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Originally fossils only provided us with relative ages because, although early paleontologists understood biological succession, they did not know the absolute ages of the different organisms. 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 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 40 K to 40 Ar potassium to argon It has a half-life of 1. 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.
Feldspar does not have any argon in it when it forms. Over time, the 40 K in the feldspar decays to 40 Ar. Argon is a gas and the atoms of 40 Ar 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 Radioactive dating of sedimentary rocks, which can detect the differences between the masses of atoms, and can therefore distinguish between 40 K and the much more abundant 39 K.
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, Radioactive dating of sedimentary rocks when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them. 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 of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock. That could be useful information, but it would not provide an accurate date for the rock in question.
It might be possible to directly 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. 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.
When radiometric techniques are applied to metamorphic rocks, the normally tell Radioactive dating of sedimentary rocks the date of metamorphism, not the date when the parent rock formed. Assume that a feldspar crystal from the granite shown in Figure 8. The proportion of 40 K remaining is 0. Using the decay curve shown on the graph below, estimate the age of the rock. An example is provided in blue for a 40 K proportion of 0.
This is determined by drawing a horizontal line from 0. See Appendix 3 for Exercise 8. Radiocarbon dating using 14 C can be applied to many geological materials, including sediments and sedimentary rocks, but the materials in question must be younger than 60 ka. Fragments of wood incorporated into young sediments are good candidates for Radioactive dating of sedimentary rocks dating, and this technique has been used widely in studies involving late Pleistocene glaciers and glacial sediments.
An example is shown in Figure 8. It is evident that the ice-front of the major glacier that occupied the Strait of Georgia was near to Campbell River at around 35 ka, near to Nanaimo and Vancouver at about 25 ka, and had reached the Victoria area by around 22 ka. Over the past decade there has been increasing use of U-Pb dating to study sedimentary rocks, not necessarily to find out the age of the rock, but to discover something about its history and origins.
All clastic sedimentary rocks contain some tiny clasts of the silicate mineral zircon ZrSiO 4derived from the weathering of the sediment parent rocks. The procedure is to isolate a few hundred tiny zircons from a rock sample, and then carry out U-Pb dating on each one of them. An example of the types of obtained are shown on Figure 8. All of the samples are from Nanaimo Gp. The Salt Spring Island sample has some zircons aged around Ma, but most are much older, at Ma and to Ma.
Figure 8. : 8. Next: 8. Share This Book Share on Twitter. Sample must contain wood, bone, or carbonate minerals; can be applied to young sediments.Radioactive dating of sedimentary rocks
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