Basic
Worldview:
103
Science, the Bible,
and Creation
Origins
- Section Four:
Radiometric Dating Basics
Origins - Section One: Introduction
and the Basics
Origins - Section Two: Premature
Dismissals
Origins - Section Two: Application
of the Basics
Origins - Section Three: Creation
Origins - Section Three: Evolution,
Origin of Life
Origins - Section Three: Evolution,
Environment for Life 1
Origins - Section Three: Evolution,
Environment for Life 2
Origins - Section Three: Evolution,
Another Planet
Origins - Section Three: Evolution,
Origin of Species
Origins - Section Three: Evolution,
Speciation Factors
Origins - Section Three: Evolution,
Speciation Rates
Origins - Section Four: Time and
Age, Redshift
Origins - Section Four: Philosophical
Preference
Origins - Section Four: Cosmological
Model 1
Origins - Section Four: Cosmological
Model 2
Origins - Section Four: Dating Methods,
Perceptions, Basics
Origins - Section Four: Global Flood
Evidence
Origins - Section Four: Relative
Dating
Origins - Section Four: Dating and
Circular Reasoning
Origins - Section Four: The Geologic
Column
Origins - Section Four: Radiometric
Dating Basics
Origins - Section Four: General
Radiometric Problems
Origins - Section Four: Carbon-14
Problems
Origins - Section Four: Remaining
Methods and Decay Rates
Origins - Section Four: Radiometric
Conclusions, Other Methods
Origins - Section Five: Overall
Conclusions, Closing Editorial
Origins - Section Five: List
of Evidences Table
Origins Debate Figures and
Illustrations
Focus
on Critical Evidence: Age and Radiometric Dating
Our
focus on radiometric dating will be broken down into several
segments. In the first segment, we will discuss the fundamental
terms, processes, categories, and types of radiometric dating,
including some of the basic criteria necessary for radiometric
dating. In the second segment, we will discuss the problems
with those required criteria in greater detail, particular
criteria surrounding isotope ratios and the obstacles those
ratios pose to radiometric dating in general, particularly
the first major category of radiometric the methods, the methods
that are used for dating igneous and metamorphic rock. In
the third section, we will discuss ratio problems facing the
potassium-argon method, the most widely used radiometric dating
method and we will close with an examination of how the evidence
for a worldwide flood also poses problems for dating igneous
and metamorphic rocks. In the fourth section, we will discuss
the ratio problems facing carbon-14, the method used to date
fossils and sedimentary rock. In the fifth section, we will
discuss the minor, remaining radiometric dating methods. In
the sixth section, we will discuss the problems with decay
rate constants, an issue which affects radiometric dating
in general. And finally, with these six segments complete,
we will conclude our focus on radiometric dating methods.
At
this point, it is also important to recall from earlier that
the terms “radiometric dating” and “absolute
dating” are synonymous and are used interchangeably.
“Geologic
Time, III DATING METHODS – In
order to determine the relative age of rock layers, scientists
use three simple principles…By matching the fossil content
of rock sequences, even across widespread geographic regions,
paleontologists believe that certain sequences are probably
about the same age. All of these methods facilitate the relative dating of rock sequences,
but do not provide absolute ages for the rocks. Geologists
have several methods for determining the actual age of a rock
layer. The most important is radiometric dating, which
uses the steady decay of radioactive elements (seeRadioactivity)
in the rock to provide a measure of age.” – "Geologic
Time," Microsoft® Encarta® Encyclopedia 99. © 1993-1998
Microsoft Corporation. All rights reserved.
“Index
Fossil, IV USE OF INDEX FOSSILS – Using index fossils and the principle of faunal and floral succession,
scientists can determine a relative chronology, or a sequence
of events. Yet, absolute
age, or the number of years that have passed since a rock
layer formed, cannot be determined using fossils alone. Absolute
age must be derived from dating methods such as radiometric
dating.” – "Index Fossil," Microsoft®
Encarta® Encyclopedia 99. © 1993-1998 Microsoft Corporation.
All rights reserved.
“Dating
Methods, II DEVELOPMENT OF RELATIVE AND ABSOLUTE METHODS
– With the methods then available, 19th-century
geologists could only construct a relative time scale.
Thus, the actual age of the earth and the
duration, in millions of years, of
the units of the time scale remained unknown until the
dawn of the 20th century.
After radioactivity was discovered,
radiometric dating methods were quickly developed. With these
new methods geologists could calibrate the relative scale
of geologic time, thereby creating an absolute one.”
– "Dating Methods," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
As
we begin with our introduction to the fundamentals of radiometric
dating, our first stop is vocabulary and basic scientific
processes. Although some of the information, which follows
may at first seem overly basic, covering such simple terms
is necessary so that the more complex concepts built on top
of them will be equally clear. And on the road to understanding
radiometric dating, we begin with the atom and its primary
components.
As
indicated by the quote below, atoms are composed of 3 types
of primary particles. Protons reside in the atom’s nucleus
and have a positive electric charge. Neutrons also reside
in the atoms nucleus but are neutral, having no electric charge.
And electrons, which have a negative electric charge,
reside outside the nucleus, in “orbits” or “clouds”
around it.
“Atom
– smallest unit into which matter can be divided without
the release of electrically charged particles. It also is
the smallest unit of matter that has the characteristic properties of
a chemical element. As such, the atom is the basic building
block of chemistry. Most of the atom is empty space. The rest
consists of a positively charged nucleus of protons and
neutrons surrounded
by a cloud of negatively charged electrons. The nucleus
is small and dense compared to the electrons, which are the
lightest charged particles in nature…Components and properties of atoms, Constituent
particles and forces – The nucleus is the positively charged centre of an atom and contains
most of its mass. It is composed of protons, which have a
positive charge, and neutrons, which have no charge…Most
properties of atoms–particularly those associated
with chemical bonds, physical forces, and the properties of
bulk matter–depend
solely on the behaviour of the electrons surrounding the nucleus.
The chemical properties of an atom depend on the arrangement
of its electrons making up the cloud around the nucleus. The
atoms of one element differ from those of other elements in
the number of their electrons…Properties of atoms, Atomic
number – The single most important characteristic
of an atom is its atomic number, which is defined as the number of units of positive charge in the nucleus. A neutral atom
has an equal number of protons and electrons, so that
the positive and negative charges exactly balance. The
atomic number determines the chemical properties of an atom,
including the kinds of molecules that can be formed and
their binding energies. Hence, the atomic number determines an atom's
characteristics as an element…Electric charge –
The normal atom is electrically neutral, meaning that
it carries a net electric charge of zero.” – Encyclopaedia Britannica
2004 Deluxe Edition
As
the quote above also indicates, atoms come in the form of
different elements such as oxygen, hydrogen, iron, calcium,
or sodium, etc. Atoms of each element have a specific quantity
of protons, electrons, and neutrons. Atoms of other elements
have different quantities of these particles. Consequently,
the different quantity of these particles causes atoms of
different elements to have different amounts of mass. Elements
with more protons, neutrons, and electrons have more mass.
Elements that have less protons, neutrons, and electrons have
less mass.
However,
the quote also states that “the single most important
characteristic of an atom is its atomic number.” The
atomic number denotes how many protons atoms of a particular
element have. But, since atoms have an equal amount of protons
and electrons, this number also represents the number of electrons
in atoms of that element. And most importantly, we note from
the quote above that an element’s chemical properties
depend on the number of protons and electrons, the particles
with electric charges. The chemical properties of an element
do not depend on its quantity of neutrons, the particles with
no electric charge.
The
fact that neutrons do not affect the chemical properties,
which distinguish one element from another, leads us to the
concept of an isotope. Because neutrons do not affect the
basic chemical properties of an element, individual atoms
of a particular element can have different numbers of neutrons
without becoming or being categorized as a different element.
Because they have the same number of protons and electrons
as all other atoms of that element, they have the same defining
chemical properties of the element. However, because they
have a different number of protons, they are considered to
be a different “isotope.” The term isotope refers
to any or all of the individual versions of each element,
including the normal form of each element predominantly found
in nature. And each isotope of an element is defined by its
specific number of neutrons. Furthermore, as indicated by
both of the quotes below, because each isotope of an element
has a different number of neutrons, it also has a different
amount of mass than other isotopes of the same element.
