Dating fossils through strata and radiometric samples
Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times.If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.
If stratum B lies above stratum A, B is the younger of the two.Feldspar does not have any argon in it when it forms.Over time, the Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms.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.However determining the actual, or absolute, age of strata (for example, 3.5 million years old) is often difficult since the age of a fossil cannot be determined directly.
The most useful tool in dating strata is radiometric dating of materials.
[SE] K-Ar is just one of many isotope-pairs that are useful for dating geological materials.
Some of the other important pairs are listed in Table 8.2, along with the age ranges that they apply to and some comments on their applications.
[SE] In order to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral.
One good example is granite, which normally has some potassium feldspar (Figure 8.15).
One of the isotope pairs widely used in geology is the decay of K is a radioactive isotope of potassium that is present in very small amounts in all minerals that have potassium in them.