ( P ) and 75.0% argon-40
that have the same number of protons and electrons but daughter atoms ( D ). This means that two half-lives have
different numbers of neutrons. This means that the different elapsed, so the age of the biotite (and the granite) is
isotopes of an element vary in atomic weight (mass number) 2.0 times 1.3 billion years, which equals 2.6 billion years.
but not in atomic number (number of protons). The useful dating ranges are also noted on FIGURE 8.11 .

214 ■ L ABOR ATORY 8