“Dating,
Absolute dating, Principles of isotopic dating –Most
elements exist in different atomic forms that are identical in their chemical properties but differ in
the number of neutral particles—i.e., neutrons—in
the nucleus. For a single element, these
atoms are called isotopes. Because isotopes
differ in mass, their relative abundance can be determined
if the masses are separated in a mass spectrometer (see below
Use of mass spectrometers).” – Encyclopaedia Britannica
2004 Deluxe Edition
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E1 Basic Theory – Isotopes
are atoms of any elements that differ in mass from that element,
but possess the same general chemical and optical properties.”
– "Dating Methods," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
In
addition, there are 2 basic kinds of isotopes, normal (non-radioactive)
isotopes and radioactive isotopes (also known as radioisotopes).
As indicated by the quotes below, radioisotopes are isotopes
whose structure is so different from the norm that it is much
more unstable. This instability causes the isotope to give
off large amounts of energy. The term “radioactive”
refers to the enormous energy emission.
“Radioactive
isotope – also
called radioisotope any of several species of the same
chemical element with different masses whose nuclei are unstable
and dissipate excess energy by spontaneously emitting radiation
in the form of alpha, beta, and gamma rays. Every chemical
element has one or more radioactive isotopes. For
example, hydrogen, the lightest element, has three isotopes with mass numbers 1, 2, and 3. Only hydrogen-3
(tritium), however,
is a radioactive isotope, the other two being stable. More
than 1,000 radioactive isotopes of the various elements are
known. Approximately
50 of these are found in nature; the
rest are produced artificially as the direct products
of nuclear reactions or indirectly as the radioactive descendants
of these products.” – Encyclopaedia Britannica
2004 Deluxe Edition
"Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E1 Basic Theory – Radioactive
elements such as uranium (U) and thorium (Th) decay
naturally to form different elements or isotopes of the same
element. (Isotopes are atoms of any elements that differ
in mass from that element, but possess the same general chemical
and optical properties.) This decay is accompanied by the emission of radiation or particles
(alpha, beta, or gamma rays) from the nucleus, by nuclear
capture, or by ejection of orbital electrons (see Atom and
Atomic Theory). A number of isotopes decay to a stable product,
a so-called daughter isotope, in a single step (for example,
carbon-14), whereas other
series involve many steps before a stable isotope is formed.
Multistep radioactive decay series include, for example, the
uranium-235, uranium-238, and thorium-232 families. If
a daughter isotope is stable, it accumulates until the parent
isotope has completely decayed. If a daughter isotope
is also radioactive, however, equilibrium is reached when
the daughter decays as fast as it is formed.” –
"Dating Methods," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
It
is also important to note from the first quote above that
only 50 radioactive isotopes are “found in nature”
while “the rest are produced artificially” as
direct or indirect “products of nuclear reactions.”
We will return to this point below when we cover the limitations
on radiometric dating.
Radioactive
isotopes are the cornerstone of radiometric dating. Dating
methods are defined as efforts to identify how much time has
elapsed since particular events occurred, such as the formation
of rock layers. And the instability of radioactive isotopes
causes them to emit excessive amounts of energy, which is
also accompanied by a loss of mass. Thus, the instability
of radioactive isotopes causes them to change over time. Depending
upon how the particular manner in which they lose mass, radioactive
isotopes will either change into another isotope of the same
element or into an atom of another element altogether. This
process of change is called “radioactive decay”
or simply “decay.” And this radioactive decay
or change from one isotope into another isotope or element
occurs at different rates that are unique to each isotope,
depending upon its particular starting structure and level
of instability.
“Dating,
Absolute dating, Principles of isotopic dating –
All absolute isotopic ages are based on radioactive decay, a process
whereby a specific
atom or isotope is converted into another specific atom or
isotope at a constant and known rate…The particles
given off during the decay process are part of a profound
fundamental change in the nucleus. To
compensate for the loss of mass (and energy), the radioactive
atom undergoes internal transformation and in most cases simply
becomes an atom of a different chemical element.”
– Encyclopaedia Britannica 2004 Deluxe Edition
As
indicated by the quote below, this process was first discovered
in the early 1900’s.
“Earth
sciences, The 20th century: modern trends and developments,
Geologic sciences, Radiometric dating – In 1905,
shortly after the discovery of radioactivity, the American chemist Bertram
Boltwood suggested that lead is one of the disintegration
products of uranium, in which case the older a uranium-bearing
mineral the greater should be its proportional part of lead.
Analyzing specimens
whose relative geologic ages were known, Boltwood found
that the ratio of lead to uranium did indeed increase with
age. After estimating the rate of this radioactive
change he calculated that the absolute ages of his specimens
ranged from 410,000,000 to 2,200,000,000 years. Though his
figures were too high by about 20 percent, their order of
magnitude was enough to dispose of the short scale of geologic
time proposed by Lord Kelvin. Versions of the modern mass
spectrometer were invented in the early 1920s and 1930s, and during World War II the device was improved
substantially to help in the development of the atomic bomb.
Soon after the war, Harold C. Urey and
G.J. Wasserburg applied the mass spectrometer to the study
of geochronology. This device separates the different isotopes
of the same element and can measure the variations in these
isotopic abundances to within one part in 10,000. By
determining the amount of the parent and daughter isotopes
present in a sample and by knowing their rate of radioactive
decay (each radioisotope has its own decay constant), the
isotopic age of the sample can be calculated. For dating minerals and rocks, investigators commonly use the following
couplets of parent and daughter isotopes: thorium-232–lead-208,
uranium-235–lead-207, samarium-147–neodymium-143,
rubidium-87–strontium-87, potassium-40–argon-40,
and argon-40–argon-39. Such techniques have had an enormous
impact on scientific knowledge of Earth history because precise
dates can now be obtained on rocks in all orogenic (mountain)
belts ranging in age from the early Archean (about 3,800,000,000 years old)
to the late Tertiary (roughly 20,000,000 years old). The oldest sedimentary and igneous rocks in the world are found
at Isua in western Greenland; they have
an isotopic age of approximately 3,800,000,000 years—a fact first established in 1972 by Stephen
Moorbath of the University
of Oxford.
Also by extrapolating backward in time to
a situation when there was no lead that had been produced
by radiogenic processes, a figure of about 4,600,000,000 years
is obtained for the minimum age of the Earth.” –
Encyclopaedia Britannica 2004 Deluxe Edition
There
are several interesting items to note from the quote above.
First, we note that radiometric dating was producing dates
that were inconsistent with the evolutionary timescale right
from the beginning. Initial radiometric dates produced an
age that was “about 20 percent too high.” Second,
from about three-quarters of the way through the quote, we
see that the common forms of radiometric dating are listed
as having been developed at this time, after World War II.
And third, we notice that the same Harold Urey of the famous
1953 Miller-Urey experiment is at the forefront of developing
radiometric dating procedure.
“Britannica, Life, The origin of life, Production
of simple organic molecules – The first deliberate
experimental simulation of these primitive conditions was
carried out in 1953 by a U.S. graduate student, S.L. Miller,
under the guidance
of the eminent chemist H.C. Urey.” –
Encyclopaedia Britannica 2004 Deluxe Edition
“Sagan, Carl Edward – Later in
the 1960s Sagan built on the work of American chemists Stanley
Miller and Harold Urey. In the 1950s Miller and Urey had combined methane,
ammonia, water vapor, and hydrogen, the probable components
of the earth's early atmosphere, in a flask.” –
"Sagan, Carl Edward," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
As
we continue forward with our discussion of basic terms and
concepts, the next stop is the term “half-life.”
As mentioned above, different radioactive isotopes decay at
different rates depending upon their individual instability.
Obviously, the faster the radioactive isotope decays (changes)
to a non-radioactive form, the less time that it takes for
half of the radioactive material to cease
being radioactive. Consequently, the amount of time that it
takes for half of the number of radioactive atoms to change
to a non-radioactive form is called that element’s “half-life.”
“Geologic
Time, III DATING METHODS – Radioactive
elements decay to form elements or isotopes (atoms of
an element that differ in mass but share the same general
chemical properties) of an element. An element's half-life is the time required
for half the number of its atoms to decay. Different elements can have dramatically different half-lives.”
