Uranium-thorium-lead dating

Radioactive decay of uranium and thorium into lead can be used as a natural clock to determine the ages of rock samples. Rocks from different locations on the earth have been dated in a wide range of ages, from 100 million to several billion years. The earth, the earth's moon, and meteorites all have a common age of about 4.6 billion years.

Radioactive Half-Life

Radioactivity was discovered by French physicist Antoine-Henri Becquerel, a professor of physics in Paris, in 1896. He found that a rock containing uranium was emitting radiation that caused photographic film to become exposed. Shortly afterward, his graduate student, French scientist Marie Curie, discovered two radioactive elements, polonium and radium, which are decay products of uranium. Over the next ten years, British chemist Ernest Rutherford and other scientists were able to unravel a whole series of radioactive processes in which uranium (element 92) decayed into its stable end product, lead (element 82).

All radioactive materials have a particular half-life, which is the time required for half the atoms to decay. In the early days of radioactivity, scientists wanted to see if the half-life of an element could be changed by various processes. For example, they heated or cooled the radioactive material, made different chemical compounds, or converted it to a gas at high pressure. In all cases, the half-life did not change. This is because radioactivity comes directly from the nucleus of an atom, while heat and pressure affect only the outer electron cloud. Therefore, the radioactive half-life is like a built-in clock, keeping time at a fixed rate. No geological process, no matter how violent, can change the half-life.

Radioactivity can be applied to dating the age of rocks under certain conditions. The isotope uranium-238, for example, has a half-life of 4.47 billion years and eventually decays into lead-206. To make the situation as simple as possible, assume a rock sample contained some uranium but no lead at all when it first solidified. This phenomenon marks the starting time of the radioactive clock (time = 0). Suppose that for a very long time period, the rock remained a closed system; that is, no uranium or any of its decay products leaked out or were added from the surroundings. Because of radioactive decay, the uranium content will decrease and the lead will gradually increase. The ratio of lead-206 to uranium-238 in the rock, which will change with time, can be used to calculate the age.

For a numerical example, suppose the rock originally contained 10 grams of uranium-238 and zero lead. After 1 billion years, one can calculate from the half-life that the 10 grams of uranium-238 must have decreased to 8.56 grams because of radioactive decay. The lead-206 will have increased from zero up to 1.24 grams. (Note that 8.56 + 1.24 = 9.80 grams. The “missing” mass of 0.20 gram has gone into the creation of alpha, beta, and gamma rays.) The ratio of lead-206 to uranium-238 after 1 billion years would equal 1.24/8.56, or 0.145. In a similar way, the ratio can be calculated for any other elapsed time. The older the rock, the more lead it will contain, so the ratio of lead to uranium gradually increases.

Isotopes

In the early 1900's, large errors were made in age calculations because one vital item of information was missing. The idea of isotopes had not yet been discovered. The mass spectrometer, whose invention may be credited to the English physicist Sir Joseph John Thomson about 1914, was later refined by Canadian-American physicist Arthur Jeffrey Dempster and others. The mass spectrometer uses a magnetic field to separate atoms of slightly different mass. Many elements were shown to be a mixture of several isotopes. Lead, for example, has four stable isotopes, with masses of 204, 206, 207 and 208. (Atom masses are expressed relative to carbon = 12.) Gradually, it became clear that uranium-238 decaying into lead-206 is only one of several radioactive decay processes. The situation is more complex. There are two other decay chains that produce lead as an end product: Thorium-232 decays into lead-208 with a half-life of 14 billion years, and another uranium isotope, uranium-235, decays into lead-207 with a half-life of 700 million years.

Radioactive dating requires that the individual isotopes are measured separately. Typically, three experimental ratios are measured with the mass spectrometer: lead-206/uranium-238, lead-208/thorium-232, and lead-207/uranium-235. Each of these three ratios is combined with the half-life for that decay process to calculate an age. If all three calculations give the same result, the age is probably quite reliable and is called concordant. If the calculations disagree, the ages are discordant, and further investigation is necessary.

Correcting for Discordant Ages

What can cause discordant ages when several radioactive clocks are compared? In the uranium-thorium-lead (U-Th-Pb) method (described above), one probable source of error is the possibility that the rock, when it first solidified, already contained some natural lead. In other words, the lead content of the rock is the sum of radiogenic lead (from radioactive decay of uranium and thorium) plus the primordial lead. Only the radiogenic lead should be used to calculate an age. A method is needed to subtract out the primordial lead, which did not come from radioactive decay.

