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Radiometric dating

RADIOMETRIC DATING OF ROCKS

Originally fossils only provided us with relative ages because, although early paleontologists understood biological succession, they did not know the absolute ages of the different organisms. It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils. In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, 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 in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time. One of the isotope pairs widely used in geology is the decay of 40 K to 40 Ar potassium to argon

Although a more difficult and expensive method, Ar—Ar is now preferred to K—Ar. The effects of alteration can be eliminated by step-heating the sample during determination of the amounts of 39 Ar and 40 Ar present by mass spectrometer. Alteration and hence 40 Ar loss occurs at lower temperatures than the original crystallisation so the isotope ratios measured at different temperatures will be different. The sample is heated until there is no change in ratio with increase in temperature a 'plateau' is reached : this ratio is then used to calculate the age.

If no 'plateau' is achieved and the ratio changes with each temperature step the sample is known to be too altered to provide a reliable date. This is a widely used method for dating igneous rocks because the parent element, rubidium, is common as a trace element in many silicate minerals.

The isotope 87 Rb decays by shedding an electron beta decay to 87 Sr with a half-life of 48 billion years. The proportions of two of the isotopes of strontium, 86 Sr and 87 Sr, are measured and the ratio of 86 Sr to 87 Sr will depend on two factors. First, this ratio will depend on the proportions in the original magma: this will be constant for a particular magma body but will vary between different bodies.

Second, the amount of 87 Sr present will vary according to the amount produced by the decay of 87 Rb: this depends on the amount of rubidium present in the rock and the age. The rubidium and strontium concentrations in the rock can be measured by geochemical analytical techniques such as XRF X-ray fluorescence.

The principle of solving simultaneous equations can be used to resolve these two unknowns. An alternative method is whole-rock dating, in which samples from different parts of an igneous body are taken, which, if they have crystallised at different times, will contain different amounts of rubidium and strontium present.

This is more straightforward than dating individual minerals as it does not require the separation of these minerals.

Isotopes of uranium are all unstable and decay to daughter elements that include thorium, radon and lead. Two decays are important in radiometric dating: U to Pb with a half-life of 4.

By measuring the proportions of the parent and daughter isotopes in the two decay series it is possible to determine the amount of lead in a mineral produced by radioactive decay and hence calculate the age of the mineral.

Trace amounts of uranium are to be found in minerals such as zircon, monazite, sphene and apatite: these occur as accessory minerals in igneous rocks and as heavy minerals in sediments. Dating of zircon grains using uranium—lead dating provides information about provenance of the sediment.

8.4 Isotopic Dating Methods

Dating of zircons has been used to establish the age of the oldest rocks in the world. Other parts of the uranium decay series are used in dating in the Quaternary. These two rare earth elements in this decay series are normally only present in parts per million in rocks.

The parent isotope is Sm and this decays by alpha particle emission to Nd with a half-life of billion years. The slow generation of Nd means that this technique is best suited to older rocks as the effects of analytical errors are less significant. The advantage of using this decay series is that the two elements behave almost identically in geochemical reactions and any alteration of the rock is likely to affect the two isotopes to equal degrees.

Common Types of Radiometric Dating Since argon is a noble gas, it would have escaped the rock-formation process, and therefore any argon in a rock. Radiometric dating. Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. What type of the use of known form of the basic principle of radiometric rocks can be used to determine the oldest known ages of. We have been dated using a.

This eliminates some of the problems encountered with Rb—Sr caused by the different reactivity and mobility of the two elements in the decay series.

This dating technique has been used on sediments to provide information about the age of the rocks that the sediment was derived from: different provenance areas, for example continental cratons of different ages, can be distinguished by analysis of mud and mudstones. Rhenium occurs in low concentrations in most rocks, but its most abundant naturally occurring isotope Re undergoes beta decay to an isotope of osmium Os with a half-life of 42 Ga.