– "Geologic Time," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
“Dating,
Absolute dating, Principles of isotopic dating –
Half-life is defined as the time period
that must elapse in order to halve the initial number of radioactive
atoms. The half-life
and the decay constant are inversely proportional because
rapidly decaying radioisotopes have a high decay constant
but a short half-life.” – Encyclopaedia Britannica
2004 Deluxe Edition
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E1 Basic Theory – Scientists
describe the radioactivity of an element in terms of half-life,
the time the element takes to lose 50 percent of its activity
by decay. This covers an extraordinary range
of time, from billions of years to a few microseconds.
At the end of the period
constituting one half-life, half of the original quantity
of radioactive element has decayed; after another half-life,
half of what was left is halved again, leaving one-fourth
of the original quantity, and so on. Every
radioactive element has its own half-life; for example,
that of carbon-14 is
5730 years and that of uranium-238
is 4.5 billion years.” – "Dating Methods,"
Microsoft® Encarta® Encyclopedia 99. © 1993-1998 Microsoft
Corporation. All rights reserved.
The
fact that each radioactive isotope decays or changes at a
different rate, depending on how unstable it was to begin
with, means that each radioactive isotope has a unique half
life. Furthermore, the half-life is inversely proportional
to the decay rate. Isotopes with faster decay or change rates
have shorter half-lives. And isotopes with slower decay or
change rates have longer half-lives. The half-life is rendered
as a measure of time, such as the 5,730 year half-life of
carbon-14 listed in the quote above. It is this measure of
time that is used by scientists to calculate age. However,
since the half-life is inversely proportional to the decay
rate, the reliability of the half-life time measurement depends
upon how constant the decay rate is and how reliably we have
identified that rate. We will discuss this issue in more depth
as we move forward thoughout our focus on radiometric dating.
Nevertheless,
the process of radioactive decay forms the basis of radiometric
dating calculations. As we will see in the second quote below,
the term “parent” refers to the radioactive isotope
before it decays or changes and, likewise, the term “daughter”
refers to the isotope or element into which the radioactive
isotope changes through the process of decay. The calculation
of age based upon radioactive decay requires the following
3 basic pieces of information, as indicated by the quotes
below. First, we need to know how many radioactive parent
atoms and how many daughter atoms were present originally
when an item, such as a rock, was formed. Second, we need
to know how long it takes for half of the radioactive atoms
to change from parent to daughter atoms. And third, we need
to know how many of the radioactive parent atoms and the daughter
atoms are currently present in the item. If we know all 3
of these factors, then the number of parent atoms that are
“missing” and the number of daughter isotopes
that have been added in comparison to the original amount
of these atoms (the number originally present when the item
was formed) tells us how much decay has occurred, how long
decay has been occurring, how many “half-lives”
have passed, and consequently how much time has passed since
the item’s formation. This is the conceptual basis for
determining age from radioactive elements.
“Fossil,
Studying fossils, Dating fossils – Paleontologists determine how old a fossil is by measuring the radioactive
isotopes in the rocks that contain the fossil. Radioactive isotopes are forms of chemical
elements that break down, or decay, to form other materials.
Scientists know the rates of decay of various radioactive
isotopes. By comparing the amount of a radioactive isotope
in a rock to the amount of the material produced by its decay,
scientists can calculate how long the decay has been taking
place. This length of time represents the age of the rock
and the fossils it contains.” – Worldbook, Contributor:
Steven M. Stanley, Ph.D., Professor of Earth and Planetary
Sciences, Johns Hopkins
University.
“Dating,
Absolute dating, Principles of isotopic dating –
In terms of the numbers of atoms present, it
is as if apples changed spontaneously into oranges at a fixed
and known rate. In this analogy, the apples would represent
radioactive, or parent, atoms, while the oranges would represent
the atoms formed, the so-called daughters. Pursuing this
analogy further, one
would expect that a new basket of apples would have no oranges
but that an older one would have many. In fact, one
would expect that the ratio of oranges to apples would change
in a very specific way over the time elapsed, since the process
continues until all the apples are converted. In geochronology the situation is identical.
A particular rock or
mineral that contains a radioactive isotope (or radio-isotope)
is analyzed to determine the number of parent
and daughter isotopes present, whereby the time since that
mineral or rock formed is calculated…This pair of
equations states rigorously what might be assumed from intuition,
that minerals formed at successively longer times in the past would have
progressively higher daughter-to-parent ratios. This follows
because, as each parent atom loses its identity with
time, it reappears as a daughter atom…In short,
one need only measure the ratio of the number
of radioactive parent and daughter atoms present, and the
time elapsed since the mineral or rock formed can be calculated,
provided of course that the decay rate is known.”
– Encyclopaedia Britannica 2004 Deluxe Edition
At
this point, having established the basic terms and processes
involved in radiometric age calculations, we are ready to
discuss the basic limitations on radiometric dating.
Now,
as we cover some of the limitations on radiometric process
it is important not to confuse limitations with problems.
These limitations are restrictions on how widely radiometric
dating can be used. These factors limit the number and kind
of items to which radiometric dating can be applied. They
are not problems that prevent the process from working at
all. There are 3 such limitations.
First,
in order to be radiometrically dated, an item must contain
not only isotopes but radioactive isotopes.
“Dating,
General considerations, Distinctions between relative-age
and absolute-age measurements – The need to correlate
over the rest of geologic time, to correlate nonfossiliferous
units, and to calibrate the fossil time scale has led to the development of a specialized
field that makes use of natural radioactive isotopes in order
to calculate absolute ages. The precise measure of geologic
time has proven to be the essential tool for correlating the
global tectonic processes (see below) that have taken place
in the past. Precise
isotopic ages are called absolute ages, since they date
the timing of events not relative to each other but as the
time elapsed between a rock-forming event and the present.”
– Encyclopaedia Britannica 2004 Deluxe Edition
Second,
there are only 50 radioactive isotopes “found in nature”
while “the rest are produced artificially” as
direct or indirect “products of nuclear reactions.”
“Radioactive
isotope – More than 1,000 radioactive isotopes of
the various elements are known. Approximately
50 of these are found in nature; the
rest are produced artificially as the direct products
of nuclear reactions or indirectly as the radioactive descendants
of these products.” – Encyclopaedia Britannica
2004 Deluxe Edition
Since
radiometric dating methods are applied to natural geologic
phenomenon, such as rock formations, this means that radiometric
dating methods can only use the 50 naturally-occuring radioactive
isotopes. The rest of the 1,000 known radioactive isotopes
are of no use in radiometric dating.
Third,
the length of half-lives also limits the application of radioactive
dating. It is not sufficient for a rock or other item to contain
just any one of isotope among the 50 naturally-occuring isotopes.
Instead, in order to be dated, an item has to have one of
the 50 radioactive isotopes whose half-life will fit within
the timeframe of the rock’s age.
“Dating,
Absolute dating, Principles of isotopic dating –
Of course, one must select geologic materials
that contain elements with long half-lives—i.e., those
for which some parent atoms would remain.” –
Encyclopaedia Britannica 2004 Deluxe Edition
“Dating,
Absolute dating, Evaluation and presentation schemes in dating,
Origin of radioactive elements used – When the elements
in the Earth were first created, many radioactive isotopes
were present. Of these, only the radioisotopes with extremely
long half-lives remain…Natural elements that are still radioactive today produce daughter products
at a very slow rate; hence, it is easy to date very old minerals
but difficult to obtain the age of those formed in the recent
geologic past. This follows from the fact that the
amount of daughter isotopes present is so small that it is
difficult to measure…Geologic
events of the not-too-distant past are more easily dated by
using recently formed radioisotopes with short half-lives
that produce more daughter products per unit time…This
aspect of geology is becoming increasingly important as researchers
try to read the global
changes that took place during the Earth's recent past
in an effort to understand or predict the future. The
most widely used radioactive cosmogenic isotope is carbon
of mass 14 (14C), which
provides a method of dating events that have occurred over
roughly the past 50,000 years.” – Encyclopaedia
Britannica 2004 Deluxe Edition
The
important factor to remember here is that the ongoing occurrence
of decay is the theoretical means of measuring time. If all
the radioactive material in an item has changed to non-radioactive
material, then decay stops occurring because the process is
complete. We then have no way to measure how long ago the
process of decay was complete and no way to know old the item
is or even whether the daughter atoms present were the result
of decay in the first place or were instead already present
in the item.
