Potassium-argon dating

Radioactive decay of potassium-40 into argon-40 is used by earth scientists as a natural clock to determine the age of rocks. A wide range of ages can be measured using this technique. Moon rocks brought back by the Apollo astronauts were shown to be more than 2 billion years old, and the volcanic lava that formed the island of Hawaii has been dated at less than 1 million years old.

Discovery of Isotopes

The idea of using radioactivity as a clock for geology was first suggested by the British physicist Ernest Rutherford in 1905. Rutherford and coworkers had shown that uranium decays into lead in a radioactive series, with helium gas formed as a by-product. If the lead and helium are retained in the pores of a rock that contains uranium, the ratio of helium to uranium or lead to uranium can be used to calculate an age. The older the rock, the more lead and helium it should contain. There were some problems, however, that could produce incorrect results. An unknown fraction of helium might have escaped over the centuries, or extra lead might be present in a rock as the result of natural lead deposits not coming from uranium decay. Experimental uncertainties were undeniable.

An important development in 1914 was the invention of the mass spectrometer by J. J. Thomson. When neon gas was analyzed with this apparatus, Thomson was able to show that it actually consisted of a mixture of two different kinds of atoms with masses of 20 and 22 units (relative to hydrogen). His experiment constituted the discovery of isotopes. Over the next several decades, all the elements in the periodic table were analyzed. The sensitivity of the apparatus was greatly improved so that even isotopes whose abundance is less than 1 percent could be measured with precision. In particular, Alfred Otto Carl Nier showed in 1935 that the element potassium (K) has three isotopes, of which potassium-40 is radioactive and has an abundance of less than 0.01 percent.

Potassium Decay

The atmosphere of the earth is known to consist of about 99 percent nitrogen and oxygen, nearly 1 percent argon, and very small amounts of other gases. It was a mystery why so much argon is present in the air. Mass spectrometer data for argon (Ar) showed that argon-40 was by far the most abundant isotope. The German physicist C. F. von Weizsäcker made the suggestion in 1937 that the unexpectedly large amount of argon in the air could have come from radioactive decay of potassium in rocks. A test of this idea would be to analyze old rocks that contain potassium to see if they also contain a higher percentage of argon-40 than that found in the air. In 1948, Nier conducted this experiment on several geologically old minerals. He showed that the argon-40 isotope was indeed greatly enriched in the old rocks. The mystery of excess argon in the air had been solved: It came from potassium decay.

Many rocks contain the element potassium. All these rocks show a slight radioactivity as the potassium slowly decays into argon. The key idea of K-Ar dating is to measure accurately the relative amounts of potassium and argon. Very old rocks contain a larger amount of argon because more time has elapsed for the argon atoms to accumulate. The point at which the rock cooled to its solid form sets the starting time for the K-Ar clock. Before the rock crystallized, the argon gas could escape, but after the rock became solid, the argon gas would be retained. In some cases, corrections have to be made in the measurements because argon may have been gained or lost over long periods of time. Some minerals retain argon gas much better than others. With experience, geologists have learned to be selective in finding those applications where the K-Ar technique has its greatest validity.

Radioactivity measurements have established that potassium-40 decays into two possible end products. About 11.2 percent of the decays become argon-40, and the other 88.8 percent of the decays become an isotope of calcium, calcium-40. The calcium-40 cannot be used in dating a rock because there is too much calcium in the rock already. Only ratios are used in actual calculations. For example, after 200 million years, the ratio of potassium-40 to argon-40 is about 80 to 1. Ratios can be calculated for any age. For meteorites with an age of 4 billion years, nearly 90 percent of the original potassium-40 has decayed. For time intervals shorter than 3 million years, the amount of argon-40 becomes very small and hard to measure, even with a mass spectrometer. As the sensitivity of the apparatus has been improved, however, it has become possible to date rocks with an age as low as 100,000 years or even less.

