{"id":814,"date":"2018-10-30T19:04:58","date_gmt":"2018-10-30T23:04:58","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/19-4-isotopic-dating-methods-2\/"},"modified":"2023-07-04T13:21:06","modified_gmt":"2023-07-04T17:21:06","slug":"isotopic-dating-methods","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/isotopic-dating-methods\/","title":{"raw":"19.4 Isotopic Dating Methods","rendered":"19.4 Isotopic Dating Methods"},"content":{"raw":"

Isotope Pairs<\/h1>\r\nIsotopes<\/strong> 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.\r\n\r\nIn 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.\r\n\r\nIsotopic 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.\r\n

How It Works: Potassium-Argon Dating<\/h2>\r\nOne of the isotope pairs commonly used to date rocks is the decay of 40<\/sup>K to 40<\/sup>Ar \u00a0(potassium-40 to argon-40). 40<\/sup>K is a radioactive isotope of potassium that is present in very small amounts in all minerals that contain potassium. It has a half-life<\/strong> of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the 40<\/sup>K atoms in a mineral or rock will decay to 40<\/sup>Ar, and over the next 1.3 Ga one-half of the remaining atoms will decay, and so forth (Figure 19.15). 40<\/sup>K is called the parent<\/strong> isotope<\/strong>, and 40<\/sup>Ar the daughter isotope<\/strong>, as the parent gives way to the daughter during radioactive decay.\r\n\r\n[caption id=\"attachment_809\" align=\"aligncenter\" width=\"577\"]\"\" Figure 19.15<\/strong> 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 40<\/sup>K will remain. The red-blue bars represent 40<\/sup>K and the green-yellow bars represent 40<\/sup>Ar. Source: Steven Earle (2015), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\n
\r\n\r\nMisconception Alert<\/strong>\r\n\r\nWhen we look at radioactive decay, we're always thinking about half-life as it applies to the remaining atoms of a radioactive isotope, and not necessarily the original amount. In other words, after the second half life, another half<\/em> of the 50% is remaining (i.e., 25%), rather than all of the remaining 50% being gone. That's why the dark coloured bars in Figure 19.15 don't go to zero for the second half-life.\r\n\r\n<\/div>\r\nIn 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 40<\/sup>K in the feldspar decays to 40<\/sup>Ar, and the atoms of 40<\/sup>Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms.\r\n\r\n[caption id=\"attachment_810\" align=\"aligncenter\" width=\"491\"]\"\" Figure 19.16<\/strong> Crystals of potassium feldspar (salmon colour) in a granitic rock are candidates for isotopic dating using the K-Ar method because they contained potassium and no argon when they formed. Source: Steven Earle (2015), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\nThe 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 40<\/sup>K and the much more abundant 39<\/sup>K. The minerals biotite and hornblende are also commonly used for K-Ar dating.\r\n

Other Isotope Pairs<\/h2>\r\nThere 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 40<\/sup>Ar 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 40<\/sup>Ar 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.\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 19.1 Commonly used isotope systems for dating geological materials. Source: Steven Earle (2015), CC BY 4.0.<\/caption>\r\n
Isotope System<\/th>\r\nHalf-life<\/th>\r\nUseful Range<\/th>\r\nComments<\/th>\r\n<\/tr>\r\n
Potassium-Argon<\/td>\r\n1.3 Ga<\/td>\r\n10 Ka - 4.57 Ga<\/td>\r\nWidely applicable because most rocks contain potassium<\/td>\r\n<\/tr>\r\n
Uranium-Lead<\/td>\r\n4.5 Ga<\/td>\r\n1 Ma - 4.57 Ga<\/td>\r\nThe rock must contain uranium-bearing minerals (felsic igneous rocks)<\/td>\r\n<\/tr>\r\n
Rubidium-Strontium<\/td>\r\n47 Ga<\/td>\r\n10 Ma - 4.57 Ga<\/td>\r\nLess precision than other methods for old rocks<\/td>\r\n<\/tr>\r\n
Carbon-Nitrogen (radiocarbon dating)<\/td>\r\n5,730 a<\/td>\r\n100 a to 60,000 a<\/td>\r\nSample must contain wood, bone, or carbonate minerals; can be applied to young sediments<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nEach 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.\r\n\r\nAnother 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.\r\n\r\nA 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.\r\n

The Meaning of a Radiometric Date<\/h1>\r\nWhen we employ isotopic methods on minerals, we're measuring an age date<\/strong>. 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<\/strong>, which is the temperature beyond which the mineral is able to lose (or gain) elements from its structure, without melting.\r\n\r\nSo 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).\r\n\r\nAbsolute 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.\r\n
\r\n\r\nCombining Absolute Ages with Relative Dating<\/strong>\r\n
\r\n\r\nThe age dates for three igneous rock layers are given. Can you figure out the age ranges for the sets of sedimentary units A, B, and C?\"The\r\n\r\nImportant first step!<\/strong>\r\n\r\nWe have to be sure that the igneous rock layers are\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> so we can use the principle of \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>. If they're \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> the \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> principle will apply instead, and this won't work as well.\r\n