Consequently,
if the item to be dated is old but the radioactive isotope
it contains has a half-life that is too short, then the decay
process will have been completed a long time ago and we will
have no way to measure how much time has passed. Likewise,
if the item to be dated is young but the radioactive isotope
it contains has a half-life that is even shorter, then once
again the process of decay will have been completed a long
time ago and we will have no way to measure how much time
has passed. Conversely, if the item to be dated is young but
the radioactive isotope it contains has a half-life that is
too long, then not enough time will have passed in order for
measurable decay to have occurred and we will have no way
to know how much time has passed. So, in order to be dated,
an item has to have one of the 50 naturally-occuring radioactive
isotopes with the right length half-life.
Because
of these limitations, “just any rock” cannot be
radiometrically dated and, in fact, “most rocks”
cannot be radiometrically dated.
“Geology,
Absolute dating – It is important to remember that
precise ages cannot be obtained for just
any rock unit but that any unit can be dated relative to a
datable unit.” – Encyclopaedia Britannica
2004 Deluxe Edition
“Dating,
General considerations, Determination of sequence –
Relative geologic ages can be deduced in rock sequences consisting
of sedimentary, metamorphic, or igneous rock units. In fact,
they constitute an essential part in any precise isotopic,
or absolute, dating program. Such is the case because most rocks simply
cannot be isotopically dated. Therefore, a geologist must first determine relative ages and then locate the most
favourable units for absolute dating.” – Encyclopaedia
Britannica 2004 Deluxe Edition
Now
that we understand the factors that limit the substances or
items to which radiometric dating can be applied, we will
move ahead to identify the criteria necessary for making radiometric
dating calculations. Later we will take a more detailed look
at this issue, but for now it is important to briefly list
these criteria as part of the basics of radiometric dating.
In
order to perform radiometric dating and calculate an absolute
age for the item, the following 4 pieces of information must
be known. First, we must know that the decay rate is constant,
not fluctuating, and we must know what that constant decay
rate is. Second, we must know the ratio of parent and daughter
elements that were already present in a rock when it originally
formed. And we must be able to identify, distinguish between,
and “correct” for those daughter-type atoms, which
were originally present, and those which result from decay.
If we don’t know how much the rock began with, we cannot
determine how much decay has occurred, or consequently, how
old the rock is. Third, we must know what the current amounts
of both parent and daughter atoms are in the item to be dated.
And fourth, we must know how many parent or daughter isotopes
migrated into or out of the rock, because such migration also
alters our perception of the original parent-to-daughter ratios
and the current parent-to-daughter ratios.
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E1 Basic Theory – Radiometric
dating techniques are based on radio-decay series with constant
rates of isotope decay. Once a quantity of a radioactive
element becomes part of a growing mineral crystal, that quantity
will begin to decay at a steady rate, with a definite percentage
of daughter products in each time interval. These "clocks
in rocks" are the geologists' timekeepers.” –
"Dating Methods," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
“Dating,
Absolute dating, Principles of isotopic dating –
Likewise, the conditions that must be met to make
the calculated age precise and meaningful are in themselves
simple: 1. The rock or mineral must have remained
closed to the addition
or escape of parent and daughter atoms since the time that
the rock or mineral (system) formed. 2.
It must be possible
to correct for other atoms identical to daughter atoms already
present when the rock or mineral formed. 3.
The decay constant must be known. 4. The measurement of the
daughter-to-parent ratio must be accurate because uncertainty
in this ratio contributes directly to uncertainty in the age.
Different schemes have been developed to deal with the critical assumptions
stated above. In uranium–lead dating, minerals virtually
free of initial lead can be isolated
and corrections made for the trivial amounts present.
In whole rock isochron methods that make use of the rubidium–strontium
or samarium–neodymium decay schemes (see below),
a series of rocks or minerals are chosen that can be assumed to have the same age and identical abundances of their
initial isotopic ratios. The results are then tested for
the internal consistency that
can validate the assumptions…Such checks include dating
a series of ancient units with closely spaced but known relative
ages and replicate analysis of different parts of the
same rock body with samples collected at widely spaced localities.”
– Encyclopaedia Britannica 2004 Deluxe Edition
Effectively,
the inability to identify the amounts of migration into or
out of a sample will create the following obstacles to dating.
Parents might be missing through migration instead of decay,
giving the mistaken impression that more decay has occurred,
that more half-lives have passed, and that the item is older.
Or, parents might be added by migration, giving the mistaken
impression that less decay has occurred, that less time has
passed, and that the item is younger. Or, daughters might
be missing through migration, giving the impression that less
decay has occurred, that less time has passed, and that the
item is younger. Or, daughters might be added through migration,
giving the impression that more decay has occurred, that more
half-lives have passed, and that the item is older.
These
4 pieces of information, the starting parent-to-daughter ratio,
the current parent-to-daughter ratio, the decay rate, and
the amount of migration are all regarded by Britannica as
“critical” and as “conditions that must
be met to make the calculated age.” If any of these
pieces of information is not known, then determining how much
decay has occurred and how much time has passed is utterly
impossible. These issues pose a significant obstacle to radiometric
dating. We have only briefly touched on them here as part
of the basic introduction to radiometric processes, however,
given the primary importance of these issues, our next segment
will return to these criteria and discuss the problems they
pose in greater detail.
Now
that we understand the limited application of radiometric
dating and the factors that must be known in order to perform
dating calculations, we can also examine the basic equipment
used in radiometric dating processes. And in this regard,
the mass spectrometer is really the essential tool of radiometric
dating. Mass spectrometers are the devices used to identify
the amount of different isotopes in a sample.
“Mass
spectrometry – Many
investigations have been conducted with the help of mass spectrometry.
These include the identification of the isotopes of the chemical
elements and determination of their precise masses and relative
abundances, the dating of geologic samples, the analysis
of inorganic and organic chemicals especially for small amounts
of impurities, structural formula determination of complex
organic substances, the strengths of chemical bonds and energies
necessary to produce particular ions, the identification of
products of ion decomposition, and the analysis of unknown
materials, such as lunar samples, for their chemical and isotopic
constituents.” – Encyclopaedia Britannica 2004
Deluxe Edition
Mass
spectrometers contain the same essential 4 or 5 parts, although
alternate descriptions list these parts slightly differently.
These parts are 1) a part to produce a vacuum, 2) a part to
introduce the sample, 3) a part to ionize the sample, 4) a
part which separates and sorts the different types of ions
according to their unique charges and masses, and 5) a part
which detects and identifies the different ions according
to their charges and masses.
“Mass
spectrometry – Mass
spectroscopes consist of five basic parts: [1] a high vacuum system; [2] a
sample handling system, through which the sample to be
investigated can be introduced; [3] an
ion source, in which a beam of charged particles characteristic
of the sample can be produced; [4] an
analyzer, in which the beam can be separated into its
components; and [5] a detector or receiver by means of which the separated ion beams can be observed
or collected.” – Encyclopaedia Britannica
2004 Deluxe Edition
In
the alternate description of a mass spectrometer below, the
term “analyte” refers to the substance that is
being analyzed by the mass spectrometer.
“Analysis
– The method usually relies on chemical reactions between
the material being analyzed (the analyte)
and a reagent that is added to the analyte.” –
Encyclopaedia Britannica 2004 Deluxe Edition
In
contrast to the 5 parts listed in the quote above, the 2 quotes
below lists only 4 parts. In effect, all the components are
the same. The “vacuum system” (which converts
the sample to a gas) and the “sample handling system”
(which introduces that sample to the ion source) have simply
been combined into a single component.
“Analysis,
Instrumental methods, Separatory methods, Mass spectrometry
– Most mass spectrometers have four major
components: [1]
an inlet system, [2] an ion source, [3] a mass analyzer, and [4] a detector. The inlet system is used
to introduce the analyte
and to convert it to a gas at reduced pressure. The gaseous
analyte flows from the inlet system into the ionic source
of the instrument where the analyte is converted to ions or ionic fragments.