Determining Potassium and Argon Content

To understand how the potassium-argon clock is used in geology in a quantitative way, it is necessary to look at the methods by which K and Ar are determined. For potassium, the general techniques of chemical quantitative analysis can be used. A rock sample may contain as much as 5 percent potassium or more. The sample is crushed and dissolved in acid; after unwanted elements are removed by heating and precipitation, the potassium is converted to an insoluble salt and the precipitate is collected by centrifuging. The amount of potassium can then be determined by weighing. A second method, which gives better precision (typically ± 1 percent), makes use of atomic spectroscopy. Potassium atoms emit a characteristic purple light with a particular wavelength. A standard solution with a known amount of potassium is prepared and vaporized in a burner flame. A photometer set for the proper wavelength measures the light intensity from the standard. The same process is repeated for the rock sample solution. The ratio of light intensities from sample and standard is used to calculate the potassium content of the rock. After the total amount of potassium in a rock sample has been determined, it is an easy step to calculate how much radioactive potassium-40 it contains because the relative isotope abundance is known.

For argon analysis, the mass spectrometer has to be used because the quantity is so small. A 5-gram rock sample with 5 percent potassium would contain one-fourth of a gram of potassium, but perhaps only a few billionths of a gram of argon-40. The older the rock, the more time has elapsed to allow a larger amount of argon-40 to accumulate. The rock is crushed and heated in a vacuum to collect the gases. The small amount of argon is separated; when it is run through a mass spectrometer, an electrical current is observed at mass number-40. How can one know the amount of argon that caused this current? A calibration standard using a known amount of another argon isotope, argon-38, is added. (The technique of adding another isotope is called isotope dilution.) A known mass of argon-38 is mixed with the argon sample from the rock, the gas mixture is run through the mass spectrometer, and the ratio of the electrical currents at mass numbers 38 and 40 is measured. Since the amount of argon-38 is known, the amount of argon-40 can then be calculated by simple proportion.

In the argon-40 determination, one possible source of error needs to be considered. Suppose the mass spectrometer shows the presence of another argon isotope at mass number 36, which cannot be caused by radioactive decay of potassium. Where did it come from? The most likely source of this contamination is the argon contained normally in the atmosphere. It could have gotten into the rock sample or it might be the result of residual air in the vacuum system used for analysis. In either case, the total argon-40 measured by the mass spectrometer would be the sum of radiogenic argon-40 (which comes from radioactive decay of potassium-40) plus atmospheric argon-40. Fortunately, the ratio of argon-36 and argon-40 in the atmosphere is well known, so the measured argon-36 can be used to subtract the atmospheric argon-40 from the total. The radiogenic argon-40 alone should be used for calculating the age of the rock.

Ar-Ar Method and Application

Once the potassium-40 and the argon-40 content of a rock have been determined, all the essential data for an age calculation are available. A relatively new development in K-Ar dating is the so-called Ar-Ar method. If a rock sample is irradiated with neutrons, some of the stable potassium atoms will be converted to a new argon isotope, argon-39. With a mass spectrometer, the ratio of argon-40 to argon-39 is then determined. The argon-39 is an indirect measure of the potassium content. It is no longer necessary to do a separate potassium analysis of the rock sample. This procedure has been used to investigate possible loss of argon from the outer layers of a rock fragment during metamorphosis. The inner part of a rock will show a larger argon-40/argon-39 ratio than the outer part because the argon-40 will be retained most effectively in the interior. In favorable cases, it is possible to reconstruct a history of the rock fragment since its time of crystallization.

An application of the Ar-Ar method of dating will be described, to show its specialized applications. The astronauts brought back a remarkable orange, glass-like rock from the moon. At first it was thought possible that it might be the product of recent volcanic activity. A sample of less than one-tenth of a gram of this glass was selected for analysis. The sample was irradiated for several days in a nuclear reactor to convert some of the stable potassium-39 into argon-39. The sample was then heated to a moderate temperature of about 650 degrees Celsius, releasing argon gas only from the outermost part of the rock. This gas was collected and analyzed in a mass spectrometer. The rock sample was heated in successive steps of 100 degrees to about 1,350 degrees Celsius, each time collecting the argon gas that was released from the more interior parts of the rock. The argon-40/argon-39 ratio was plotted against the temperature of release. The resulting graph showed that the ratio varied but reached a constant plateau at higher temperatures. Using the argon ratio where it became constant, an age of 3.7 billion years was calculated. Variation in the argon ratio at lower release temperature was attributed to gain or loss of argon in the outer layers of the sample. The Ar-Ar method made it possible to discard erroneous data from the outer part of the rock when determining the age.

