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Radiometric Clocks

When in 1907 Dr. B.B. Boltwood of Yale University suggested that the decay of radioactive elements could provide a method for dating rocks, it was thought that this was the absolute method that scientists had been hoping for. Soon determinations of lead-uranium ratios in minerals were being made by chemical methods and age estimates calculated. However, the modern development of radiometric dating methods was made possible following the application by Aston in 1927 of the mass spectrograph to study isotopic composition of lead. In 1936 Dr. A.O.C. Nier further improved this technique by developing the mass spectrometer. With this instrument it is possible to measure the ratios of the different isotopes of an element in a rock or mineral. Since these early developments, the radiometric dating methods have multiplied in number and become the principal basis for the great-age chronology which makes the earth's age some 4.5 billion years.

The Radioactive Decay Curve and Half-Life

The Theory of radioactive decay depends upon the assumption that in a sample of atoms of any radioactive parent isotope each atom has an equal probability of decaying within a given length of time to produce an atom of the daughter isotope. If this is the case, the probable number of atoms decaying per unit of time will proportional to the number of atoms remaining. If the initial number of atoms is very large (so that the random variations from the probable rate of decay become relatively very small) the number of remaining atoms decrease with what is termed an exponential decay curve which is shown by the graph in figure 8-3.

figure 8-3. Decay curve of a hypothetical sample of 1,000,000 radioactive atoms.

In this illustration the initial number of radioactive atoms is taken to be 1,000,000. The probability of a particular atom's decaying in one second is taken as 10-4, i.e., 1/10,000. This is called the specific decay rate. Thus at time zero, with the initial one million atoms present, the probable number of atoms decaying in the first second is 1,000,000/10,000 = 100.

When 6,933 seconds have passed, half of the parent atoms will have decayed and the sample will consist of 500,000 parent atoms and 500,000 daughter atoms. Therefore, the time span of 6,933 seconds is called the half-life for this particular kind of radioactive isotope. When one half-life has passed and just one-half of the parent atoms remained, the probable number of radioactive decompositions per second is 50, just half of the initial rate, since only half as many parent atoms remain. After the passage of the third half-life period of 6,933 seconds, there will be one-quarter of the initial number of parent atoms or 250,000 remaining, with 750,000 daughter atoms. At this time the expected rate of decay will be 25 atoms per second.

It is easy to see how, given certain assumptions, a radioactive parent-daughter couple can be used to date a rock or mineral specimen. If one knows the decay probability or half-life of the parent isotope, by analyzing the sample for the number of parent atoms and daughter atoms, the interval back to time zero can be calculated. It is assumed that the initial numbers of parent and daughter atoms are known, that the decay rate has not varied, and that neither parent nor daughter atoms has entered or left the specimen.

For example, if it is assumed that only parent and no daughter atoms were present at time zero, then an analysis of 250,000 parent and 750,000 daughter atoms would be interpreted as meaning that an interval of two half-lives or 13,866 seconds has passed since time zero. Or if the rate of decomposition (i.e., counts per second) is measured and found to be 25 counts per second, assuming an initial 1,000,000 parent atoms, the same age of two half-lives is calculated.

There are many radioactive isotopes, each with its own decay rate or half-life, which could theoretically be used as radiometric clocks. Some have fast decay rates, others slow rtes. Their different uses can be compared with those of a stop watch and an ordinary clock, one used to measure periods of seconds, the other of hours. As a concrete example let us consider the substance bismuth-210.

Bismuth-210 has a half-life of five days. During a period of five days a sample of bismuth will give off enough radiation to change one-half of the original bismuth atoms into thallium-206 atoms plus an equal number of atoms of helium-4. The block diagram depicts the course of radioactive decay of two differently sized samples of bismuth-210.

Sample "A" is all bismuth and has an original weight of 16 grams; Sample "B" is all bismuth and has an original weight of 40 grams. At the end of five days Sample "A" will consist of 8 grams of bismuth-210 and 7.85 grams of thallium-206, and 0.15 grams of helium-4 will have been released, for a total weight of 16 grams. At the end of five days Sample "B" will consist of 20 grams of bismuth-210 and 19.62 grams of thallium-206, and 0.38 grams of helium-4 will have been released, for a total weight of 40 grams. After another five days Sample "B" will consist of 10 grams of bismuth-210 and 29.43 grams of thallium-206, and 0.57 grams of helium-4 will have been released, for a total weight of 40 grams. So we see that in one half-life of five days one-half of the bismuth has decomposed and in ten days, or two half-lives, three quarters has decomposed.

