Geologic Time: Concepts and Principles - The Changing Earth: Exploring Geology and Evolution - page 453

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Figure 17.21 Crystallization of Magma Containing Radioactive Parent and Stable Daughter Atoms

Magma Mineral

crystallizing
from magma

Igneous rock

Radioactive parent atoms
Stable daughter atoms

a Magma contains both radioactive
parent atoms and stable daughter
atoms. The radioactive parent atoms
are larger than the stable daughter
atoms.

b As magma cools and begins to
crystallize, some of the radioactive
atoms are incorporated into certain
minerals because they are the right
size and can fi t into the crystal
structure. In this example, only the
larger radioactive parent atoms fi t
into the crystal structure. Therefore,
at the time of crystallization, minerals
in which the radioactive parent atoms
can fi t into the crystal structure will
contain 100% radioactive parent
atoms and 0% stable daughter atoms.

c After one half-life, 50% of the
radioactive parent atoms will have
decayed to stable daughter atoms,
such that those minerals that had
radioactive parent atoms in their
crystal structure will now have 50%
radioactive parent atoms and 50%
stable daughter atoms.

magma. If the ages do not closely agree, then they are said to
be disconcordant , and other samples must be taken and ratios
measured to see which, if either, date is correct.
Recent advances and the development of new techniques
and instruments for measuring various isotope ratios have en-
abled geologists to analyze not only increasingly smaller sam-
ples, but with a greater precision than ever before. Presently,
the measurement error for many radiometric dates is typically
less than 0.5% of the age, and in some cases is even better than
0.1%. Thus, for a rock 540 million years old (near the begin-
ning of the Cambrian Period), the possible error could range
from nearly 2.7 million years to less than 540,000 years.

Long-Lived Radioactive Isotope Pairs
Table 17.1 shows the five common, long-lived parent-
daughter isotope pairs used in radiometric dating. Long-
lived pairs have half-lives of millions or billions of years.
All of these were present when Earth formed and are still
present in measurable quantities. Other shorter-lived ra-
dioactive isotope pairs have decayed to the point that only
small quantities near the limit of detection remain.
The most commonly used isotope pairs are the uranium-
lead and thorium-lead series, which are used principally
to date ancient igneous intrusives, lunar samples, and some

TABLE 17.1 Five of the Principal Long-Lived Radioactive Isotope Pairs Used in Radiometric Dating
ISOTOPES Half-Life of Effective Dating Minerals and Rocks That

Parent Daughter Parent (years) Range (years) Can Be Dated

Uranium 238 Lead 206 4.5 billion 10 million to 4.6 billion Zircon

Uraninite

Uranium 235 Lead 207 704 million

Thorium 232 Lead 208 14 billion

Rubidium 87 Strontium 87 48.8 billion 10 million to 4.6 billion Muscovite

Biotite

Potassium feldspar

Whole metamorphic or igneous rock

Potassium 40 Argon 40 1.3 billion 100,000 to 4.6 billion Glauconite Hornblende

Muscovite Whole volcanic rock

Biotite