The samarium-neodymium method has also been shown to be more resistant to being disturbed or re-set by metamorphic heating events, so for some metamorphosed rocks the samarium-neodymium method is preferred. For a rock of the same age, the slope on the neodymium-samarium plots will be less than on a rubidium-strontium plot because the half-life is longer.
However, these isotope ratios are usually measured to extreme accuracy--several parts in ten thousand--so accurate dates can be obtained even for ages less than one fiftieth of a half-life, and with correspondingly small slopes. The lutetium-hafnium method uses the 38 billion year half-life of lutetium decaying to hafnium This dating system is similar in many ways to samarium-neodymium, as the elements tend to be concentrated in the same types of minerals. Since samarium-neodymium dating is somewhat easier, the lutetium-hafnium method is used less often. The rhenium-osmium method takes advantage of the fact that the osmium concentration in most rocks and minerals is very low, so a small amount of the parent rhenium can produce a significant change in the osmium isotope ratio.
The half-life for this radioactive decay is 42 billion years. The non-radiogenic stable isotopes, osmium or , are used as the denominator in the ratios on the three-isotope plots. This method has been useful for dating iron meteorites, and is now enjoying greater use for dating Earth rocks due to development of easier rhenium and osmium isotope measurement techniques. Uranium-Lead and related techniques. The uranium-lead method is the longest-used dating method.
It was first used in , about a century ago. The uranium-lead system is more complicated than other parent-daughter systems; it is actually several dating methods put together. Natural uranium consists primarily of two isotopes, U and U, and these isotopes decay with different half-lives to produce lead and lead, respectively. In addition, lead is produced by thorium Only one isotope of lead, lead, is not radiogenic. The uranium-lead system has an interesting complication: Each decays through a series of relatively short-lived radioactive elements that each decay to a lighter element, finally ending up at lead.
Since these half-lives are so short compared to U, U, and thorium, they generally do not affect the overall dating scheme. The result is that one can obtain three independent estimates of the age of a rock by measuring the lead isotopes and their parent isotopes. Long-term dating based on the U, U, and thorium will be discussed briefly here; dating based on some of the shorter-lived intermediate isotopes is discussed later.
The uranium-lead system in its simpler forms, using U, U, and thorium, has proved to be less reliable than many of the other dating systems. This is because both uranium and lead are less easily retained in many of the minerals in which they are found.
Yet the fact that there are three dating systems all in one allows scientists to easily determine whether the system has been disturbed or not. Using slightly more complicated mathematics, different combinations of the lead isotopes and parent isotopes can be plotted in such a way as to. One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. Some of these techniques allow scientists to chart at what points in time metamorphic heating events have occurred, which is also of significant interest to geologists.
The Age of the Earth. We now turn our attention to what the dating systems tell us about the age of the Earth. The most obvious constraint is the age of the oldest rocks. These have been dated at up to about four billion years. But actually only a very small portion of the Earth 's rocks are that old. From satellite data and other measurements we know that the Earth's surface is constantly rearranging itself little by little as Earth quakes occur. Such rearranging cannot occur without some of the Earth's surface disappearing under other parts of the Earth's surface, re-melting some of the rock.
So it appears that none of the rocks have survived from the creation of the Earth without undergoing remelting, metamorphism, or erosion, and all we can say--from this line of evidence--is that the Earth appears to be at least as old as the four billion year old rocks. When scientists began systematically dating meteorites they learned a very interesting thing: These meteorites are chips off the asteroids.
When the asteroids were formed in space, they cooled relatively quickly some of them may never have gotten very warm , so all of their rocks were formed within a few million years. The asteroids' rocks have not been remelted ever since, so the ages have generally not been disturbed. Meteorites that show evidence of being from the largest asteroids have slightly younger ages. The moon is larger than the largest asteroid. Most of the rocks we have from the moon do not exceed 4.
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The samples thought to be the oldest are highly pulverized and difficult to date, though there are a few dates extending all the way to 4. Most scientists think that all the bodies in the solar system were created at about the same time. Evidence from the uranium, thorium, and lead isotopes links the Earth's age with that of the meteorites.