It does not matter how much of a material one starts with, for during each half-life period one-half of the remaining radioactive "parent" substance will be converted to the "daughter" substance. The values of radioactive half-lives have not been observed to be greatly affected by any known chemical or physical conditions commonly observed in nature. Because most half-lives are known to a fair degree of certainty and have not been observed to fluctuate substantially, it is reasonable to assume that they are indeed constant. If this is the case over long times spans, then radioactive elements may serve well as clocking materials if they meet the other requirements of accurate timing. On the other hand, it was reported in Chapter-6 that the speed of light may have decreased since the creation, and this would mean that rates of radioactive have also changed.

figure 8-4. A block of pure bismuth-210 is transformed to thallium-206 by radioactive decomposition. Half of the remaining bismuth changes every five days, so the half-life of bismuth-210 is said to be five days.

Another factor that must be considered is the setting of the radiometric "clock" at zero time. The half-life data for a radioactive material gives the rate at which the clock runs, but how can we be sure of the amounts of parent and daughter isotopes present at zero time? If another piece of material is found to have a composition of 8 grams of bismuth-210 and 7.85 grams of thallium-206, one really cannot be sure the material is only five days old unless there is some way of knowing its exact composition five days ago. Was it all bismuth? Or was there already some thallium present? The two elements may have been combined just before the measurements were begun. And there are other factors that might lead one to incorrect conclusions about its age.

It is obvious that a number of possible factors may have masked the real age of this material, now apparently five days old, before it was given to you for analysis. The same kinds of problems would be true for obtaining the age of any material containing radioactive elements. We cannot know the original composition of a material containing radioactive elements, because they continually change by radioactive decay and also may move into or out of any rock of mineral. Their value for age determination is based on assumptions about the original composition of the sample and of physical and chemical conditions which may affect the composition of the sample throughout its existence.

To recapitulate, the fundamental assumptions inherent in the use of the radiometric dating methods are the following:

1. The rate of radioactive decomposition of the parent isotope has remained constant.

2. The initial composition of the rock of mineral crystal is known.

3. The rock or mineral crystal has been a closed system, so that no parent or daughter isotope has been either added or lost.

The first assumption appears to be reasonably secure, but the second and third are impossible to verify with certainty. As will be seen, they often can be shown to be false, with the result that the calculated age is erroneous.

Some Common Radiometric Methods

Now let us consider some of the radioactive isotopes commonly used in dating moon and earth rocks and meteorites. A few of these are listed in the following table:

table 8-1. Parent/Daughter Isotope Pairs

Scientists have determined the "ages" of meteorites, moon rocks, and earth rocks by measuring the ratio of one of the above parent/daughter isotope pairs and then calculating age by means of the half-life principle explained above using the example of Bi-210/Th-206.

The original radiometric estimates of the age of the earth made use of the uranium/lead system, in which the parent isotopes, U-238 and U-234, decompose through a number of intermediate steps to produce the terminal daughter isotopes in the series, Pb-206 and Pb-207. What is called the "lead-lead isochron" method was applied to certain meteorites to obtain an age determination of about 4.5 billion years. Information from meteorites was also used to determine a supposed original isotopic composition of the lead in the solar system and therefore in the earth. This information made it possible to correct uranium-lead ages of earth rocks for the original lead contained in any particular rock or mineral sample. The interpretation of the data is complex and not direct, often involving the assumption of one or more catastrophic events which reset the radiometric clocks. In our section below entitled "A General Creationist Explanation for Radiometric 'Ages'?" we discuss in some detail these interpretive techniques and show that they do not necessarily lead to correct results. However, geochronologists accept the resulting earth age of about 4.54 billion years as probably close to the true value.²⁹ The age of the formation of the oldest earth rocks is thought to be about 3.6 billion years.

Many creationists hold that the earth and solar system are much younger, of the order of only some thousands rather than billions of years. In this view the ratios of daughter to parent isotopes basically reflect not radioactive decay over vast time spans, but conditions of the initial creation modified by subsequent events over a relatively very short time span measured in thousands of years.

It is certainly true that a large structure of radiometric dating measurements has been built up in the past several half century which exhibits many ages of earth rocks, moon rocks and meteorites which accord with the great-age evolutionary time frame. On the other hand, there are numerous examples of discordances and inconsistencies which raise serious questions about the validity of the methods. Three categories of problems are anomalous ages; discordant ages within one rock and between methods, and disequilibrium between members of radio decay series. Examples will be cited to illustrate each type of problem. In what follows "BY" is an abbreviation for "billion years."