This dating technique has been used mainly on sulphide ore bodies and basalts, but there have also been some successful attempts to date the depositional age of mudrocks with a high organic content. Osmium isotopes in seawater have also been shown to have varied through time. Radiometric dating is the only technique that can provide absolute ages of rocks through the stratigraphic record, but it is limited in application by the types of rocks which can be dated.

The age of formation of minerals is determined by this method, so if orthoclase feldspar grains in a sandstone are dated radiometrically, the date obtained would be that of the granite the grains were eroded from.

Radioactive dating works best with what type of rocks?

It is therefore not possible to date the formation of rocks made up from detrital grains and this excludes most sandstones, mudrocks and conglomerates. Limestones are formed largely from the remains of organisms with calcium carbonate hard parts, and the minerals aragonite and calcite cannot be dated radiometrically on a geological time scale.

Hence almost all sedimentary rocks are excluded from this method of dating and correlation. An exception to this is the mineral glauconite, an authigenic mineral that forms in shallow marine environments: glauconite contains potassium and may be dated by K—Ar or Ar—Ar methods, but the mineral is readily altered and limited in occurrence.

The formation of igneous rocks usually can be dated successfully provided that they have not been severely altered or metamorphosed. Intrusive bodies, including dykes and sills, and the products of volcanic activity lavas and tuff may be dated and these dates used to constrain the ages of the rocks around them by the laws of stratigraphic relationships.

Dates from metamorphic rocks may provide the age of metamorphism, although complications can arise if the degree of metamorphism has not been high enough to reset the radiometric 'clock', or if there have been multiple phases of metamorphism. General stratigraphic relations and isotopic ages are the principal means of correlating intrusive igneous bodies. Geographically separate units of igneous rock can be shown to be part of the same igneous suite or complex by determining the isotopic ages of the rocks at each locality.

Radiometric dating can also be very useful for demonstrating correspondence between extrusive igneous bodies. The main drawbacks of correlation by this method are the limited range of lithologies that can be dated and problems of precision of the results, particularly with older rocks. For example, if two lava beds were formed only a million years apart and there is a margin of error in the dating methods of one million years, correlation of a lava bed of unknown affinity to one or the other cannot be certain.

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Popular Tags Blog Archives. In this case, fossils can be useful tools for understanding the relative ages of rocks.

Each fossil species reflects a unique period of time in Earth's history. The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4.

Fossils occur for a distinct, limited interval of time. In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil.

The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence — when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i. For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed.

If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box.

Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. Because they are often rare, primate fossils are not usually good index fossils.

Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly. Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time Figure 4.

If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time. Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas. All elements contain protons and neutronslocated in the atomic nucleusand electrons that orbit around the nucleus Figure 5a.

In each element, the number of protons is constant while the number of neutrons and electrons can vary. Atoms of the same element but with different number of neutrons are called isotopes of that element.

Radiometric dating types of rock

Each isotope is identified by its atomic masswhich is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon 12 12 Ccarbon 13 13 Cand carbon 14 14 C Figure 5a.

C 12 and C 13 are stable. The atomic nucleus in C 14 is unstable making the isotope radioactive. Because it is unstable, occasionally C 14 undergoes radioactive decay to become stable nitrogen N The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope. Most isotopes found on Earth are generally stable and do not change.

However some isotopes, like 14 C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both.

Radiometric dating of minerals in metamorphic rocks usually indicates the age of A number of elements have isotopes (forms of the element that have different. Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which. Using relative and radiometric dating methods, geologists are able to answer To establish the age of a rock or a fossil, researchers use some type of clock to.

This change is called radioactive decay. For example, unstable 14 C transforms to stable nitrogen 14 N. The atomic nucleus that decays is called the parent isotope. The product of the decay is called the daughter isotope. In the example, 14 C is the parent and 14 N is the daughter.