That is often accomplished by bombarding the analyte with
electrons or by allowing the analyte to undergo collisions
with other ions. The ions that are formed in the ionic source
are accelerated into the mass analyzer by a system of electrostatic
slits. In the analyzer the ions are subjected to an electric or magnetic field
that is used to alter their paths. In the most common
mass analyzers the ions are separated
in space according to their mass-to-charge ratios. In
time-of-flight mass analyzers, however, no electric or magnetic
field is employed, and the time required for ions of varying
m/z that are accelerated to the same kinetic energy to pass
through a flight tube is measured. The detector is placed at the end of the mass analyzer and measures
the intensity of the ionic beam.” – Encyclopaedia
Britannica 2004 Deluxe Edition
“Mass
Spectrometer, I INTRODUCTION
– All mass spectrometers have four
features in common: (1) a system for introducing the substance
to be analyzed into the instrument; (2)
a system for ionizing the substance; (3) an accelerator that
directs the ions into the measuring apparatus; and (4)
a system for separating the constituent ions and recording
the mass spectrum of the substance.” – "Mass
Spectrometer," Microsoft® Encarta® Encyclopedia 99. ©
1993-1998 Microsoft Corporation. All rights reserved.
The
image below illustrates all of these essential parts of a
mass spectrometer.

“[PHOTO
CAPTION] Mass Spectrometer – In a mass spectrometer,
a sample of gas is ionized by an electron
beam, and the ions are accelerated toward a magnet, which
separates the ions according to their mass (upper right).
Ions of a certain mass strike the detector; the detector is usually
connected to a computer or other electronic device to process
the data (bottom). © Microsoft Corporation. All Rights Reserved.”
– "Mass Spectrometer," Microsoft® Encarta®
Encyclopedia 99. © 1993-1998 Microsoft Corporation. All rights
reserved.
Now
that we have identified what each of the main components of
a mass spectrometer are, we can discuss what each component
does. The vacuum system is necessary to reduce pressure in
order transform the sample into a gas so that it can flow
throughout the rest of the mass spectrometer’s apparatus.
The ion source is necessary to convert the atoms and molecules
of the gaseous sample into ions. Depending upon the specific
mass spectrometer, converting the sample into ions is accomplished
in one of 2 ways, either bombarding the gaseus sample with
electrons or causing it to collide with existing ions from
another source. Converting the sample to ions is necessary
in order for the different atoms and molecules in the sample
to be separated and sorted by the mass spectrometer into their
individual kinds.
The
analyzer is necessary to alter the path of the ions in the
gaseous sample. This is accomplished either by passing the
gaseous sample through an electric field or a magnetic field.
Because each different kind of atom and molecule has a different
mass and a particular ionic electric charge depending upon
its components, they each respond differently to the electric
or magnetic field. This is known as the “mass-to-charge”
ratio. Consequently, passing the ionized, gaseous sample through
an electric or magnetic field separates and sorts the different
atoms and molecules by altering their trajectory (or path)
through the apparatus according to their particular mass and
nuclear structure.
“Analysis,
Instrumental methods, Separatory methods, Mass spectrometry
– In the analyzer the ions are subjected to
an electric or magnetic field that is used to alter their
paths. In the most common mass analyzers the ions are separated
in space according to their mass-to-charge ratios.”
– Encyclopaedia Britannica 2004 Deluxe Edition
“Chemical
compound, Organic compounds, General considerations, Spectroscopy
of organic compounds, Carbon-13 magnetic resonance spectroscopy
– The mass analyzer contains a
strong magnetic field through which the molecular ions must
pass. As the ions pass through the magnetic field, they
are deflected into a curved path that is dependent on both
their charge and mass. Ions of different mass travel along
a different trajectory before reaching a detector, which records
the intensities and masses of the ions that strike it. The
mass spectrum that is recorded shows the mass-to-charge ratio
(m/z) along the horizontal axis and ion abundance along the
vertical axis.” – Encyclopaedia Britannica 2004
Deluxe Edition
“Dating,
Absolute dating, Instruments and procedures, Use of mass spectrometers
– For isotopic dating with a mass spectrometer, a beam of charged atoms, or ions, of a single element from the sample
is produced. This beam is passed through a strong magnetic
field in a vacuum, where it is separated into a number of
beams, each containing atoms of only the same mass.”
– Encyclopaedia Britannica 2004 Deluxe Edition
“Mass
Spectrometer, I INTRODUCTION – Although many different kinds of mass spectrometers are in use today, they are all
related to a device developed by the British physicist Francis
William Aston in 1919. In Aston's instrument, a thin
beam of positively charged ions was first deflected by an
electric field and then deflected in the opposite direction
by a magnetic field. The amount of deflection of the particles
as registered on a photographic plate depended on their mass
and velocity: the greater the mass or velocity of the ion,
the less it was deflected. Aston measured the molecular weights of
the isotopes of many elements
as well as the relative abundance of these isotopes in
nature.” – "Mass Spectrometer," Microsoft®
Encarta® Encyclopedia 99. © 1993-1998 Microsoft Corporation.
All rights reserved.
“Mass
Spectrometer, II THE MAGNETIC-DEFLECTION MASS SPECTROMETER
– In a magnetic-deflection mass spectrometer, ions with
a positive unit charge are created from the sample and accelerated
by an electrostatic field. A magnetic field deflects the ions according to their mass. Ions with
a certain mass strike the detector. Less massive ions are
deflected too much and miss the detector, and more massive
ions are not deflected enough and miss the detector. The strength
of the magnetic field slowly varies so the detector can measure
the relative proportions of all the constituents of the sample.
The detector is often connected to a computer that processes the data.” – "Mass Spectrometer,"
Microsoft® Encarta® Encyclopedia 99. © 1993-1998 Microsoft
Corporation. All rights reserved.
“Mass
spectrometry – Mass spectrometers use electric fields
and magnetic fields (areas of electric charge and magnetic
force) to separate ions A magnetic sector mass spectrometer, the
most basic kind, uses an electric field to accelerate the
ions and a magnetic field to deflect them onto a detector.
The amount that an ion is deflected depends on how heavy it is-that
is, on the ratio of its mass to its charge. By varying the intensity of the magnetic field, the device causes different
ions to hit the detector at different times. This results
in a graph called a mass spectrum that shows the relative
numbers of ions with different ratios of mass to charge.
Most mass spectrometers
send this information to a computer that can store, manipulate,
and interpret the data.” – Worldbook, Contributor:
Julie A. Leary, Ph.D., Professor of Chemistry, University of California,
Berkeley.
Once
the individual atoms and molecules have been sorted according
to their mass and nuclear structure, the detector can identify
and count the number of each different type of atom and molecule
by its trajectory. Since mass and nuclear structure are the
basis of separating and identifying each type of atom or molecule,
effectively, the atoms and molecules are being identified
by their atomic weight or mass-to-charge ratios.
When
a sample has been run through a mass spectrometer, the end
result is a graph called a “mass spectrum.” The
graph represents where different ions struck the detector.
This data is usually complicated enough that it requires a
computer to process and interpret it.
“Analysis,
Instrumental methods, Separatory methods, Mass spectrometry
– The detector is placed at the end of
the mass analyzer and
measures the intensity of the ionic beam. A mass spectrum
is a plot of the ionic beam intensity as a function of the
mass-to-charge ratio of the ionic fragment.” –
Encyclopaedia Britannica 2004 Deluxe Edition
“Mass
Spectrometer, II THE MAGNETIC-DEFLECTION MASS SPECTROMETER
– The strength of the magnetic field slowly varies
so the detector can measure the relative proportions of all
the constituents of the sample. The detector is often
connected to a computer that processes the data.”
– "Mass Spectrometer," Microsoft® Encarta®
Encyclopedia 99. © 1993-1998 Microsoft Corporation. All rights
reserved.
“Mass
spectrometry – By
varying the intensity of the magnetic field, the device causes
different ions to hit the detector at different times. This
results in a graph called a mass spectrum that shows the relative
numbers of ions with different ratios of mass to charge.
Most mass spectrometers
send this information to a computer that can store, manipulate,
and interpret the data.” – Worldbook, Contributor:
Julie A. Leary, Ph.D., Professor of Chemistry, University
of California, Berkeley.