This would make the Earth 4. There is another way to determine the age of the Earth. If we see an hourglass whose sand has run out, we know that it was turned over longer ago than the time interval it measures. Similarly, if we find that a radioactive parent was once abundant but has since run out, we know that it too was set longer ago than the time interval it measures. There are in fact many, many more parent isotopes than those listed in Table 1. However, most of them are no longer found naturally on Earth--they have run out. Their half-lives range down to times shorter than we can measure.
Every single element has radioisotopes that no longer exist on Earth! Many people are familiar with a chart of the elements Fig.
Radioactive dating - The Australian Museum
Nuclear chemists and geologists use a different kind of figure to show all of the isotopes. It is called a chart of the nuclides. Figure 7 shows a portion of this chart. It is basically a plot of the number of protons vs. Recall that an element is defined by how many protons it has. Each element can have a number of different isotopes, that is,. A portion of the chart of the nuclides showing isotopes of argon and potassium, and some of the isotopes of chlorine and calcium.
Isotopes shown in dark green are found in rocks. Isotopes shown in light green have short half-lives, and thus are no longer found in rocks.
Short-lived isotopes can be made for nearly every element in the periodic table, but unless replenished by cosmic rays or other radioactive isotopes, they no longer exist in nature. So each element occupies a single row, while different isotopes of that element lie in different columns. For potassium found in nature, the total neutrons plus protons can add up to 39, 40, or Potassium and are stable, but potassium is unstable, giving us the dating methods discussed above. Besides the stable potassium isotopes and potassium, it is possible to produce a number of other potassium isotopes, but, as shown by the half-lives of these isotopes off to the side, they decay away.
Now, if we look at which radioisotopes still exist and which do not, we find a very interesting fact. Nearly all isotopes with half-lives shorter than half a billion years are no longer in existence. For example, although most rocks contain significant amounts of Calcium, the isotope Calcium half-life , years does not exist just as potassium, , , etc.
Just about the only radioisotopes found naturally are those with very long half-lives of close to a billion years or longer, as illustrated in the time line in Fig. The only isotopes present with shorter half-lives are those that have a source constantly replenishing them. Chlorine shown in Fig. In a number of cases there is. Some of these isotopes and their half-lives are given in Table II. This is conclusive evidence that the solar system was created longer ago than the span of these half lives! On the other hand, the existence in nature of parent isotopes with half lives around a billion years and longer is strong evidence that the Earth was created not longer ago than several billion years.
The Earth is old enough that radioactive isotopes with half-lives less than half a billion years decayed away, but not so old that radioactive isotopes with longer half-lives are gone. This is just like finding hourglasses measuring a long time interval still going, while hourglasses measuring shorter intervals have run out.
Principles of isotopic dating
Years Plutonium 82 million Iodine 16 million Palladium 6. Unlike the radioactive isotopes discussed above, these isotopes are constantly being replenished in small amounts in one of two ways. The bottom two entries, uranium and thorium, are replenished as the long-lived uranium atoms decay. These will be discussed in the next section. The other three, Carbon, beryllium, and chlorine are produced by cosmic rays--high energy particles and photons in space--as they hit the Earth's upper atmosphere.
Very small amounts of each of these isotopes are present in the air we breathe and the water we drink. As a result, living things, both plants and animals, ingest very small amounts of carbon, and lake and sea sediments take up small amounts of beryllium and chlorine The cosmogenic dating clocks work somewhat differently than the others. Carbon in particular is used to date material such as bones, wood, cloth, paper, and other dead tissue from either plants or animals. To a rough approximation, the ratio of carbon to the stable isotopes, carbon and carbon, is relatively constant in the atmosphere and living organisms, and has been well calibrated.
Once a living thing dies, it no longer takes in carbon from food or air, and the amount of carbon starts to drop with time. Since the half-life of carbon is less than 6, years, it can only be used for dating material less than about 45, years old. Dinosaur bones do not have carbon unless contaminated , as the dinosaurs became extinct over 60 million years ago. But some other animals that are now extinct, such as North American mammoths, can be dated by carbon Also, some materials from prehistoric times, as well as Biblical events, can be dated by carbon The carbon dates have been carefully cross-checked with non-radiometric age indicators.
For example growth rings in trees, if counted carefully, are a reliable way to determine the age of a tree. Each growth ring only collects carbon from the air and nutrients during the year it is made. To calibrate carbon, one can analyze carbon from the center several rings of a tree, and then count the rings inward from the living portion to determine the actual age.