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Petrology Tulane University Prof. Stephen A. Nelson 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. Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Then, in , radioactivity was discovered.

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.

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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.

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Other isotope studies demonstrate that Earth formed within, at most, a few tens of millions of years. History at your fingertips.

Rb sr dating

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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.

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.

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. Although the unreliability of the Rb-Sr method for dating is readily demonstrated, 1, 2 it is important that alternative models are developed to explain the observed patterns of strontium isotopes. The ratios of 88 Sr, 86 Sr, and 84 Sr, relative to each other, are commonly believed to be .

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.

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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.

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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.

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

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.

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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. 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.

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If we divide equation 4 through by the amount of 86 Sr, then we get:. Note also that equation 5 has the form of a linear equation, i.

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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.

Radiometric Dating

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. 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. Another possible scenario involves U leakage, again possibly as a result of a metamorphic event. U leakage would cause discordant points to plot above the cocordia.

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The Age of the Earth A minimum age of the Earth can be obtained from the oldest known rocks on the Earth. So far, the oldest rock found is a tonalitic Gneiss metamorphic rock rock from the Northwest Territories, Canada, with an age of 3.

This gives us only a minimum age of the Earth. Is it likely that we will find a rock formed on the Earth that will give us the true age of the Earth? From the Pb-Pb isochron equation 11 we can make some arguments about meteorites.

First, it appears that meteorites have come from somewhere in the solar system, and thus may have been formed at the same time the solar system and thus the Earth formed. If all of the meteorites formed at the same time and have been closed to U and Pb since their formation, then we can use the Pb-Pb isochron to date all meteorites.

First, however, we need to know the initial ratios of the Pb isotopes.

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We recognize two major types of meteorites: Fe- meteorites and stony or chondritic meteorites The Fe meteorites contain the mineral troilite FeS that has no U. Since the mineral troilite contains no U, all of the Pb present in the troilite is the Pb originally present, and none of it has been produced by U decay. We can then determine the Pb ratios in other meteorites and see if they fall on a Pb-Pb isochron that passes through the initial ratios determined from troilite in Fe-meteorites.

Apr 22,   Rb-Sr dating of sphalerites from Tennessee and the genesis of Mississippi Valley-type ore deposits. Nature, , - 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. 87 Sr* = 87 Rb (e ? t-1) (3) but, 87 Sr t = 87 Sr 0 + 87 Sr* or. 87 Sr* = 87 Sr t - 87 Sr 0. 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.

The slope of this isochron, known as the Geochron, gives an age of 4. K-Ar Dating 40 K is the radioactive isotope of K, and makes up 0.



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