Below
are two examples of mass spectrums, followed by captions explaining
each one. The first was produced when isotopes of the element
“osmium” were passed through a mass spectrometer.
Notice that although the two mass spectrums below are formatted
slightly differently, in both cases the “peaks”
on the graph represent the amount of each individual kind
of ion.

“[PHOTO
CAPTION] Figure 7: The mass spectrum
of osmium. This recorder trace was obtained with an electron
multiplier detecting OsO3 −. The
leftmost and rightmost peaks were recorded with the detector
gain set at a value 100 times that used for the rest of the
spectrum; this change is marked by the change in the baseline position.
The small satellite
peaks to the left are those of the low abundance oxygen isotopes
17O and 18O; the osmium isotopes are, from left to right,
184Os, 186Os, 188Os, 189Os, 190Os, and the satellite peaks
of 192Os.” – Encyclopaedia Britannica 2004
Deluxe Edition
The
second example is a mass spectrum produced when a chemical
called “butanone” was passed through a mass spectrometer.

“Chemical
compound – [PHOTO CAPTION] Figure
37: The mass spectrum
of 2-butanone.” – Encyclopaedia Britannica 2004
Deluxe Edition
“Organic
compounds, General considerations, Spectroscopy of organic
compounds, Carbon-13 magnetic resonance spectroscopy –
The mass spectrum that is recorded shows the mass-to-charge ratio (m/z)
along the horizontal axis and ion abundance along the vertical
axis. For ions bearing a single positive charge, z equals
1, and the horizontal axis shows the masses of the fragments
directly. The mass spectrum of the ketone 2-butanone is shown
as an example in Figure 37. The strongest peak in the spectrum is known as the base peak, and its
intensity is arbitrarily set at a value of 100. The peak at
m/z = 72 is the molecular ion and as such gives the molecular
mass of the molecule. In high-resolution mass spectrometry,
the mass of the molecular ion can be measured to an accuracy
of four ppm. In such an instrument, the molecular ion of 2-butanone
would appear at m/z = 72.0575, which would unambiguously establish
its molecular formula as C4H8O. High-resolution mass spectrometry
is an excellent method for determining the molecular formulas
of organic compounds.” – Encyclopaedia Britannica
2004 Deluxe Edition
In
conclusion, there are several points worth restating concerning
mass spectrometers. First, as we saw earlier, there are several
pieces of information that must be known in order for radiometric
dating to be performed and an absolute age calculated. Those
factors included the decay rate, the original parent-to-daughter
ratio when the item was formed, the current parent-to-daughter
ratio, and the amount of migration of parent or daughter isotopes
into or out of the item since it was formed. Of these 4 factors
that must be known in order to even perform radiometric dating
calculations, mass spectrometers only test for and only provide
one factor, the current ratio or amount of parent and daughter
atoms.
Mass
spectrometers cannot tell us how many parent or daughter atoms
were in the item originally or how much migration has occurred
since the item formed. And decay rates, particularly slower
decay rates, cannot be observed either because their half-lives
become too long to verify. Consequently, these other required
pieces of information that must be known to perform radiometric
dating calculations remain unknown and must be assumed. There
is no such device that will tell us the original parent-to-daughter
ratio or the amount of migration or that can directly detect
the decay rate. We will discuss more about decay rates in
a later segment. However, the fact that these other pieces
of information must be assumed is stated plainly by Britannica
Encyclopedia when listing these required factors. In fact,
as Britannica explains, evolutionary scientists have had to
“develop” “different schemes” to “deal
with these critical assumptions.” The quote then lists
the “isochron method” as one such scheme for “correcting”
for these assumptions and notes that even the isochron method
relies on “assuming” that rocks or minerals “have
the same age and identical initial isotopic ratios.”
The isochron method will also be discussed in more detail
below, including how it “corrects” for unknown
factors that must be assumed to calculate an age.
“Dating,
Absolute dating, Principles of isotopic dating –
Likewise, the conditions that must be met to make the calculated
age precise and meaningful are in themselves simple: 1.
The rock or mineral must have remained closed
to the addition or escape of parent and daughter atoms
since the time that the rock or mineral (system) formed. 2. It must be possible to
correct for other atoms identical to daughter atoms already
present when the rock or mineral formed. 3.
The decay constant must be known. 4. The measurement of the daughter-to-parent ratio must be accurate
because uncertainty in this ratio contributes directly to
uncertainty in the age. Different
schemes have been developed to deal with the critical assumptions
stated above. In uranium–lead dating, minerals virtually
free of initial lead can be isolated
and corrections made for the trivial amounts present.
In whole rock isochron methods that make use of the rubidium–strontium or samarium–neodymium
decay schemes (see below), a series of rocks or minerals
are chosen that can
be assumed to have the same age and identical abundances of
their initial isotopic ratios.” – Encyclopaedia
Britannica 2004 Deluxe Edition
Finally,
having covered the basic concepts, processes, limitations,
criteria, and tools of radiometric dating, we are ready to
move on and discuss the types of radiometric dating methods.
When we refer to types, we are referring to the actual isotopes
used in dating. In particular, as stated above, radiometric
dating is concerned with the parent isotope and the daughter
isotope that is produced from the parent through radioactive
decay. Thus, radiometric dating utilizes different pairs of
parent and daughter isotopes. The following isotope pairs
constitute the most prominent types of radiometric dating.
“Earth
sciences, The 20th century: modern trends and developments,
Geologic sciences, Radiometric dating – For dating
minerals and rocks, investigators commonly use the following
couplets of parent and daughter isotopes: thorium-232–lead-208,
uranium-235–lead-207, samarium-147–neodymium-143,
rubidium-87–strontium-87, potassium-40–argon-40,
and argon-40–argon-39.” – Encyclopaedia
Britannica 2004 Deluxe Edition
Concerning
the isotope pairs listed above, the most prominent radiometric
dating methods are potassium-argon dating and carbon-14 dating.
As stated in the quotes below, Carbon-14 dates items (organic
items) 50,000 years or less while potassium-argon dates items
100,000 years or older all the way back to 4 billion years,
which is the age of the oldest datable rocks.
“Geologic
Time, III DATING METHODS – The
two radioactive decay sequences most useful to geologists
are the decay of carbon-14 into nitrogen-14 and the decay
of potassium-40 into argon-40. Carbon-14, or radiocarbon,
dating works for organic materials less than about 50,000 years old…Geologists
can use potassium-argon dating to determine ages of rocks
from about 100,000 years old to as old as the earth itself.”
– "Geologic Time," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
“Archeology,
VIII DETERMINING THE AGE OF FINDS, B Absolute Dating, B4 Potassium-Argon
Dating – Potassium-argon
dating provides approximate dates for sites in early prehistory.
Geologists use this method to date volcanic
rocks that may be as much as 4 billion to 5 billion years
old.” – "Archaeology," Microsoft®
Encarta® Encyclopedia 99. © 1993-1998 Microsoft Corporation.
All rights reserved.
“Dating,
Absolute dating, Major methods of isotopic dating, Potassium–argon
methods – The radioactive decay scheme involving
the breakdown of potassium of mass 40 (40K) to argon gas of
mass 40 (40Ar) formed the basis of the first widely used isotopic dating
method. Since radiogenic argon-40 was first detected in
1938 by the American geophysicist Lyman T. Aldrich and A.O.Nier,
the method has evolved into one of the most versatile and widely employed
methods available. Potassium
is one of the 10 most abundant elements that together make
up 99 percent of the Earth's crust and is therefore a
major constituent of many rock-forming minerals.” –
Encyclopaedia Britannica 2004 Deluxe Edition
“Prehistoric
People, Placing prehistoric people in time – By
measuring the amount of each isotope in a fossil, scientists
can determine how long the decay has been going on and therefore
how old the fossil is. The most commonly used dating methods of
this type are radiocarbon dating and potassium-argon dating.”
– Worldbook, Contributor: Alan E. Mann, Ph.D., Professor
of Anthropology, Princeton
University.
However,
since carbon-14 typically dates items that are 40 to 50 thousand
years old and younger while potassium-argon dates items that
are 100 thousand years old or older, there is a gap from about
40 to 50 thousand years up to 100 thousand years that cannot
be checked by either of the 2 most-prominent dating methods.
