Chapter 8 Measuring Geological Time 8.
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.
One of the isotope pairs widely used in geology is the decay of 40K to 40Ar potassium to argon It has a half-life of 1. The red-blue bars represent 40K and the green-yellow bars represent 40Ar. One good example is granite, which normally has some potassium feldspar Figure 8.
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Feldspar does not have any argon in it when it forms. Over time, the 40K in the feldspar decays to 40Ar.
Argon is a gas and the atoms of 40Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms. The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between 40K and the much more abundant 39K. 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. A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.
Dating Rocks and Fossils Using Geologic Methods, Learn Science at Scitable
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.
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.
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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. The proportion of 40K remaining is 0. Using the decay curve shown on this graph, estimate the age of the rock.
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An example is provided in blue for a 40K proportion of 0. This is determined by drawing a horizontal line from 0.
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.
Relative Dating of Rock Layers