Anomalous Radiometric Ages

1. Volcanic rocks produced by lava flows which occurred in Hawaii in the years 1800-1801 were dated by the potassium-argon method. Excess argon produced apparent ages ranging from 160 million to 2.96 BY.³⁰ In contrast with this, some moon rocks are considered to have lost up to 48 percent of their argon, and their K/Ar ages are judged to be too low. On the other hand, many lunar rocks contain such large quantities of what is considered to be excess argon that dating by K/Ar is not even reported.³¹

2. Some lunar rocks and soil from the Apollo 16 mission yielded "highly discordant" ages exceeding six billion years by lead methods. This is unacceptably high for current theories of lunar origins and disagrees with measurements made on other moon materials.³²

3. recent rocks from active volcanic sites studied in Russia, perhaps only thousands of years old, gave ages from 50 million to 14.6 BY, depending upon which methods, samples, and corrections were used.³³

4. A rock from Apollo 16 contains 85 percent excess lead which gives uncorrected ages ranging from seven to 18 BY by three lead methods. Removal of lead by acid treatment makes possible a date of 3.8 BY which is considered acceptable.³⁴

Discordant Ages Within One -Rock and Between Different Methods

1. Granite from the Black Hills in South Dakota yielded the following ages by the several methods: Sr/Rb, 1.16 BY; Pb²⁰⁶/U²³⁸, 1.68 BY; Pb²⁰⁷/U²³⁵, 2.1 BY; Pb²⁰⁷/Pb²⁰⁶, 2.55 BY; and Pb²⁰⁸/Th²³², 1.55 BY.³⁵

2. A series of volcanic rocks from Réunion Island in the Indian Ocean gave K/Ar ages ranging from 100,000 to 2 million years, whereas the Pb²⁰⁶/U²³⁸ ages range from 2.18 to 3.00 BY, and Pb²⁰⁶/Pb²⁰⁷ ages(not considered by geochronologists to represent the age of formation of the rock) range narrowly from 4.42 to 4.45 BY. Thus the factor of discordance between K/Ar and Pb/U and Pb/Pb "ages" ranges from 1,300 to 42,000.³⁶

The general explanation offered by secular geologists for these discrepancies is that these lavas brought with them the uranium and lead concentrations which had evolved during billions of years in former rocks which were then melted to form the magma source for the Réunion Island lava flows. When the lava solidified on the island the K/Ar clock took over to record the time since that most recent crystallization. On the other hand, Pb/Pb clock gives roughly the time since the earth was formed. But it in these rocks the Pb/U ratios were transported in with the original magma and tell nothing of the age of the present rocks, how can one have confidence that any "age" determined by these methods is a true age? Or from another perspective, when do we know for sure that the K/Ar clock is correct and when the Pb/U and Pb/Pb clocks are reading right?

3. Certain rocks from Apollo 12, dated by Sr/b and several lead methods, yielded ages ranging from 2.3 to 4.9 BY. The effort to explain the results involves hypothesized second and third events which reset the radiometric clocks at different times in the past.³⁷

4. Lunar soil collected by Apollo 11 gave discordant ages by different methods: Pb²⁰⁷/Pb²⁰⁶, 4.67 BY; Pb²⁰⁶/U²³⁸, 5.41 BY; Pb²⁰⁷/U²³⁵, 4.89 BY; and Pb²⁰⁸/Th232, 8.2 BY. Rocks from the same location yielded ages of around 2.3 BY.³⁸

Disequilibrium Between Members of Radioactive Decay Series

In the radioactive decay series in which U²³⁸ is transformed into Pb²⁰⁶ there are fourteen successive decay steps and, therefore, thirteen intermediate radioactive nuclides. Once decay of uranium in a rock or mineral grain begins, the concentrations of the intermediates begin to build up, but they also begin to decay as soon as they are formed. After a certain time the members of the series reach concentrations which are in the proportions, approximately, of their half-lives. The radioactive decay series is then said to be in equilibrium. In the U²³⁸ series the equilibrium ratio of U²³⁴/U²³⁸ is 5.5x10^-5.

