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The radioactive decay of rubidium 87 Rb to strontium 87 Sr was the first widely used dating system that utilized the isochron method. Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium compared with the stable isotopes. A ratio for average continental crust of about 0. This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70, it is quite significant. Dissolved strontium in the oceans today has a value of 0. Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited.

The decay schemes are as follows. Note that the present ratio of. If these two independent dates are the same, we say they are concordant.

We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates. The Concordia curve can be calculated by defining the following:.

### SVT - TS - Datation radiochronologie Rb/Sr : partie 1

Zircon has a high hardness 7. Zircon can also survive metamorphism. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced. Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene. Discordant dates will not fall on the Concordia curve.

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Sometimes, however, numerous discordant dates from the same rock will plot along a line representing a chord on the Concordia diagram. Such a chord is called a discordia.

We can also define what are called Pb-Pb Isochrons by combining the two isochron equations 7 and 8. Since we know that theand assuming that the Pb and Pb dates are the same, then equation 11 is the equation for a family of lines that have a slope. The answer is about 6 billion years.

This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. It does, however, give a maximum age of the Earth. Is this the age of the Earth?

Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Modern Oceanic Pb - i.

Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon. Thus, our best estimate of the age of the Earth is 4. The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks.

# Rb-sr dating limitations

Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40 K is important in dating rocks. But this scheme is not used because 40 Ca can be present as both radiogenic and non-radiogenic Ca. Since Ar is a noble gas, it can escape from a magma or liquid easily, and it is thus assumed that no 40 Ar is present initially.

Note that this is not always true. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped. If this happens, then the date obtained will be older than the date at which the magma erupted.

For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar. Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.

The dating equation used for K-Ar is:. Some of the problems associated with K-Ar dating are Excess argon. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Minerals should not contain any excess Ar because Ar should not enter the crystal structure of a mineral when it crystallizes.

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Thus, it always better to date minerals that have high K contents, such as sanidine or biotite. If these are not present, Plagioclase or hornblende.

If none of these are present, then the only alternative is to date whole rocks.

Atmospheric Argon. Some 40 Ar could be absorbed onto the sample surface. This can be corrected for. Metamorphism or alteration.

Most minerals will lose Ar on heating above o C - thus metamorphism can cause a loss of Ar or a partial loss of Ar which will reset the atomic clock.

If only partial loss of Ar occurs then the age determined will be in between the age of crystallization and the age of metamorphism.

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If complete loss of Ar occurs during metamorphism, then the date is that of the metamorphic event. The problem is that there is no way of knowing whether or not partial or complete loss of Ar has occurred.

Examples of questions on this material that could be asked on an exam. Radiometric Dating. Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state.

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Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus. Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.

Some examples of isotope systems used to date geologic materials. If we divide equation 4 through by the amount of 86 Sr, then we get:.

Rubidium-strontium dating, method of estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium formed by the decay of the unstable isotope rubidium that was present in the rock at the time of its formation. Rubidium comprises percent of the total atomic abundance of rubidium, and of the four isotopes of strontium, only. The Rb-Sr beta-decay dating system is one of the most attractive tools in geochronology, as Rb is sufficiently abundant in common K-bearing minerals like biotite, muscovite and K-feldspar. This. Rb/Sr Age Dating Summary Average 3, Maximum 27, Minimum -3, Difference 31, Table 16 Of the 21 isotopic ratios, seven were below million years old. Two were over six billion years old. History Of The Pasamonte AchondriteAuthor: Paul Nethercott.

Note also that equation 5 has the form of a linear equation, i. How can we use this? In nature, however, each mineral in the rock is likely to have a different amount of 87 Rb. Thus, once the rock has cooled to the point where diffusion of elements does not occur, the 87 Rb in each mineral will decay to 87 Sr, and each mineral will have a different 87 Rb and 87 Sr after passage of time.

The discordia is often interpreted by extrapolating both ends to intersect the Concordia. Pb leakage is the most likely cause of discordant dates, since Pb will be occupying a site in the crystal that has suffered radiation damage as a result of U decay.

The Rb/Sr System. by ? decay. The neutron emits an electron to become a proton. For this decay reaction, ? = x /yr, ? 1/2 = x 10 10 yr at present, of natural Rb is 87 Rb. If we use this system to plug into equation (2), then. The Rb-Sr method. usually the isochron method is employed to determine the age and initial. Sr/ Sr ratio of a suite of rock samples. 87Rb/86Sr y = 0 5 10 15 20 25 30 35 8 7 S r / 8 6 S r Rb-Sr isochron diagram for a series of cogenetic rock samples formed at the same time ( Sr/ 86 Sr) 0 = Dating - Dating - Rubidium-strontium method: The radioactive decay of rubidium (87Rb) to strontium (87Sr) was the first widely used dating system that utilized the isochron method. Rubidium is a relatively abundant trace element in Earth's crust and can be found in many common rock-forming minerals in which it substitutes for the major element potassium.

U would have been stable in the crystallographic site, but the site is now occupied by by Pb. An event like metamorphism could heat the crystal to the point where Pb will become mobile.

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The radioactive decay of rubidium 87Rb to strontium 87Sr was the first widely used dating system that utilized Learn More in these related Britannica articles:. The radioactive decay of rubidium 87 Rb to strontium 87 Sr was the first widely used dating system that utilized the isochron method.

Rubidium-strontium Rb-Sr dating was the first technique in which the whole-rock isochron method was extensively employed. Certain rocks that cooled quickly at the surface were found to give precisely defined linear isochrons, but many others did not.

Some studies have shown that rubidium is very. For example, the concentrations of rubidium and the strontium into which it decays, or those of samarium and its decay product neodymium, indicate that the oldest meteorites formed some 4. Other isotope studies demonstrate that Earth formed within, at most, a few tens of millions of years. History at your fingertips.

The rubidium-strontium (Rb-Sr) method is used to determine ages of geologic events, and it serves as a tracer of geochemical processes. The method is based upon the radioactive decay of Rb, which is present in trace abundances (but never as a major element) in many common minerals. Nov 17, Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. Basaltic rocks are more altered than granitic rocks therefore those are not preferred for dating. DRAWBACKS- Uses of Rb-Sr Dating geochronology Isotope geochemistry Strontium Isotope stratigraphy. Apr 22, The Rb-Sr dating technique is among the most widely used and most powerful dating tools available in Earth sciences. It is an effective means of dating igneous rocks or metamorphic events and, under special circumstances, can be applied to sedimentary sequences, ore deposits, and past volcanic eruptions or to date fluid events in the deep crust.

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