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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.
U and Th are found on the extremely heavy end of the Periodic Table of Elements. Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium Section 2. We can therefore assume that the Pb is directly formed by the U, the Pb from U-pb dating methods U and the Pb from the Th. The ingrowth equations for the three radiogenic Pb isotopes are given by: 5.
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No notes for slide. Uranium lead method of rock dating 1. By examining layers of sedimentary rock, geologists developed a time scale for dividing up earth history. Earlier in the 20th century, radiometric-dating techniques allowed scientists to put absolute dates on divisions in the geologic time scale.
How do geologists determine how old rocksHow do geologists determine how old rocks are? Relative dating -- determine whether the rock is older or younger than other rocks 2.
Absolute dating -- use radiometric dating techniques to determine how long ago the rock formed in the exact of years 4. The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity.
For example, if one starts with grams of radiumwhose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes Radioisotopes decay through time, and stable isotopes are formed. Determining the ratio of parent isotope to daughter product reveals the of half-lifes that has elapsed.
Common isotopes used in age dating U-Pb -- half-life of U is 4. K-Ar -- half-life of K is 1. Rb-Sr -- half-life of Rb is 47 b. Carbon 14 -- half-life of U-pb dating methods is yrs 8. Diagram indicates the radioactive disintegration of an isotope. In principle, any U-Th bearing minerals such as apatite, xenotime, allanite, uraninite or thorite can be used for dating. U-Pb whole rock analyses have also been used for age determinations.
Monazite Monazite grain cm sized Zircon grain Zircon The average uranium concentrations of the Zircons are below those of zircons from granitic rocks. Dating Of Igneous rocks Isotopic age measurement of U-Pb and Rb-Sr methods on meteorites have yielded solidification age close to m.
The oldest rock and soils from the moon yield U-Pb and Rb-Sr ages close to m.
This method has almost exclusively been used for U-Pb dating of zircons, while other geochronologically important minerals e. The chemical Th-U-total Pb isochron method was applied to the age determination of monazite, and zircon. The chemical Th-U-total Pb isochron ages coincide well with mineral and whole rock ages isotopically determined for the samples.
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