Since it can measure dates from about 40 thousand to 1 million
years, fission-track dating is the method that is ued to fill
in this gap.
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E7 Fission-Track Dating – The method works best
for micas, tektites, and meteorites. It has been used to help date the period from about 40,000 to 1 million years ago,
an interval not covered by carbon-14 or potassium-argon methods.”
– "Dating Methods," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
And
as we can see from these 2 prominent dating methods, younger
items require an isotope with a short half-life and older
items require an isotope with a long half-life. Consequently,
the different types of radiometric dating pairs listed above
can be classified into 2 categories according to whether or
not they date young items or old items. As indicated above
and in the last line of the first quote, the dividing line
between young and old is typically 50,000 years, the upper
limit for carbon-14 dating. Incidentally, we should also notice
from both quotes below that the term “cosmogenic”
refers to carbon-14 dating, because as we will see, the carbon-14
isotope is produced from cosmic rays.
“Dating,
Absolute dating, Evaluation and presentation schemes in dating,
Origin of radioactive elements used – When the elements
in the Earth were first created, many radioactive isotopes
were present. Of these, only the radioisotopes with extremely
long half-lives remain…Natural elements that are still radioactive
today produce daughter products at a very slow rate; hence,
it is easy to date very old minerals but difficult to obtain
the age of those formed in the recent geologic past. This
follows from the fact that the amount of daughter isotopes present
is so small that it is difficult to measure…Geologic events of the not-too-distant past are more easily dated by
using recently formed radioisotopes with short half-lives
that produce more daughter products per unit time. Two
sources of such isotopes exist. In one case, intermediate
isotopes in the uranium
or thorium decay chain can become isolated in certain
minerals due to differences in chemical properties and, once
fixed, can decay to new isotopes, providing a measure of the
time elapsed since they were isolated…Another
special type of dating employs recently formed radioisotopes
produced by cosmic-ray bombardment of target atoms at the
Earth's surface or in the atmosphere. The amounts produced,
although small, provide insight into many near-surface processes
in the geologic past. This aspect of geology is becoming increasingly
important as researchers try to read the global changes that
took place during the Earth's recent past in an effort to
understand or predict the future. The
most widely used radioactive cosmogenic isotope is carbon
of mass 14 (14C), which provides a method of dating events
that have occurred over roughly the past 50,000 years.”
– Encyclopaedia Britannica 2004 Deluxe Edition
“Dating,
Absolute dating, Major methods of isotopic dating, Uranium-series
disequilibrium dating – The isotopic dating methods
discussed so far are all based on long-lived
radioactive isotopes that have survived since the elements
were created or on short-lived isotopes that were recently
produced by cosmic-ray bombardment. The long-lived isotopes
are difficult to use on young rocks because the extremely
small amounts of daughter isotopes present are difficult to
measure. A third
source of radioactive isotopes is provided by the uranium-
and thorium-decay chains. As noted in Table 3, these
uranium–thorium series radioisotopes, like the cosmogenic
isotopes, have short half-lives and are thus suitable for
dating geologically young materials.” – Encyclopaedia
Britannica 2004 Deluxe Edition
Furthermore,
not only can radiometric dating methods be categorized according
to whether or not they date young items or old items, but
they can also be categorized according to what types of rocks
they date. This categorization actually corresponds to the
first. Igneous and metamorphic rocks contain isotopes with
long half-lives and are dated by the methods used on older
items. Sedimentary rocks, which have a much more limited capacity
for radiometric dating, contain isotopes with short half-lives
and so are dated by either the uranium-thorium chain (which
dates seafloor sediments) or, more commonly, carbon-14. In
short, dating methods can be categorized into those methods
that use isotopes of long half-lives to date igneous and metamorphic
rock and those methods that use isotopes of short half-lives
to date sedimentary rock.
The
resulting categories of isotope pairs are as follows. Dating
methods used for younger ages and applied to sedimentary rock
include the uranium-thorium decay chain and carbon-14. Dating
methods used for older ages and applied to igneous and metamorphic
rock include uranium-led, rubidium-strontium, samarium-neodymium,
rhenium-osmium, and potassium-argon. And fission-track dating
is used for dating items in the gap between these other methods.
Isotope Dating Chart Figure 1
provides a summary of these categories. In the chart, in the
column to the far right, green designates methods used to
date once-living items and sedimentary rocks and red designates
methods used to date igneous or metamorphic rock. In the second
column from the right, the color turquoise designates methods
used to date up to 50,000 years ago, blue designates methods
used to date between 40,000 and 1 million years ago, and purple
designates methods used to date between 100,000 and 4 to 5
billion years ago.
The
details of the Isotopic Dating Chart are stated in the following
quotes. The first quotes below address carbon-14 dating and
the parts of the uranium-thorium series, which are the methods
used on sedimentary rocks and once-living items. The quote
below states plainly that carbon-14 is used to date organic
(i.e. once-living) materials that are less than 50,000 years
old.
“Geologic
Time, III DATING METHODS –
Carbon-14, or radiocarbon, dating works for organic materials less than about 50,000 years old.” –
"Geologic Time," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
The
next 2 quotes state that the uranium-thorium decay-chain series
can be used to date deep-sea sedimentary rocks and corals
with ages up to 400,000 years.
“Dating
Methods, III ABSOLUTE DATING METHODS, E Radiometric Dating,
E5 Methods Involving Thorium-230 – In
the ionium-deficiency method, the age of fossil shell or coral
from 10,000 to 250,000 years old is based
on the growth of ionium toward equilibrium with uranium-238
and uranium-224, which entered the carbonate shortly after
its formation or burial.” – "Dating Methods,"
Microsoft® Encarta® Encyclopedia 99. © 1993-1998 Microsoft
Corporation. All rights reserved.
“Ionium-thorium
dating – method
of establishing the time of origin of marine sediments according
to the amount of ionium and thorium they contain…One
of these thorium isotopes, thorium-230 (also known as ionium),
has a half-life of about 80,000 years, which makes it suitable
for dating sediments as old as 400,000 years.” –
Encyclopaedia Britannica 2004 Deluxe Edition
The
next few quotes focus on fission-track dating, the method
used to span the gap created between the two major groupings
of dating methods. Fission-track dating is used to date micas
and tektites, which are igneous and metamorphic rocks and
the age range is between 40,000 and 1 million years.
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E7 Fission-Track Dating – The method works best
for micas, tektites, and meteorites. It has been used to help date the period from about 40,000 to 1 million years ago,
an interval not covered by carbon-14 or potassium-argon methods.”
– "Dating Methods," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
“Mica,
Origin and occurrence – Micas
may originate as the result of diverse processes under several
different conditions. Their occurrences, listed below, include
crystallization from consolidating magmas, deposition by fluids
derived from or directly associated with magmatic activities,
deposition by fluids circulating during both contact and regional
metamorphism, and formation as the result of alteration
processes—perhaps
even those caused by weathering—that involve minerals
such as feldspars…The
common rock-forming micas are distributed widely. The more important occurrences follow: Biotite occurs in many igneous rocks (e.g., granites and granodiorites),
is common in many pegmatite masses, and constitutes one of the chief components of many metamorphic rocks
(e.g., gneisses, schists, and hornfelses). It alters rather
easily during chemical weathering and thus is
rare in sediments and sedimentary rocks.” –
Encyclopaedia Britannica 2004 Deluxe Edition
“Tektite
– any of a class of small, natural
glassy objects that are found only in certain areas of
the Earth's surface. The term is derived fromthe Greek word
tektos, meaning “melted,” or “molten.” Tektites
have been the subject of intense scientific scrutiny throughout
much of the 20th century owing to their unknown and possibly
extraterrestrial origins, but they
are now recognized as having formed from the melting and rapid
cooling of terrestrial rocks that have been vaporized by the
high-energy impacts of large meteorites, comets, or asteroids
upon the surface of the Earth. The extremely high temperatures
and enormous pressures generated by such impacts melted the
rocks at the site, producing clouds of molten silicate droplets
that quickly cooled to a glassy form before falling back
to Earth.” – Encyclopaedia Britannica 2004 Deluxe
Edition
The
next series of quotes all focus on the methods used to date
igneous and metamorphic rocks. Potassium-argon, the most widely-used
method, is used to date igneous and metamorphic rocks between
100,000 and 4 to 5 billion years.