Successful K-Ar Applications

One successful application of K-Ar dating was to determine the ages of the individual Hawaiian Islands. The islands were formed by volcanic activity, and small rocks in the lava would have cooled rather rapidly after an eruption. With rapid cooling, it is possible that some atmospheric argon might have become trapped in the rocks, so adjustments had to be made in the calculations. The K-Ar measurements gave the following results: The island of Kauai is the oldest, at about 5 million years; Hawaii is the youngest, at less than 1 million years; and the other islands show a regular sequence of ages in between. Scientists concluded that the volcanic activity started in Kauai and gradually migrated to Hawaii, about 300 miles to the southeast, forming a chain of islands at regular time intervals over the 5 million years.

As another example, several hundred ages have been measured for granitic rock samples from the Sierra Nevada region (near Yosemite National Park) using K-Ar dating. The time when these igneous rocks formed, before they rose to the surface, is of significance for a geological understanding of the region. Samples from Half Dome, Cathedral Peak, and other sites gave initial results which were in the range of 80 million years. The data, however, had to be corrected for loss of argon as a result of reheating from later molten rock intrusions that moved upward toward the surface. With corrections, the age of the Sierra Nevada is estimated to be in the range of 140-210 million years.

Age measurements of a much longer time span have been taken for some rock samples brought back from the surface of the moon by the Apollo astronauts in the 1970's. Radioactive dating by K-Ar, as well as uranium and rubidium decay, gave ages of about 4 billion years for some of the rocks from the lunar exploration. Four billion years is equal to about three half-lives for potassium, so almost 90 percent of the original potassium 40 would have decayed. It has been suggested that some moon rocks may be even older than the measured 4 billion years, because a portion of the argon may have been lost as a result of a high-temperature episode in the early history of the solar system.

As a final example of the K-Ar dating method, consider an application to archaeology. In 1959, L. S. B. Leakey and his wife, Mary, discovered the fossil of a humanoid skull in the east African nation of Tanzania. The remains were found in an area of volcanic deposit suitable for dating by radioactivity, and rock samples lying in strata near the fossil remains were dated. At first, there was controversy about the results because some samples selected for analysis came from rock strata that were not accurately correlated with the fossils. In addition, some data had to be discarded because weathering and possible contamination by water were a problem. General agreement has now been reached that the fossil remains, dated by K-Ar of properly chosen nearby rock samples, are about 1.75 million years old. This age has been confirmed by other methods of radioactive dating. When close correlation is obtained by independent methods, one can conclude with confidence that the age determination is valid.

Scientific and Practical Value

Dating with potassium-argon has been applied to many geologically interesting questions. Among them are the early history of the solar system, the age of meteorites, the exploration of the surface of the moon, the dates when the earth's magnetic field reversed, the general geological history of various mountain ranges, the eruption of volcanoes, and the dates of fossil remains for archaeology. In some cases, the Ar-Ar technique using two different isotopes can be used to analyze the history of a rock sample through periods of cooling and reheating.

The problem of storing high-level radioactive wastes began to receive increased attention as more electricity was generated by nuclear power plants worldwide. Nuclear power has an advantage over coal because it does not generate acid rain or carbon dioxide. Some environmental groups now find nuclear power plants less damaging to the environment than coal or oil plants. The disadvantage of nuclear power is the radioactive waste that is produced. The information obtained by geologists from K-Ar dating has made a contribution to the development of waste storage technology because it has been shown that certain rocks can retain radioactivity for very long periods of time. Research is being done on embedding radioactivity in synthetic rocks (the SYNROC process): The radioactive waste is to be incorporated in the rock material itself, where it would be relatively impervious to water and weathering. Geological study of K-Ar dating on very old rocks can thus help scientists to find an acceptable solution to the very practical problem of nuclear waste storage.

