Radiometric dating – internal clocks in rocks Geochronology: the science of dating geologic materials. Radioactive decay occurs at an exponential rate, meaning that it can be described in terms of a half life. After one half live, half of the original radioactive isotope material in the system under consideration decays. Another half life and half of the remaining material decays, and so on. This is for unforced decay. Forced decay is when the isotopic material is packed densely enough that a decay in one unstable atom sends out a particle that hits another atom and causes it to decay. If it is packed too densely there is a run away reaction and one of those unpopular mushroom clouds or meltdowns. Normal concentrations of radioactive material on earth are well below the levels where forced decay occurs so we can use the relatively simple mathematics of exponential decay to describe the process.
Clocks in the Rocks
Uranium—lead dating , abbreviated U—Pb dating , is one of the oldest  and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4. The method is usually applied to zircon. This mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead when forming.
Give examples of other isotopes used in radioactive dating. assumes that all of the lead present came from the decay of uranium
Radiometric Dating Activity. This hands-on activity is a simulation of some of the radiometric dating techniques used by scientists to determine the age of a mineral or fossil. The activity uses the basic principle of radioactive half-life, and is a good follow-up lesson after the students have learned about half-life properties. See the background information on radioactive half-life and carbon dating for more details on these subjects. Students will use half-life properties of isotopes to determine the age of different “rocks” and “fossils” made out of bags of beads.
Through this simulation, they will gain an understanding of how scientists are able to use isotopes such as U and Pb to determine the age of ancient minerals. National Science Education Standards :. Mathematics is important in all aspects of scientific inquiry. Science and technology are reciprocal. Science helps drive technology, as it addresses questions that demand more sophisticated instruments and provides principles for better instrumentation and technique.
Technology is essential to science, because it provides instruments and techniques that enable observations of objects and phenomena that are otherwise unobservable due to factors such as quantity, distance, location, size, and speed. Technology also provides tools for investigations, inquiry, and analysis.
RADIOMETRIC TIME SCALE
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers. To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating. Radiometric dating is largely done on rock that has formed from solidified lava. Lava properly called magma before it erupts fills large underground chambers called magma chambers.
Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios.
U and Th are found on the extremely heavy end of the Periodic Table of Elements. Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium Section 2. We can therefore assume that the Pb is directly formed by the U, the Pb from the U and the Pb from the Th.
The ingrowth equations for the three radiogenic Pb isotopes are given by: 5. The corresponding age equations are: 5. This assumption cannot be made for other minerals, young ages, and high precision geochronology. The corresponding age equations then become: 5. This built-in redundancy provides a powerful internal quality check which makes the method arguably the most robust and reliable dating technique in the geological toolbox. The initial Pb composition can either be determined by analysing the Pb composition of a U-poor mineral e.
Note that isotopic closure is required for all intermediary isotopes as well. Initially, the U-Pb method was applied to U-ores, but nowadays it is predominantly applied to accessory minerals such zircon and, to a lesser extent, apatite, monazite and allanite. Note that these are only a function of time. Equations 5. The Pb-Pb method has the following advantages over conventional U-Pb dating: There is no need to measure uranium.
The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes:. Note that uranium and uranium give rise to two of the natural radioactive series , but rubidium and potassium do not give rise to series. They each stop with a single daughter product which is stable.
Some of the decays which are useful for dating, with their half-lives and decay constants are:. The half-life is for the parent isotope and so includes both decays.
Lead, Bismuth, Carbon14, Nitrogen14, 5, Uranium, Lead, ,, Potassium40, Argon40, 1,,, Uranium, Lead
Carbon has a large number of stable isotopes. All carbon atoms contain six protons and six electrons, but the different isotopes have different numbers of neutrons. The amount of carbon in the atmosphere has not changed in thousands of years. Even though it decays into nitrogen, new carbon is always being formed when cosmic rays hit atoms high in the atmosphere.
Plants absorb carbon dioxide from the atmosphere and animals eat plants. This means all living things have radioactive carbon in them. When an organism, eg a tree, dies it stops taking in carbon dioxide.
Uranium , for example, accounts for more than 99 percent of all naturally occurring uranium. In breeders, approximately 70 percent of this isotope can be utilized for power production. Conventional reactors, in contrast, can extract less than….
Uranium–Lead dating is the geological age-determination method that uses the (Pb/U, Pb/U, Pb/Th) and a radiogenic lead isotope.
Misconceptions and Confusions in U-Pb dating. Selva Harris published an essay on the web which is reproduced below claiming that U-Pb dating supports the hypothesis of a Young Earth:. This is a response to that extraordinary claim. U-Pb dating uses the relative presence of parent isotopes of uranium U and U and their daughter species of lead Pb and Pb respectively to determine the age of crystallisation of certain minerals.
Together they provide two separate decay schemes to determine ages of crystallisation of minerals ranging from about 10 million years, up to and beyond the age of the earth at 4. U-Pb dating is commonly carried out on a limited set of minerals, namely zircon, quartz and apatite. Zircon is a particularly valuable mineral for this purpose for the following reasons:.
Harris claims that U-Pb data indicates a young earth. In so doing he makes a number of errors of understanding, interpretation and fact.
FAQ – Radioactive Age-Dating
Radiometric dating or radioactive dating is any technique used to date organic and also inorganic materials from a process involving radioactive decay. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The radioactive decay law states that the probability per unit time that a nucleus will decay is a constant, independent of time.
Uranium, Lead, billion years Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures.
The ratio of the amounts of U and Pb in a rock sample enables the age of the rock to be estimated using the technique of radiometric dating. U forms a decay chain in which it undergoes a sequence of 8 alpha and 6 beta decays:. It moves back in the periodic table until the isotope falls in the band of stability at Pb Each step has its own individual half – life but the first decay to Th is about 20, times slower than the other decay steps.
Those of you who are familiar with chemical kinetics will know that it is the slowest step in a mechanism which determines the overall rate of reaction, the so – called “rate determining step”. The half – life of U is about 4. As time passes, the ratio of Pb to U will increase and it is this which enables the age of the rock to be estimated. Since the decay of 1 U atom will result in the formation of 1 atom of Pb we can say that:.