Radiometric Dating
Radioactive decay provides a way of estimating the ages of some objects. Estimating the age of an object based on the radioactive isotopes it contains is called radiometric dating. One of the most useful and important methods of radiometric dating is radiocarbon dating, which involves carbon-14, a radioactive isotope of the element carbon. Most of the carbon on our planet is carbon-12, a stable (non-radioactive) isotope that has 6 protons and 6 neutrons. Carbon-14 has two extra neutrons, and it decays into nitrogen-14 via beta minus decay. The ages of old bones and other organic (carbon-based) remains can be estimated by measuring the proportion of carbon-14 to carbon-12 that they contain. The older the bones are, the less carbon-14 they’ll contain, because more of the carbon-14 will have decayed into nitrogen.
Of course, we can’t estimate the age of the bones in this way unless we know how much carbon-14 the bones contained when the animal died. So, how do we figure that out? Here’s how. While an animal is alive, the matter that constitutes its body is continually replaced by new matter, which comes from the food it eats. The carbon in an animal’s body comes primarily from carbon dioxide in the air, which is absorbed by plants, which in turn are eaten by the animal. For this reason, the ratio of carbon-14 to carbon-12 in a living plant or animal tends to match that of the atmosphere. Surprisingly, the ratio of carbon-14 to carbon-12 in the atmosphere doesn’t change much, even over long periods of time. Although carbon-14 atoms eventually decay into nitrogen-14, new carbon-14 atoms are created when high-energy particles from outer space (called cosmic rays) collide with nitrogen-14 atoms in Earth’s atmosphere. So, the ratio of carbon-14 to carbon-12 in an animal’s body remains approximately constant while the animal is alive but begins to decrease after the animal dies.
Other radiometric dating techniques can be used to measure the ages of certain kinds of rocks and minerals. One of the best techniques is the uranium-lead method. When magma cools into rock, various kinds of mineral crystals form. The mineral most often used in uranium-lead dating is zircon, which consists of crystals of zirconium silicate (ZrSiO4). As these crystals form in cooling magma, uranium atoms from the magma are incorporated into the crystal structure; but atoms of the element lead remain in the magma (and are eventually incorporated into other minerals as the magma continues to cool). If you find a zircon crystal with some lead atoms in it, you can conclude that these atoms came from the decay of uranium: no lead was in the crystal when it formed. Since the half-lives of uranium and other isotopes on its decay chain are known, it is possible to calculate how long ago the zircon crystal formed, just by measuring the ratio of lead to uranium in the crystal.
In a mass spectrometer, a tiny sample of material is ionized (e.g. by heating the sample until it becomes a plasma). An electric field is then applied to accelerate the charged ions and shoot them through a magnetic field, which deflects them sideways. (Recall from chapter 2 that charged particles experience a sideways force when passing through a magnetic field.) Lighter isotopes of any given element are deflected more than heavier isotopes. After passing through the magnetic field, the deflected ions reach a sensitive detector, which gives an exact count of how many atoms of each isotope were in the sample.
The uranium-lead method is more reliable if you measure the specific isotopes of lead and uranium. (This can be accomplished using a mass spectrometer—a device that ionizes the atoms and then shoots them through an electric or magnetic field to sort them by mass.) The decay chain of uranium-238 culminates with lead-206, while the decay chain of uranium-235 ends with lead-207. So, you can compare the ratio of lead-206 to uranium-238 to calculate the age of the crystal, and you can also calculate the crystal’s age by comparing the ratio of lead-207 to uranium-235. If you get the same answer from both calculations, you can be pretty confident that you’ve determined the true age of the crystal.
Radiometric dating is useful in geology, paleontology, and other sciences, as we’ll see in later chapters. All radiometric dating techniques have their limitations, however. The methods, accuracy, and limitations of radiometric dating will be further discussed in chapters 9 and 10.