1. The collection of volcanic rocks from Russia mentioned in item 3 of Anomalous Ages above presents only three example of equilibrium (See Footnote 33). The deviations from equilibrium range from 55 percent deficiency to 36 percent excess. in U²³⁴. Ages obtained for these rocks by the lead methods range from 50 million to 5 billion years, depending upon the correction techniques used. But equilibrium should be attained in only two million years. the volcanic rock is reportedly only thousands of years old according to the geologic evidence, but the lead dating methods say tens of millions to billions. So the disequilibrium mixture of U²³⁸ and U²³⁴ must have entered the melt before the rock solidified.

But presumably there were billions of years for the two isotopes of uranium to come into equilibrium before that. How can disequilibrium of up to 55 percent be explained? The proposal that two chemically identical isotopes, U²³⁸ and U²³⁴, could be in equilibrium in a melt but deposited in the crystalline rock in ratios up to 55 percent out of equilibrium cannot be supported by present knowledge of physics and chemistry. At present there is no explanation other than the view that the initial creation a few thousand years ago brought the uranium isotopes into being in various degrees of disequilibrium in different places in the earth.

2. Ten rock samples from Faial Azores, Tristan de Cunha, and Mt Vesuvius display similar disequilibrium in different places in the earth.³⁹

3. Certain moon rocks have been found with some radioactive isotopes out of equilibrium by up to 10 percent.⁴⁰

A Last Look at the Moon

The Apollo lunar landers brought back to earth several hundred pounds of rock and soil samples from the lunar surface. Hundreds of different samples have been analyzed by laboratories all over the world. A large number of age determinations have been made using various of the radiometric dating methods. As in the case with the earth, a large body of these age determinations can be arranged to show agreement. However, a significant body of the results display striking discordant ages, often within the same sample of rock, soil or mineral crystal. For example, the application of the uranium/lead, thorium/lead methods and the potassium/argon methods to twelve different samples yielded ages from 0.7 BY to 28.1 BY.⁴¹ Results from several particular samples are given in the table below.

table 8-2. Discordant age determinations by the application of radiometric dating methods to lunar rocks and soil returned to earth by Apollo landers.

Sample Uranium-Thorium-Lead Method Potassium/Argon Age Ranges

As one reads the research reports and the lunar conference reports concerning moon rocks, the impression is gained that the collection of a great mass of facts has resulted in the raising of more questions than have been answered. It would seem that one unanswered question is the age of the moon.

The examples given above indicate some of the problems that are found with radiometric dating methods. They suggest that it is not uncommon for radioactive parent and/or daughter nuclides to be added to or lost from rocks and mineral crystals. They show that as yet unexplained disequilibrium exists between nuclides in decay series. These examples also reveal the magnitude of the discordances which occur between different methods and between different minerals within a single rock specimen.

References

29. Tera, F., Reassessment of the "age of the earth." Annual Report of the Director, Dept. of Terrestrial Magnetism, Carnegie Institute(Washington, D.C.); 1979-1980: pp. 524-531.

30. Funkhouser, J.G. and J.J. Naughton, Journal of Geopohysical Research, Vol. 73, 15 July 1968, p. 4601.

31. Turner, Grenville, Science, Vol. 167, 30 Jan. 1970, p. 466.

32. Tera, F. and G.J. Wasserburg, Earth and Planetary Science Letters, Vol. 17, 1972, p. 36.

33. Cherdyntsev, V.V., et al., Geological Institute, Academy of Sciences, USSR, Earth Science Section, Vol. 172, p. 178. The data is reproduced by Sidney P. Clemmentson in Creation Research Society Quarterly, Vol. 7, Dec. 1970, p. 140.

34. Nunes, P.D. and M. Tatsumoto, Science, Vol. 182, 30 Nov. 1973, p. 916.

35. Zartman, R.E., et al., Science, Vol. 145, 31 July 1964, p. 479.

36. Oversby, V.M., Geochimica et Cosmochimica Acta, Vol. 36, Oct. 1972, p. 1167.

37. Tera, F. and G.J. Wasserburg, Earth and Planetary Science Letters, Vol. 14, 1972, p. 281.

38. Wang, R.K., et al., Science, Vol. 167, 30 Jan. 1970, p. 479.

39. Oversby, V.M. and P.W. Gast, Earth and Planetary Science Letters, Vol. 5, 1968, p. 199.

40. Tatsumoto, M. and John N. Rosholt, Science, Vol. 167, 30 Jan. 1970, p. 461.

41. Read, John G., What Is the Scientific Certainty of Evolution?, Scientific-Technical Presentations, P.O. Box 1284, Culver City, Calif.; Tera, F. and G.J. Wasserburg, Earth and Planetary Science Letters, Vol. 14, 1972, pp. 288-291.