Fortunately, a good method to correct for primordial lead is available. Three of the lead isotopes come from the decay of uranium and thorium; there is a fourth lead isotope, lead-204, which is not formed by a radioactive decay process. If a rock contains any lead-204, it means that it already must have contained some lead at the time it was formed. Ordinary lead contains a mixture of isotopes whose normal relative abundance has been measured. The normal ratio of lead-206 to lead-204 is about 17 to 1. Suppose a rock contains 50 milligrams of lead-206 and 1 milligram of lead-204. The investigator would subtract 17 milligrams of lead-206 from the total, which leaves a net excess of 50 – 17, or 33 milligrams of lead-206 that must have come from the decay of uranium-238. A similar correction can be made for lead-207 in normal lead.

There is an uncertainty in the correction factor. One cannot be sure that the natural lead, when it was incorporated into the rock, had the same relative abundance of lead isotopes as lead does today. It is very likely that the normal ratio of lead-206 to lead-204 was smaller than 17 to 1 at an early time in the history of the earth because lead-206 has gradually been added as a result of decay. The most accurate age measurements are obtained if the natural lead correction is small—that is, if most of the lead in a rock sample under study came from radioactive decay.

Sometimes discordant ages can be corrected in a systematic way to calculate a consistent result. For example, some samples of the mineral zircon from the Montevideo area of southern Minnesota gave uranium-lead ages that vary from 2.6 to 3.3 billion years. Suppose a loss of lead occurred, perhaps because of temperature or weathering. It is reasonable to assume that all the lead isotopes decreased by the same percentage because they are chemically identical. One can extrapolate backward in time to show that the zircon samples must have been formed about 3.55 billion years ago and probably had a lead loss resulting from regional heating at a later time. Fortunately, zircon crystallization strongly rejects lead, so any lead content is probably radiogenic unless the crystalline structure has been damaged so that lead can escape.

Loss of Intermediate Elements

Another reason that discordant ages sometimes are measured may be that intermediate elements escape in the decay chain between the starting element, uranium or thorium, and the end product, lead. For example, it may happen that radium forms a chemical compound that is relatively soluble in water and is leached out from a rock sample. Also, radon gas may escape. (Scientists know this happens at least some of the time because of the radon buildup in the basements of houses in various parts of the country.)

Any loss of intermediate elements means that too little of the lead end product accumulates. Therefore, the apparent age of a rock sample would be calculated to be too short. An experienced investigator will try to select those minerals for analysis whose crystal structures are known to be relatively impervious to losses. Also, it is desirable to analyze many samples from an area. If most of the results are consistent, one may be able to reject those ages which are discordant for some reason.

Determining Earth's Age

Radioactive age determinations made before 1930 are considered to be unreliable because the mixture of isotopes with different half-lives was not well understood. With improvements in the mass spectrometer in the 1930's, it was shown that uranium consists of two main isotopes: about 99 percent uranium-238 and less than 1 percent uranium-235. Rutherford suggested how such data could be used to estimate at least a rough upper limit for the age of the earth. His reasoning was as follows: The isotope uranium-235 has such a very low abundance presently because it has a relatively short half-life; most of it has decayed away. The amount of uranium-235 that existed on Earth 700 million years ago (one half-life) would have been twice as much as currently; 1.4 billion years ago, the amount of uranium-235 would have been four times as much. By comparison, the amount of uranium-238 would have been only a little greater than at present because of its long half-life. If one calculates back far enough, the amounts of uranium-235 and uranium-238 would have been equal about 6 billion years ago; that is, uranium would have been a fifty-fifty mixture of these two isotopes. It is very unlikely that uranium-235 ever was more abundant than uranium-238 because odd isotopes in general are less abundant in nature than are even ones. Therefore, 6 billion years sets an upper limit for the age of the earth.

A much-improved procedure to determine the age of the earth was developed in the 1950's. As described by American geochemist Harrison Brown, the lead isotopes in the Canyon Diablo meteorite (which created the famous Meteor Crater in Arizona) were analyzed. The ratio of lead-206 to lead-204 was only about 9.4, much lower than any samples on Earth. The argument is made that this ratio represented primordial lead, uncontaminated by any radiogenic lead from uranium decay. Over the history of the earth, this ratio should gradually increase for terrestrial samples because additional lead-206 is produced from uranium, but lead-204 remains constant. Samples that are representative of modern lead on Earth contain about twice as much lead-206 as the meteorite. This amount of extra lead-206 would have required about 4.5 billion years to accumulate. The data about primordial lead in meteors, when combined with the accumulated radiogenic lead from terrestrial samples, give the most reliable result for the age of the earth—about 4.5 billion years.

