Isotope Pairs

Isotopes of an element have different numbers of neutrons. Sometimes this just makes one of the isotopes slightly lighter or heavier than others, but other times it makes the element unstable, causing it to undergo radioactive decay. Unstable elements paired with the elements they decay to are used in isotopic dating methods.

In most cases, we can’t use isotopic techniques to directly date fossils or the sedimentary rocks in which they are found, 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 within them, is based upon the fact that we know the decay rates of certain unstable isotopes of elements, and that these decay rates have been constant throughout geological time. It is also based on the premise that when the atoms of an element decay within a mineral or a rock, they remain trapped in the mineral or rock, and don’t escape.

How It Works: Potassium-Argon Dating

One of the isotope pairs commonly used to date rocks 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 contain potassium. 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 forth (Figure 19.15). ⁴⁰K is called the parent isotope, and ⁴⁰Ar the daughter isotope, as the parent gives way to the daughter during radioactive decay.

In order to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral. In granite, we can use the mineral potassium feldspar (salmon-coloured crystals in Figure 19.16). When potassium feldspar forms, it has no argon. But over time, the ⁴⁰K in the feldspar decays to ⁴⁰Ar, 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. The minerals biotite and hornblende are also commonly used for K-Ar dating.

Other Isotope Pairs

There are many isotope pairs that can be employed in dating igneous and metamorphic rocks (Table 19.1), each with its strengths and weaknesses. In the above example, the daughter isotope ⁴⁰Ar is naturally a gas, and can escape the potassium feldspar quite easily if the feldspar is exposed to heating during metamorphism, or interaction with hydrothermal fluids. This means we have to examine the feldspar mineral closely first to see if there is any evidence of alteration. If some ⁴⁰Ar has been lost, but the sample is dated anyway, the age we get will be too young, because it will look like the argon has been accumulating for less time than it really has.

Each parent isotope has a certain half-life, which ranges from microseconds to billions of years, depending on the isotope. In dating rocks, we need to select an isotope pair with a parent isotope that has a reasonable half-life for our sample. If the half-life is too short, then most of the parent isotope will have decayed to form the daughter isotope. If we can’t measure the amount of parent isotope very accurately, which will be impossible to do if there is only the tiniest amount of parent isotope left, our calculated age will have huge errors associated with it. The same applies if the half-life is too long. In this case, very little of the daughter isotope will have formed, and our inability to measure the small amount of daughter isotope accurately will again result in huge errors in our calculated age.

Another complicating factor is whether the mineral of interest incorporated any of the daughter isotope into its structure at the time of formation. When we select a mineral and an isotope pair to date that mineral, we make the assumption that all of the daughter isotope we find in the mineral was produced in the mineral by radioactive decay of the parent isotope. But if the mineral formed with some daughter isotope already present in its structure, then the age we calculate will be too old.

A more robust mineral to use to date certain types of igneous and metamorphic rocks is zircon. Zircon is a mineral that incorporates uranium into its structure at the time of formation. One of the isotopes of uranium decays to lead with a long half-life (4.5 Ga). Zircon is a mineral of choice for dating because it takes no lead into its structure when it forms, so any lead present is due entirely to the radioactive decay of the uranium parent. Another reason is because zircon is a very resistant mineral. It can handle exposure to hydrothermal fluids, and all but the highest grades of metamorphism, and not lose any of the parent or daughter isotopes. One drawback is that zircon tends to form only in felsic igneous rocks, so if we’re trying to date a mafic rock, we need to use a different mineral.

The Meaning of a Radiometric Date

When we employ isotopic methods on minerals, we’re measuring an age date. Generally, an age date refers to the time since a mineral crystallized from molten rock, when the elements that make up the mineral got locked into the mineral’s structure. But as we’ve already seen, elevated temperatures can cause elements to escape from a mineral, without the mineral melting. This means that when we date a mineral, we might actually be dating the time since the mineral last experienced a period of heating above its Curie point, which is the temperature beyond which the mineral is able to lose (or gain) elements from its structure, without melting.

So we have to know something about the rock before we forge ahead to measure an age. We may choose a mineral and isotope pair that are very resistant to metamorphism, so that we can “see through” the metamorphism, and determine the original age that the mineral crystallized from a melt. Or we may be interested in the age of the metamorphic event itself, so choose a mineral and isotope pair that is susceptible to resetting the isotopic clock during metamorphism (such as by losing all of the daughter isotope).

Absolute age dating is a powerful tool for unraveling the geological history of a region, but we must ultimately rely upon igneous rocks (that may have later metamorphosed) for the minerals that we are able to date (more on issues with dating sedimentary rocks below). Another issue with absolute age dating is that it is expensive, and a single analysis can cost hundreds of dollars. This is why geologists never forget their relative age dating principles, and are always applying them in the field to determine the sequence of events that formed the rocks in a region.

Isotope Dating Techniques and Sedimentary Rocks

A clastic sedimentary rock (e.g., conglomerate, Figure 19.17) is made up of older rock and mineral fragments. These fragments were derived from weathering and erosion of pre-existing rocks. The process of forming a sedimentary rock from sediments generally occurs at low temperatures, so the minerals are not heated beyond their Curie points. This means the minerals still preserve their original ages (either igneous crystallization age, or a metamorphic age), but what does that actually say about the age of the sedimentary rock?

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 sedimentary rock. The most that such ages gleaned from a sedimentary rock can tell us is a maximum age of the sedimentary 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.

Radiocarbon Dating

Radiocarbon dating (using ¹⁴C) can be applied to many geological materials, including sediment and sedimentary rocks, but only if the materials in question are younger than ~60 ka, and contain organic material. Beyond this time, there is so little ¹⁴C left that it cannot be measured accurately, resulting in unreliable age dates. Fragments of wood incorporated into young sediment are good candidates for carbon dating, and this technique has been used widely in studies involving late Pleistocene glaciers and glacial sediments. Figure 19.18 shows radiocarbon dates from wood fragments in glacial sediments have been used to estimate the time of the last glacial advance along the Strait of Georgia.

References

Clague, J. (1976). Quadra Sand and its relation to late Wisconsin glaciation of southeast British Columbia. Canadian Journal of Earth Sciences, 13, 803-815.