“Geologic
Time, III DATING METHODS – Geologists
can use potassium-argon dating to determine ages of rocks
from about 100,000 years old to as old as the earth itself.”
– "Geologic Time," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
“Archeology,
VIII DETERMINING THE AGE OF FINDS, B Absolute Dating, B4 Potassium-Argon
Dating – Potassium-argon
dating provides approximate dates for sites in early prehistory.
Geologists use this method to date volcanic
rocks that may be as much as 4 billion to 5 billion years
old.” – "Archaeology," Microsoft®
Encarta® Encyclopedia 99. © 1993-1998 Microsoft Corporation.
All rights reserved.
“Archeology,
VIII DETERMINING THE AGE OF FINDS, B Absolute Dating, B4 Potassium-Argon
Dating – Geologists
use this method to date volcanic rocks that may be as much
as 4 billion to 5 billion years old…Thus,
geologic layers rich in volcanic deposits lend themselves
to potassium-argon dating. Prehistoric archaeological
sites such as the Koobi Fora area of East Turkana, Kenya, and Olduvai Gorge in Tanzania, both of which formed during periods of intense volcanic activity, have
been dated using the potassium-argon method..” –
"Archaeology," Microsoft® Encarta® Encyclopedia
99. © 1993-1998 Microsoft Corporation. All rights reserved.
As
indicated by the subheading for the article excerpt below,
argon-argon is an alternate version of potassium-argon dating.
As such it is used on igneous and metamorphic rocks with a
timeframe between 100,000 and 4 to 5 billion years.
“Dating,
Absolute dating, Major methods of isotopic dating, Potassium–argon
methods – In conventional potassium–argon dating,
a potassium-bearing sample is split into two fractions:
one is analyzed for its potassium content, while the other
is fused in a vacuum to release the argon gas…A
method designed to avoid such complexities was introduced
by the geochronologists Craig M. Merrihue and Grenville Turner
in 1966. In this technique,
known as the argon-40–argon-39 method, both parent
and daughter can be determined in the mass spectrometer as
some of the potassium atoms in the sample are first converted
to argon-39 in a nuclear reactor.” – Encyclopaedia
Britannica 2004 Deluxe Edition
Rubidium-strontium
are used to check dates generated by the potassium-argon method.
Consequently, rubidium-strontium is also a method used to
date igneous and metamorphic rocks and covering the same timeframe,
from 100,000 to 4 or 5 billion years.
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E4 Rubidium-Strontium Method – Used
to date ancient igneous and metamorphic terrestrial rocks
as well as lunar samples, this method is based
on disintegration by beta decay of rubidium-87 to strontium-87.
The method is frequently used to check potassium-argon
dates, because the strontium daughter element is not diffused
by mild heating, as is argon.” – "Dating
Methods," Microsoft® Encarta® Encyclopedia 99. © 1993-1998
Microsoft Corporation. All rights reserved.
Similarly,
samarium-neodymium is theoretically identical to rubidium-strontium.
In particular, while rubidium-strontium is used for igneous
and metamorphic rocks in the crust, samarium-neodymium is
used for igneous and metamorphic rocks from the mantle. As
these correspondences indicate, samarium-neodymium covers
the same timeframe.
“Dating,
Absolute dating, Major methods of isotopic dating, Samarium–neodymium
method – In
theory, the samarium–neodymium method is identical to
the rubidium–strontium approach (see above). Both use
the isochron method to display and evaluate data.”
– Encyclopaedia Britannica 2004 Deluxe Edition
“Dating,
Absolute dating, Major methods of isotopic dating, Samarium–neodymium
method – Unlike
rubidium, which is enriched over strontium in the crust, samarium
is relatively enriched with respect to neodymium in the mantle.”
– Encyclopaedia Britannica 2004 Deluxe Edition
Both
of the quotes below discuss that uranium-lead dating is complicated
by reheating events, which applies only to igneous and metamorphic
rocks. In addition, the first quote cites micas as an example.
As we saw earlier, micas are igneous and metamorphic rock
types. The second quote cites titanite as an example, which
is also an igneous rock type. Consequently, uranium-lead dating
is a method used for igneous and metamorphic rocks. And, as
indicated by the last 2 quotes below, the timeframe for uranium-lead
dating is the Precambrian age, between 540 million years ago
and the formation of the earth (i.e. 4 to 5 billion years
ago.)
“Dating,
Absolute dating, Evaluation and presentation schemes for dating,
Multiple ages for a single rock; the thermal effect –
Fossils record the initial, or primary, age of a rock unit.
Isotopic systems,
on the other hand, can
yield either the primary age or the time of a later event,
because crystalline materials are very specific in the
types of atoms they incorporate, in terms of both the atomic
size and charge…All it takes for such an element to
be purged from the mineral is sufficient heat to allow solid
diffusion to occur…In this case, the host mineral could have an absolute age very much older than is
recorded in the isotopic record…Taken in perspective,
it is evident that many parts of the Earth's
crust have experienced reheating temperatures above 300°
C—i.e., reset
mica ages are very common in rocks formed at deep crustal
levels. Vast areas within the Precambrian shield, which
have identical ages reflecting a common cooling history, have
been identified. These are called geologic provinces. ”
– Encyclopaedia Britannica 2004 Deluxe Edition
“Dating,
Absolute dating, Major methods of isotopic dating, Importance
of zircon in uranium-lead dating – Given the two
related uranium–lead parent–daughter systems, it is possible
to determine both the
time of the initial, or primary, rock-forming event and the
time of a major reheating, or secondary, event. This is
illustrated in Figure 3. Here, the uranium–lead isotopes
in the mineral titanite
(CaTiSiO5) from a series of rocks that have a common geologic
history plot on a straight line. The minerals first formed 1,651 million years ago but were later heated
and lost varying amounts of lead 986 million years ago.”
– Encyclopaedia Britannica 2004 Deluxe Edition
“Sphene
– Sphene, calcium titanosilicate, formerly
called titanite, chemical formula CaOTiO2SiO2…Sphene occurs as a microscopic accessory mineral in different types
of igneous rocks-especially granite-and in larger crystals
in pegmatites.” – "Sphene," Microsoft®
Encarta® Encyclopedia 99. © 1993-1998 Microsoft Corporation.
All rights reserved.
“Dating
Methods, III ABSOLUTE DATING METHODS. E Radiometric Dating,
E6 Methods Involving Lead – In the uranium-lead method,
age in years is calculated for geologic material based
on the known radioactive decay rate of uranium-238 to lead-206
and of uranium-235 to lead-207…The method is most applicable to materials Precambrian
in age.” – "Dating Methods," Microsoft®
Encarta® Encyclopedia 99. © 1993-1998 Microsoft Corporation.
All rights reserved.
“Precambrian
time – period
of time that extends from a little more than 3.9 billion years
ago, which is the approximate age of the oldest known
rocks, to the beginning of the Cambrian Period, roughly 540 million years ago.
The Precambrian era thus represents more than 80 percent of
the whole of geologic time.” – Encyclopaedia Britannica
2004 Deluxe Edition
Lastly,
there is one outlying method, which does not fit into either
of the 2 categories for radiometric dating methods. Radon-lead
dating, which is a member of the uranium-decay scheme, is
used to date glacial ice.
“Dating,
Absolute dating, Major methods of isotopic dating, Uranium-series
disequilibrium dating, Lead-210 dating – The presence
of radon gas as a member of the uranium-decay
scheme provides a unique method for creating disequilibrium.
The gas radon-222 (222Rn)
escapes from the ground and decays rapidly in the atmosphere
to lead-210 (210Pb), which falls quickly to the surface where
it is incorporated in glacial ice and sedimentary materials.
By assuming that the present deposition rate also prevailed
in the past, the age of a given sample at depth can be estimated
by the residual amount of lead-210.” – Encyclopaedia
Britannica 2004 Deluxe Edition
At
this point, we have concluded not only our exploration of
the various types of radiometric dating methods, but we have
also completed our introduction to the basics of radiometric
dating. Consequently, with the fundamental concepts established,
we are ready to procede to our next segment, a discussion
of the problems facing radiometric dating methods in general
as a result of the factors required to calculate radiometric
ages.