Major improvements in the sensitivity of mass spectrometers have made it possible to measure the abundance of both parent and daughter isotopes. For example, fine surface material and small rocks brought back from the moon by the Apollo 11 astronauts in 1971 were analyzed for uranium and lead isotopes. The results were in good agreement (concordant) for the uranium-238/lead-206 and the uranium-235/lead-207 decay chains. The so-called moon-dust was dated to be between 4.6 and 4.7 billion years old. It appears that the age of the earth, the moon, and meteorites all cluster around 4.6 billion years. This value would be representative for the age of the solar system.

Determining Evolution of Geographic Regions

In general, rocks that solidified much later in the evolution of the earth contain considerable uranium and thorium. Their lead content is largely radiogenic. The ratio of lead isotopes to uranium and thorium will vary greatly, depending on the time of solidification.

Uraninite is a radioactive mineral containing uranium (in the form of UO₂) and thorium. It is similar to the “pitchblende” that was used by Curie in her famous experiment to isolate the new element radium. In a typical age analysis, samples of uraninite from the Black Hillsof South Dakota were dated using three different isotopes, with the following results: uranium-238/lead-206 gave an age of 1.58 billion years, uranium-235/lead-207 gave 1.6 billion years, and thorium-232/lead-208 gave 1.44 billion years. The three measurements agree fairly closely and therefore are said to be concordant. The overall goal of such age measurements is to understand the stages of geological evolution for a whole geographical region on the earth's surface.

Locating Uranium Deposits

Another application of U-Th-Pb dating has been to study the worldwide distribution of uranium resources. All over the surface of the earth, the crust contains about one part per million of uranium. Because of the combined action of high temperature, chemical reactions, and water flow, concentrated uranium mineral deposits were formed when suitable geological conditions existed. Some high-grade uranium ore from Gabon, on the west coast of Africa, contains more than 20 percent uranium. This deposit took place about 2 billion years ago, according to uranium-lead dating. Much larger deposits, but with a much lower percentage yield, are located in northern Canada at Elliot Lake and at Witwatersrand, South Africa. These deposits occurred considerably earlier, about 2.5 billion years ago. In the United States, the major deposits are located in the Colorado plateau extending from Wyoming to Texas, with a relatively recent age of less than 200 million years. Such information about the age of uranium deposits is useful to understand the process of mineralization and possibly to locate new deposits.

Nuclear power plants in 1989 contributed about 16 percent of the world's electricity. Coal-burning plants generate acid rain and carbon dioxide in the environment, and the oil supply is limited, so it is likely that nuclear power will continue to be used, especially in Europe, Japan, Russia, Canada, and the United States. The location and size of the world's major uranium deposits are of great importance to supply the necessary fuel. The mining industry needs to know as much as possible about uranium ore deposits so that the present resources can be estimated accurately and exploration for new deposits can receive helpful guidance.

Investigating Radon Release

Another area where the uranium-lead decay chain plays an important role is in regard to the radon hazard. Uranium in the soil decays in several steps into radium, which in turns decays into the radioactive gas radon. Because it is a gas, radon mixes with the air in small quantities and is ingested into the lungs. In the open, this natural radioactivity in the air is very dilute, so it is not a hazard. The problem comes when radon seeps into the basement of a house through cracks in the floor or through a sump hole. If the house is located in a geographic region where the soil contains considerable uranium, the radon level may be hazardous to the occupants.

The radon problem came to national attention in 1984 when an engineer at the Limerick nuclear power plant in Pennsylvania set off a radiation alarm when he entered the plant, not when he was leaving. The radioactivity was traced to the engineer's home. The radiation level in that house was found to be about one hundred times greater than exposures permitted for workers in uranium mines. Other homes in the area were also found to have relatively high levels. Surveys of radon levels have been made recently in other areas of the United States to investigate the extent of the problem. The Environmental Protection Agency (EPA) has estimated that radon in homes may be responsible for between 5,000 and 20,000 cancer fatalities per year—a cause for great concern.

Another application of radon release may be in earthquake prediction. In Uzbekistan, the city of Tashkent is in a major earthquake zone. The radon content of well water was monitored in the area. A graph of the data starting in 1956 showed a low level of radon at first, increasing slowly for several years. After 1964, the rate of increase became very steep, until the earthquake came in 1966. Immediately after the quake, the radon decreased rapidly. The explanation for this phenomenon is based on the idea that stresses in the ground cause microfracturing of rocks with release of radon from the pores. This method of study is very promising, but more work needs to be done to see if the radon signal can predict the magnitude and epicenter of a quake with any quantitative accuracy.

