Radiometric Dating: How Does It Work?
The discovery of radioactive decay in the early 20th century led to a number of advances in physics, medicine, and power generation. One area in which radioactive materials have been particularly useful is in geology.
Since radioactive elements have a measurable half-life - the period of time needed for an amount of the substance to decrease by half - they can be used to measure the ages of rocks containing them. Over the past century this technique, known as radiometric dating, has allowed geologists to precisely date rocks from the Earth, the Moon, Mars, and meteorites, allowing them to establish the age of the Earth and our Solar System. By dating the many rock strata containing ancient fossils, this technique has also allowed scientists to determine when different creatures lived on our planet and even how the continents were oriented at the time.
This article provides a brief introduction to the technique, using a specific example of a recently-dated formation to illustrate how radiometric dating is used to establish Earth's geologic history.
Common Dating Methods
700 million years
4.47 billion years
50 billion years
106 billion years
1.3 billion years
Radiometric Dating Methods
Radioactive elements decay into daughter isotopes over time. By analyzing a rock sample in a mass spectrometer and measuring the ratio of mother to daughter isotopes in a rock, scientists can estimate how long ago the rock formed. Geologists use several different pairs of parent-daughter isotopes to establish the ages of rocks. When possible, multiple methods are used on the same rock or formation so that they will provide a check on each other. Although it is not possible to go back in time and measure the amount of parent isotope that was present in a rock, scientists can use the chemical properties of different elements to estimate the original ratio.
Uranium-Lead dating provides a good example of this. Zircon (zirconium silicate (ZrSiO4) is a common mineral in igneous rocks such as granite. When magma cools and crystallizes, zircon crystals formed will often contain trace amounts of uranium in place of zirconium. Lead, however, will be strongly rejected by the crystallizing zircon, as has been demonstrated in laboratory experiments.
As uranium decays over millions of years, it eventually turns into lead. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years, and uranium-235 decays to lead-207 with a half-life of 704 million years. Measuring the proportion of lead to uranium thus provides a very accurate date when the rock solidified. Since there are two uranium isotopes and two lead isotopes with two different half lives, this method of dating also checks itself, providing two calculable ages for each sample.
Another method commonly used is Potassium-Argon dating. While potassium is a common element found in many rock minerals, argon is a noble gas that does not bind with other atoms. Potassium-40 decays to argon-40 with a half-life of 1.3 billion years. Since argon cannot form part of the lattice of minerals in igneous rocks and thus should not be present in them, argon-40 that is found in rocks forms there due to the decay of potassium-40. For rocks formed and preserved under ideal conditions the K-40/Ar-40 ratio will preserve a record of the rock's age.
However, not all rocks form and preserve under ideal conditions. Since argon is present in the atmosphere, tiny air bubbles left in a rock that cooled rapidly or formed from volcanic ash may preserve atmospheric argon, making the rock appear older than it is. On the other hand, extreme temperatures and pressures, such as during tectonic events, can fracture rocks and allow decay-product argon to escape, making the rock appear too young.
The argon-argon dating method, to be discussed in the next section, was created to correct for these errors. Instead of measuring the total amount of argon-40 and assuming that the rock started with zero argon content, this more complicated technique dates the rock using the ratio of different argon isotopes to each other.
Location of the Jiufotang Formation
An Example of Radiometric Dating
Northeastern China has become a fossil-hunting mecca in recent decades, with rich deposits yielding new species of fish, dinosaurs, amphibians, and early birds from the Tertiary Period. One of the richer deposits is the Jiufotang Formation near the city of Chaoyang, Liaoning Province, which has yielded fascinating fossils such as the four-winged dinosaur Microraptor gui and the pterosaur Saperonis .
Precisely dating these fossil finds had been difficult for many years, however, as there had not been much known about the rocks containing them. One piece of this information gap was filled by a study published in 2004 by a team led by Dr. Huaiyu He of the Institute of Geology and Geophysics, Chinese Academy of Sciences. Dr. He and her team conducted argon-argon and uranium-lead radiometric dating tests on a sample of tuff - rock formed from volcanic ash - in order to establish a minimum age for the fossil-bearing layers below.
The sample He's team used came from the lower of two layers of volcanic rock, named Tuff L3001. Once the sample, named Lx9, was taken to the laboratory at the Institute of Geology and Geophysics, it was crushed into small pieces. The purest crystals of K-feldspar, a potassium-bearing mineral, were then hand-selected, cleaned, and heated in a vacuum chamber to remove as much atmospheric gas as possible.
Sample Lx9 was then irradiated with neutrons in a nuclear reactor. This procedure causes potassium-39, a normally stable isotope, to decay into argon-39. Finally, the sample was placed in a mass spectrometer and heated with a laser to higher and higher temperatures, causing the crystal to break and the argon gas to be released. Based on the ratio of argon-40 to argon-39, the age of the sample was determined to be 120.3 million years old.
In order to confirm this date, zircons were selected from the same piece of rock and analyzed using uranium-lead dating. This method yielded a slightly older date - 124 million years, but with a four-million-year margin of error. The dating of this volcanic formation at 120 million years placed a minimum age of 120 million years for all of the fossils beneath it in the Jiufotang Formation. It also places a minimum age of 120 million years on fossils of similar types found elsewhere in the region.
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The Limitations of Radiometric Dating
Radiometric dating is not a perfect method of measurement. All dating techniques have an inherent margin of error, and even the most precise measurements are only estimates. And even though many dating techniques have self-correcting mechanisms, their results can be skewed by metamorphic processes or contamination that changed the chemistry of the rocks being studied.
Despite these limitations, careful sample selection and advances in technology have vastly improved the accuracy of radiometric dating over the past century. Radiometric dating techniques have given geologists, paleontologists, archaeologists and other scientists studying the past a very useful timeline of the history of our planet.
Sources and Further Information
- Bureau Laboratories: Argon Lab: Methods
The isotopes the KAr system relies on are Potassium (K) and Argon (Ar). Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals.
- Timing of the Jiufotang Formation (Jehol Group) in Liaoning, northeastern China, and its implication
H. Y. He, et al. Geophys. Res. Lett., 31, L12605, 2004. The timing of the Jiufotang Formation remains speculative despite recent progress in the study of the Jehol Biota. In this paper we contribute to this topic with 40Ar/39Ar dating on K-feldspar
- Radiometric Dating and the Geological TimeScale
This document discusses the way radiometric dating and stratigraphic principles are used to establish the conventional geological time scale.
- Isochron Dating
A brief introduction to isochron dating methodology. The technique (and related ones) is widely used in isotope geology.
- Radioactive dating
Certain radioactive lelments decay a predictable rates and may be used to date earth rocks and minerals.
- Direct Test of the Time-Independence of Fundamental Nuclear Constants Using the Oklo Natural Reactor
Alexander I. Shlyakhter. ATOMKI (Debrecen, Hungary) Report A/1 (1983)
- Geologic Time: Radiometric Time Scale
The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the French physicist, opened new vistas in science.