68 8.4 Isotopic Dating Methods — Physical Geology – 2nd Edition

Figure 8.4.1 The decay of ⁴⁰K over time. Each half-life is 1.3 billion years, so after 3.9 billion years (three half-lives) 12.5% of the original ⁴⁰K will remain. The red-blue bars represent ⁴⁰K and the green-yellow bars represent ⁴⁰Ar. [Image Description]

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. It is also based on the premise that when the atoms of an element decay within a mineral or a rock, they stay there and don’t escape to the surrounding rock, water, or air. One of the isotope pairs widely used in geology is the decay of ⁴⁰K to ⁴⁰Ar (potassium-40 to argon-40). ⁴⁰K is a radioactive isotope of potassium that is present in very small amounts in all minerals that have potassium in them. It has a half-life of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the ⁴⁰K atoms in a mineral or rock will decay to ⁴⁰Ar, and over the next 1.3 Ga one-half of the remaining atoms will decay, and so on (Figure 8.4.1).

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 8.4.2). 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.

Figure 8.4.2 Crystals of potassium feldspar (pink) in a granitic rock are candidates for isotopic dating using the K-Ar method because they contained potassium and no argon when they formed.

Why can’t we use isotopic dating techniques to accurately date sedimentary rocks?

Figure 8.4.3

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.g., ⁴⁰Ar in the case of the K-Ar method). 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. 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.  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 8.2, along with the age ranges that they apply to and some comments on their applications. 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.

Assume that a feldspar crystal from the granite shown in Figure 8.4.2 was analyzed for ⁴⁰K and ⁴⁰Ar. The proportion of ⁴⁰K remaining is 0.91. Using the decay curve shown on the graph below, estimate the age of the rock.

Figure 8.4.4 [Image Description]

An example is provided (in blue) for a ⁴⁰K proportion of 0.95, which is equivalent to an age of approximately 96 Ma. This is determined by drawing a horizontal line from 0.95 to the decay curve line, and then a vertical line from there to the time axis.

See Appendix 3 for Exercise 8.3 answers.

Figure 8.4.5 Radiocarbon dates on wood fragments in glacial sediments in the Strait of Georgia.

Radiocarbon dating (using ¹⁴C) 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 carbon dating, and this technique has been used widely in studies involving late Pleistocene glaciers and glacial sediments. An example is shown in Figure 8.4.5; radiocarbon dates from wood fragments in glacial sediments have been used to estimate the timing of the last glacial advance along the Strait of Georgia.  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 (ZrSiO₄), derived from the weathering of the sediment parent rocks.  Zircon always has some uranium in it (but no lead) so it is a good candidate for U-Pb dating, and it isn’t too difficult to separate the grains of zircon from the other grains in a sandstone.  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 results obtained are shown on Figure 8.5.6.  All of the samples are from Nanaimo Gp. rocks on Vancouver Island and nearby Salt Spring Island.

The three samples from Vancouver Island have zircons aged around 90 Ma, 118 Ma and 150 Ma.  The Salt Spring Island sample has some zircons aged around 150 Ma, but most are much older, at 200 Ma and 340 to 360 Ma.  It is interpreted that the younger zircons (90 to 150 Ma) are mostly derived from granitic rocks in the Coast Range, while the older ones (>200 Ma) are from older rocks on Vancouver Island (Huang, 2018).

Figure 8.4.6 U-Pb dates for zircon samples from the Nanaimo Gp. (after Krause, 2018) a: a typical zircon clast (this one is about 1/4 mm long).  b: plots of zircon ages for 4 sandstone samples.

Image Descriptions

[Return to Figure 8.4.1]

[Return to Figure 8.4.4]

Media Attributions

• Figures 8.4.1, 8.4.2, 8.4.3, 8.4.4: © Steven Earle. CC BY.
• Figure 8.4.5: © Steven Earle. CC BY. From J. Clague, 1976, Quadra Sand and its relation to late Wisconsin glaciation of southeast British Columbia, Can. J. Earth Sciences, V. 13, p. 803-815.
• Figure 8.4.6 (left): “Zircon microscope” © Chd. CC BY-SA.
• Figure 8.4.6b: © Steven Earle. CC BY. From data in Huang, C, 2018, Refining the chronostratigraphy of the lower Nanaimo Group, Vancouver Island, Canada, using Detrital Zircon Geochronology, MSc thesis, Department of Earth Science, Simon Fraser University, 74 p.

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