Principal Terms

common lead: ordinary lead as it was formed at the time when all the elements in nature were created; also called primordial lead

concordant age: a situation in which several naturally radioactive elements, such as uranium, thorium, strontium, and potassium, all give the same age for a rock sample

discordant age: a situation in which several radioactive elements do not give the same age because of gain or loss of decay products from a rock sample

half-life: the time for half the atoms in a radioactive sample to decay, having a different value for each radioactive material

isotope: atoms of the same element but with different masses as a result of extra neutrons in the nucleus, such as the two uranium isotopes uranium-235 and uranium-238

mass spectrometer: an apparatus that is used to separate the isotopes of an element and to measure their relative abundance

radiogenic lead: lead formed from uranium or thorium by radioactive decay

Bibliography

Allison, Ira S., and Donald F. Palmer. Geology. 7th ed. New York: McGraw-Hill, 1980. A college-level introductory textbook in geology that has gone through many revisions since the first edition was published in the 1930's. Chapter 5 gives a clearly written and up-to-date overview of how the ages of rocks and geologic time can be measured.

Dosseto, Anthony, Simon P. Turner, and James A. Van-Orman, eds. Timescales of Magmatic Processes: From Core to Atmosphere. Hoboken, N.J.: Wiley-Blackwell, 2010. Covers many aspects of the earth's history from the formation and differentiation of the earth, to magma ascent, cooling, and degassing. Uranium series Isotopes are referenced multiple times in evaluating the timescales of multiple concepts.

Durrance, E. M. Radioactivity in Geology. New York: Halsted Press, 1986. The author shows the wide scope of radioactivity measurements in geological investigations. Up-to-date information is presented on environmental radioactivity (including the radon hazard), heat generation, and various isotope-dating procedures. A bibliography of articles published in professional as well as popular journals follows each chapter.

Eicher, Don L. Geological Time. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1976. This thin volume of six chapters gives a historical overview of various methods to estimate age and time sequence in the evolution of the earth. The evidence from heat loss, rock strata, fossils, and eventually radioactivity is described in a nontechnical narrative style.

Faure, Gunter. Isotopes: Principles and Applications. 3rd ed. New York: John Wiley & Sons, 2004. An intermediate-level book, originally titled Principles of Isotope Geology, addressed to students of geology as well as to practicing geologists who may not be trained in this area of investigation. Both radioactive and stable isotope analyses are described. After each chapter, some numerical problems with actual experimental data are given. Numerous references to published scientific articles are listed.

‗‗‗‗‗‗‗‗‗. Origin of Igneous Rocks: The Isotopic Evidence. New York: Springer, 2010. Descriptions of multiple radioactive isotope dating methods are contained within this book. Principles of isotope geochemistry are explained early, making this book accessible to undergraduates. Data are presented in diagrams, there are more than 400 original drawings, and a long list of references is included at the end.

Russell, R. D., and R. M. Farquhar. Lead Isotopes in Geology. New York: Interscience Publishers, 1960. A compact discussion of methodology, followed by a 120-page appendix giving specific data on many samples taken worldwide. The authors are particularly concerned about discordant age measurements and how to interpret occasional large variations in lead-isotope ratios.

Skinner, Brian J., and S. C. Porter. Physical Geology. New York: John Wiley & Sons, 1987. A widely used college-level textbook for an introductory course in geology. One chapter deals with geological time and its determination, using radioactivity and other physical methods. The uranium/lead and thorium/lead techniques are described in a readable way.

Wagner, Gunther A., and S. Schiegl. Age Determination of Young Rocks and Artifacts: Physical and Chemical Clocks in Quaternary Geology and Archaeology. New York: Springer, 2010. The authors cover various materials and dating methods. Well organized, accessible to advanced undergraduates and graduate students.

Walker, Mike. Quaternary Dating Methods. New York: Wiley, 2005. This text provides a detailed description of current dating methods, followed by content on the instrumentation, limitations, and applications of geological dating. Written for readers with some science background, but clear enough for those with no prior knowledge of dating methods.

Walther, John Victor. Essentials of Geochemistry. 2d ed. Jones & Bartlett Publishers, 2008. Contains chapters on radioisotope and stable isotope dating and radioactive decay. Geared more toward geology and geophysics than toward chemistry; this text provides content on thermodynamics, soil formation, and chemical kinetics.