Principal Terms

atomic spectroscopy: a method to identify various elements by the unique spectrum of light waves that each one emits

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

igneous rocks: rocks formed by solidification of molten magma from within the earth

isotopes: atoms of the same element but having different masses as a result of extra neutrons in the nucleus

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

photometer: a device to measure light intensity, using a light meter with a numerical output reading

radiogenic: an isotope formed by radioactive decay

Bibliography

Burchfield, Joe D. Lord Kelvin and the Age of the Earth. Chicago: University of Chicago Press, 1990. Lord Kelvin (1824-1907) was widely regarded as the greatest physicist of his era. He published some articles in the 1860's that suggested an age of 100 million years or less for the earth. The scientific basis for and the general acceptance of this estimate by most geologists before 1900 are documented. Eventually, Kelvin's conclusion was overthrown by the new technique of radioactive dating. Many original documents are cited after each chapter.

Criss, Robert E. Principles of Stable Isotope Distribution. New York: Oxford University Press, 1999. Criss describes isotopes and their properties with clarity. In addition to well-written text, the book features diagrams and illustrations that present a clear picture of the different phases of isotopes and isotope distribution. Includes a bibliography and index.

Dalrymple, G. Brent; and Marvin A. Lanphere. Potassium-Argon Dating: Principles, Techniques, and Applications to Geochronology. San Francisco: W. H. Freeman, 1969. The two authors are scientists with the U.S. Geological Survey, writing from their extensive personal experience with K-Ar measurements. Sample preparation, instrumentation, sources of error, and useful applications are discussed with careful attention to detail. The best technical overview of K-Ar dating, compiled into a compact volume of about 250 pages.

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.

Emsley, John. The Elements. 3rd ed. Oxford: Oxford University Press, 1998. Emsley discusses the properties of elements and minerals, as well as their distribution in the earth. Although some background in chemistry would be helpful, the book is easily understood by the high school student.

Faure, Gunter. Isotopes: Principles and Applications, 3rd ed. New York: John Wiley & Sons, 2004. Originally titled Principles of Isotope Geology. An excellent, though technical, introduction to the use of radioactive and stable isotopes in geology, including a thorough treatment of the Rb-Sr technique. The work is well illustrated and well indexed. Suitable for college-level readers.

Fleischer, Robert Louis. Tracks to Innovation: Nuclear Tracks in Science and Technology. New York: Springer, 1998. The author explains the method of fission track dating and describes experiments done to compare it with other dating techniques. The book also emphasizes the mechanism of track formation and the use of solid-state track detectors to determine the charge and energy of each particle. Designed to acquaint geologists with the technique of fission track dating, it is a suitable introduction for general readers.

Kerr, Richard A. “Two Geologic Clocks Finally Keeping the Same Time.” Science 320 (2008): 434-435. This article compares argon-argon radiometric dating with potassium-argon dating. It discusses the recalibration of Ar-Ar dating.

Levin, Harold L. The Earth Through Time. 9th ed. Philadelphia: Saunders College Publishing, 2009. This college-level text contains a brief, clear description of the Rb-Sr method. A diagram of a whole-rock isochron is included. Five other radiometric dating techniques are discussed, and background information on absolute age and radioactivity is provided. Includes review questions, a list of key terms, and references.

Parker, Sybil P., ed. McGraw-Hill Encyclopedia of the Geological Sciences. 2d ed. New York: McGraw-Hill, 1988. This source contains entries on radioactivity and radioactive minerals. The entry on dating methods includes a brief section on the Rb-Sr method. Includes the formula for radioactive decay and a table of principal parent and daughter isotopes used in radiometric dating. The entry on rock age determination has a longer discussion of the Rb-Sr dating and includes an isochron diagram. For college-level audiences.

Skinner, Brian J., and Stephen 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 methods. Both the K-Ar method and the more recent argon-40/argon-39 ratio are described in a readable way.

Tuniz, Claudio, et al., eds. Accelerator Mass Spectrometry: Ultrasensitive Analysis for Global Science. Boca Raton, Fla.: CRC Press, 1998. This book looks at the processes involved with accelerator mass spectrometry and the instruments required. There is also a substantial amount of care given to radioactive dating and its protocols, principles, and usefulness. Bibliographic references and index.

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.