Feldspar does not have any argon in it when it forms. Radiocarbon dating can be used on sediments or sedimentary rocks that contain carbon, but it cannot be . The best rocks to use for radiometric dating are igneous rocks. As newly deposited magma/lava cools, it forms igneous rock. In this type of rock, the See full. Each type of radioactive isotope has a half-life, a length of time that it will take with minerals containing radioactive isotopes, it is as though the rock crystallizes .

Some minerals in rocks and organic matter e. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1.

The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay argon, a gas, escapes into the atmosphere while the lava is still molten.

When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral. The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope Figure 5b. When the quantities of the parent and daughter isotopes are equal, one half-life has occurred.

How Carbon Dating Works

If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated. For example, if the measured abundance of 14 C and 14 N in a bone are equal, one half-life has passed and the bone is 5, years old an amount equal to the half-life of 14 C.

If there is three times less 14 C than 14 N in the bone, two half lives have passed and the sample is 11, years old.

However, if the bone is 70, years or older the amount of 14 C left in the bone will be too small to measure accurately. Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past.

Luckily, there are methods, such as the commonly used potassium-argon K-Ar methodthat allows dating of materials that are beyond the limit of radiocarbon dating Table 1. Comparison of commonly used dating methods. Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material. Dating methods like thermoluminescenceoptical stimulating luminescence and electron spin resonancemeasure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material.

If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material.

These methods are applicable to materials that are up to aboutyears old. However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged. The Earth is like a gigantic magnet.

It has a magnetic north and south pole and its magnetic field is everywhere Figure 6a. Just as the magnetic needle in a compass will point toward magnetic north, small magnetic minerals that occur naturally in rocks point toward magnetic north, approximately parallel to the Earth's magnetic field.

Because of this, magnetic minerals in rocks are excellent recorders of the orientation, or polarityof the Earth's magnetic field. Small magnetic grains in rocks will orient themselves to be parallel to the direction of the magnetic field pointing towards the north pole. Black bands indicate times of normal polarity and white bands indicate times of reversed polarity. Through geologic time, the polarity of the Earth's magnetic field has switched, causing reversals in polarity.

The Earth's magnetic field is generated by electrical currents that are produced by convection in the Earth's core. During magnetic reversals, there are probably changes in convection in the Earth's core leading to changes in the magnetic field. The Earth's magnetic field has reversed many times during its history.

When the magnetic north pole is close to the geographic north pole as it is todayit is called normal polarity. Reversed polarity is when the magnetic "north" is near the geographic south pole. Using radiometric dates and measurements of the ancient magnetic polarity in volcanic and sedimentary rocks termed paleomagnetismgeologists have been able to determine precisely when magnetic reversals occurred in the past.

Combined observations of this type have led to the development of the geomagnetic polarity time scale GPTS Figure 6b.

The GPTS is divided into periods of normal polarity and reversed polarity. Geologists can measure the paleomagnetism of rocks at a site to reveal its record of ancient magnetic reversals. Every reversal looks the same in the rock record, so other lines of evidence are needed to correlate the site to the GPTS.

Information such as index fossils or radiometric dates can be used to correlate a particular paleomagnetic reversal to a known reversal in the GPTS. Once one reversal has been related to the GPTS, the numerical age of the entire sequence can be determined. Using a variety of methods, geologists are able to determine the age of geological materials to answer the question: "how old is this fossil?

These methods use the principles of stratigraphy to place events recorded in rocks from oldest to youngest. Absolute dating methods determine how much time has passed since rocks formed by measuring the radioactive decay of isotopes or the effects of radiation on the crystal structure of minerals.

Paleomagnetism measures the ancient orientation of the Earth's magnetic field to help determine the age of rocks. Deino, A. Evolutionary Anthropology 6 : Faure, G. Isotopes: Principles and Applications.

Third Edition. New York: John Wiley and Sons Gradstein, F. The Geologic Time Scale2-volume set.

Waltham, MA: Elsevier Ludwig, K. Geochronology on the paleoanthropological time scale, Evolutionary Anthropology 9, McDougall I.

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