It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils. In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers.
Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time. Biotite and hornblende are also commonly used for K-Ar dating. An important assumption that we have to be able to make when using isotopic dating is that when the rock formed none of the daughter isotope was present e.
It might be possible to date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks. Radiocarbon dating can be used on sediments or sedimentary rocks that contain carbon, but it cannot be used on materials older than about 60 ka. Some of the other important pairs are listed in Table 8. When radiometric techniques are applied to metamorphic rocks, the results normally tell us the date of metamorphism, not the date when the parent rock formed.
8.4 Isotopic Dating Methods
Uranium–lead radiometric dating involves using one based on uranium's decay to lead with a half-life. Isotopic dating of rocks, or the minerals within them, is based upon the fact that we know the decay rates of certain unstable isotopes of elements, and that these. Radiometric dating is used to estimate the age of rocks and other objects based on the fixed decay rate of radioactive isotopes. Learn about half-life and how it is.