Which rock types are most commonly used in radiometric dating

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The oldest in the Solar System are 4. Some commonly used radiometric systems: Note that the effective range of these dating systems is limited by the degree of error in measurement. Which rocks are useful for radiometric dating?


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When you radiometrically date a mineral grain you are determining when it crystallized. Thus, you would like to use rocks whose crystals are roughly the same age. The easiest are igneous rocks in which all crystals are roughly the same age, having solidified at about the same time. The age of new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating. The problem is that metamorphism - the pressure-cooking of rocks - can occur over long intervals. Thus, different crystal grains can yield different ages.

With sedimentary rocks, one would end up dating the individual grains of sediment comprising the rock, not the rock as a whole. These grains could have radically different ages. So, geologists prefer to work with igneous rocks. Useful to archaeologists, maybe, but system is not typically used on rocks at all.

Thus, sedimentary and metamorphic rocks can't be radiometrically dated. Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded. Magnetostratigraphy The Earth generates a magnetic field that encompasses the entire planet. In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field.

It is, in essence a form of relative dating. Some magnetic minerals, such as magnetite occur naturally in igneous rocks. When their grains form, they align themselves with the Earth's magnetic field. The Earth's magnetic field changes quickly i. Nevertheless, because of the orientation of their magnetic minerals, their intrinsic magnetic field records the orientation of the Earth's field as it existed when they formed. Such ancient magnetic fields are called remnant or paleomagnetism. The Earth's magnetic field has a north and south pole.

For unknown reasons, at intervals of very roughly , years, the north and south poles trade places. The result is that the paleomagnetic polarity of igneous rocks is either: Magnetic north coincides roughly with geographic north. Magnetic north coincides roughly with geographic south. If we drill a core form layers of rocks with paleomagnetism, and color-code ones with normal and reverse polarity, we get a pattern like a bar code.

Any interval of time we designate will display a unique pattern of paleomagnetic reversals. What kinds of rocks retain paleomagnetism: Igneous, for reasons noted. Some sedimentary rocks retain paleomagentism when they contain minerals derived form earlier igneous rocks. Three requirements need to be met: Sediments consist of very small grains that settle slowly from water Sediments include magnetic minerals Sediments were deposited in very quiet body of water, like a lake.

The fact that sediments can record paleomagnetism is very useful. Remember, we have no means of directly measuring the radiometric age of sediments that aren't preserved in association with igneous rocks. We can , however, hang a numerical age on them if their paleomagnetic "fingerprint" can be matched with that of a sequence of igneous rocks that can be radiometrically dated.

By studying paleomagnetic polarity of rocks of different ages, geologists have developed a paleomagnetic time scale that is correlated with the regular time scale.

The scale consists of chrons a. The study of the history of paleomagnetic reversals is called magnetostratigraphy. The utility of paleomagnetism: Radiometric dates are always subject to margins of error, whereas a rock's paleomagnetic polarity is absolute. Knowing the paleomagnetic polarity of a sample can, therefore, give an independent means of constraining its age. Most rocks that preserve paleomagnetism igneous can also be radiometrically dated. Because some sedimentary rocks can also retain paleomagnetism, then by knowing their polarity, we can assign them more reliable absolute dates by correlating them with igneous rocks of the same paleomagnetic chron.

Radiometric or Absolute Rock Dating

While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life , usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product.

In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain , 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 temperature , pressure , chemical environment, or presence of a magnetic or electric field.

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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 clock to measure the time from the incorporation of the original nuclides into a material to the present.

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 alteration. 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 isochron. This can reduce the problem of contamination. In uranium—lead dating , the concordia diagram 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.

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Radiometric dating - Wikipedia

For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement except as described below under "Dating with short-lived extinct radionuclides" , the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material.

The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. 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.

If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion , setting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system.

These temperatures are experimentally determined in the lab by artificially resetting sample minerals 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 thermochronology or thermochronometry. The mathematical expression that relates radioactive decay to geologic time is [12] [15]. 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. 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 invented by Ernest Rutherford as a method by which one might determine the age of the Earth.

In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. It operates by generating a beam of ionized atoms from the sample under test.

The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization. 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.

Radiometric dating

Uranium—lead radiometric dating involves using uranium or uranium to date a substance's absolute age. 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 mineral zircon ZrSiO 4 , though it can be used on other materials, such as baddeleyite , as well as monazite see: 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. 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.


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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. This involves the alpha decay of Sm to Nd with a half-life of 1. Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable. This involves electron capture or positron decay of potassium to argon Potassium has a half-life of 1. This is based on the beta decay of rubidium to strontium , with a half-life of 50 billion years.

This scheme is used to date old igneous and metamorphic rocks , and has also been used to date lunar samples. Closure temperatures are so high that they are not a concern.