biblestudying.net

Printer Friendly Version

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

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.

“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 – 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.