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 datings of fossils red us, 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.
The red-blue bars represent 40K and the green-yellow bars represent 40Ar. 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. 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.