How does radioactive dating work quizlet
The final decay product, lead Pb , is stable and can no longer undergo spontaneous radioactive decay. All ordinary is made up of combinations of , each with its own , indicating the number of in the. Additionally, elements may exist in different , with each isotope of an element differing in the number of in the nucleus.
A particular isotope of a particular element is called a. Some nuclides are inherently unstable.
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That is, at some point in time, an atom of such a nuclide will undergo and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including emission of and emission, emission, or. Another possibility is into two or more nuclides. While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays at a rate described by a parameter known as the , usually given in units of years when discussing dating techniques.
In many cases, the daughter nuclide itself is radioactive, resulting in a , eventually ending with the formation of a stable nonradioactive daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e.
For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant. It is not affected by external factors such as , , chemical environment, or presence of a or.
How can radioactive dating be used to determine the age of rocks
The only exceptions are nuclides that decay by the process of electron capture, such as , , and , whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a to measure the time from the incorporation of the original nuclides into a material to the present.
Accuracy of radiometric dating used in radiometric dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of.
Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an.
This can reduce the problem of. In , the is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon has a half-life of 5, years.
After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. The temperature at which this happens is known as the or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by using a high-temperature furnace.
As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. This field is known as or thermochronometry. The age equation plotted of samples from the ,. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis. The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value N o.
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. However, construction of an does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.
Radiometric dating has been carried out since when it was by as a method by which one might determine the. In the century since then the techniques have been greatly improved and expanded. Dating can now be performed on samples as small as a nanogram using a. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s.
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It operates by generating a beam of from the sample under test. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams. Uranium—lead dating method A concordia diagram as used in , with data from the ,.
All the samples show loss of lead isotopes, but the intercept of the errorchron straight line through the sample points and the concordia curve shows the correct age of the rock. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. Uranium—lead dating is often performed on the ZrSiO 4 , though it can be used on other materials, such as , as well as see:.
Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for , but strongly reject lead. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. In situ micro-beam analysis can be achieved via laser or techniques. One of its great advantages is that any sample provides two clocks, one based on uranium's decay to lead with a half-life of about million years, and one based on uranium's decay to lead with a half-life of about 4.
This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. Samarium—neodymium dating method Main article: This involves or decay of potassium to argon Potassium has a half-life of 1. Rubidium—strontium dating method Main article: This is based on the beta decay of to , with a half-life of 50 billion years.
This scheme is used to date old and , and has also been used to date. Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample. Uranium—thorium dating method Main article: A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years.
It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years. While is water-soluble, and are not, and so they are selectively precipitated into ocean-floor , from which their ratios are measured. The scheme has a range of several hundred thousand years.
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A related method is , which measures the ratio of thorium to thorium in ocean sediment. Carbon is a radioactive isotope of carbon, with a half-life of 5, years, which is very short compared with the above isotopes and decays into nitrogen. In other radiometric dating methods, the heavy parent isotopes were produced by in supernovas, meaning that any parent isotope with a short half-life should be extinct by now.
Carbon, though, is continuously created through collisions of neutrons generated by with nitrogen in the and thus remains at a near-constant level on Earth. The carbon ends up as a trace component in atmospheric CO 2. A carbon-based life form acquires carbon during its lifetime. Plants acquire it through , and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years.
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The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon an ideal dating method to date the age of bones or the remains of an organism. The carbon dating limit lies around 58, to 62, years. The rate of creation of carbon appears to be roughly constant, as cross-checks of carbon dating with other dating methods show it gives consistent results. However, local eruptions of or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates.
The releases of carbon dioxide into the as a consequence of have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground tests that were conducted into the early s. Also, an increase in the or the Earth's above the current value would depress the amount of carbon created in the atmosphere.
In years is a result obtained because juvinas is used by using radioactive dating.
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