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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. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: It provided another source of heat, not considered by Kelvin, which would mean that the cooling time would have to be much longer.

It provided a means by which the age of the Earth could be determined independently.

Model ages can be calculated for an individual rock from a single pair of Sm-Nd isotopic ratios. The basis of all such model ages is an assumption about the isotopic composition of the mantle source region from which the samples were originally derived. Care must be exercised in their interpretation. A key strength of Sm-Nd dating is that it involves two rare earth elements (REE). The behavior of the REE during geologic processes is relatively well understood because, with the exception of Eu Author: Richard W. Carlson. The decay of Sm to Nd allows Nd/Nd to be used to trace Sm/Nd fractionation in long time?scale geologic processes. Nd/Nd has been measured in .

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.

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The energies involved are so large, and the nucleus is so small that physical conditions in the Earth i. T and P cannot affect the rate of decay. The rate of decay or rate of change of the number N of particles is proportional to the number present at any time, i. So, we can write.

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After the passage of two half-lives only 0. This can only be done for 14 C, since we know N 0 from the atmospheric ratio, assumed to be constant through time. For other systems we have to proceed further. The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock.

We can see how do deal with this if we take a particular case. The neutron emits an electron to become a proton.

Nov 01, By measuring the Sm-Nd and Rb-Sr isotopic compositions of harzburgite and gabbro from Shimian ophiolite suite, we got the whole rock Sm-Nd isochron age of (±30) Ma (2?), and the ?Nd of ± (2?), which shows that the ophiolite was formed at the Early Neoproterozoic. The obvious change (-) of ISr values of the ophiolite is caused by the meteoric hydrothermal Cited by: 9. Sm-Nd Isotopic Systematics According to the article20this specimen [Enderby Land, East Antarctic] was dated in by scientists from the Australian National University, Canberra, and the Bureau of Mineral Resources, Canberra. According to the article20the age of the sample is 3, million bullbreedadvisoryservice.com: Paul Nethercott. Jul 07, Whole rock Rb-Sr and Sm-Nd analyses. For whole rock Rb-Sr and Sm-Nd isotopic analyses, the sample preparation and purification methods were the same as in Pandey et al. The Rb-Sr and Sm-Nd measurements were performed at the .

We still don't know 87 Sr 0the amount of 87 Sr daughter element initially present. Thus, 86 Sr is a stable isotope, and the amount of 86 Sr does not change through time.

So, applying this simplification. The reason for this is that Rb has become distributed unequally through the Earth over time. For example the amount of Rb in mantle rocks is generally low, i. Thus we could tell whether the rock was derived from the mantle or crust be determining its initial Sr isotopic ratio as we discussed previously in the section on igneous rocks.

Two isotopes of Uranium and one isotope of Th are radioactive and decay to produce various isotopes of Pb. 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:.

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.

Sm - Nd Dating. Sm > Nd. ? = 6 Which isotopic systems are most useful for radiometric dating and what are the limitations of each? What is an isochron and what information can be obtained from an isochron? Why is zircon the preferred mineral for obtainting U - Pb dates? Jun 29, The results of Sm-Nd and Ar-Ar isotopic dating are presented in Table 1 and 2, respectively, and plotted in isochron diagram in Fig. 5 and 6, respectively,. The Sm-Nd dated samples are from different locations in the Nuri deposit, thus the .

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.

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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. 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. 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.

### Evolution of the Solar System Inferred from Sm Nd Isotopic Studies - Lars Borg (SETI Talks 2016)

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. They have relatively homogeneous trace-element signatures, and therefore their isotopic evolution can model the evolution of the whole Solar system and of the "bulk Earth". This is called the epsilon notation, whereby one epsilon unit represents a one part per 10, deviation from the CHUR composition. Since epsilon units are finer and therefore a more tangible representation of the initial Nd isotope ratio, by using these instead of the initial isotopic ratios, it is easier to comprehend and therefore compare initial ratios of crust with different ages.

In addition, epsilon units will normalize the initial ratios to CHUR, thus eliminating any effects caused by various analytical mass fractionation correction methods applied. This fractionation would then cause a deviation between the crustal and mantle isotopic evolution lines. The intersection between these two evolution lines then indicates the crustal formation age.

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The T CHUR age of a rock can yield a formation age for the crust as a whole if the sample has not suffered disturbance after its formation. This therefore allows crustal formation ages to be calculated, despite any metamorphism the sample has undergone.

The composition of the depleted reservoir relative to the CHUR evolution line, at time Tis given by the equation. Sm-Nd model ages calculated using this curve are denoted as TDM ages.

From Wikipedia, the free encyclopedia. Redirected from Samarium-neodymium dating. Bibcode : Sci Geophysical Research Letters. Bibcode : GeoRL Radiogenic Isotope Geology2nd ed.

Cambridge: Cambridge University Press. Neodymium isotopes in the Colorado Front Range and crust - mantle evolution in the Proterozoic. Nature- Periods Eras Epochs.

# Sm-nd isotopic dating

Canon of Kings Lists of kings Limmu. Chinese Japanese Korean Vietnamese. Lunisolar Solar Lunar Astronomical year numbering.

Deep time Geological history of Earth Geological time units. Chronostratigraphy Geochronology Isotope geochemistry Law of superposition Luminescence dating Samarium-neodymium dating. Amino acid racemisation Archaeomagnetic dating Dendrochronology Ice core Incremental dating Lichenometry Paleomagnetism Radiometric dating Radiocarbon Uranium-lead Potassium-argon Tephrochronology Luminescence dating Thermoluminescence dating.