Age-dating of rocks

The basic idea in radioactive age-dating of rocks (from the Earth, Moon and meteorites) is to find the ratio of daughter to parent in an isolated system. Thus the age inferred is usually the solidification age which is the time since the last occasion when chemical fractionation was halted by solidification. (K-Ar dating gives a gas-retention age which can be slightly shorter.) 

In general, the daughter nucleus will have an initial abundance at time zero, which can be assumed to bear a constant ratio to another isotope of the same 

A few meteorites have significantly younger ages these are believed to come from the Moon and in some cases from Mars, rather than from asteroids. 

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There are a multitude of papers concerning the age dating of rocks by mass spectrometry on the basis of different geochronological systems with well defined constants of radioactive decay. They include such systems as Rb-Sr, K-Ar, K-Ca, Re-Os, Nd-Sm, U-Pb, Th-Pb, Pb-Pb or Lu-Hf, which are discussed in detail in the literature.26 Therefore only a few relevant examples were briefly sketched here. 

A specific example of applications in the second category is the dating of rocks. Age determination is an inverse problem of radioactive decay, which is a first-order reaction (described later). Because radioactive decay follows a specific law relating concentration and time, and the decay rate is independent of temperature and pressure, the extent of decay is a measure of time passed since the radioactive element is entrapped in a crystal, hence its age. In addition to the age, the initial conditions (such as initial isotopic ratios) may also be inferred, which is another example of inverse problems. 

Defining the movement of water in rocks of low permeability by chemical methods, such as the use of tracers, age dating of water, and isotope ratios, has met with limited success for the same reasons as have the physical methods. That is, they are incapable of describing the permeability distribution in an adequately large area. 

Radioactive dating of rocks brought back by the. / llo astronauts yields an absolute age (the time since the rocks solidified) of the highlands as 3.9 to 3.8 billion years, with the final lava flow around 3 billion years o. 

Huyge, D., Watchman, A., De Dapper, M. Marchi, E. (2001) Dating Egypt s oldest art AMS 14C age determinations of rock varnishes covering petroglyphs at El-Hosh (Upper Egypt). Antiquity 75, 68-72. 

It is clear, for example from radio-carbon dating of rocks in the earth s surface, that the solar system must be very much older than the Kelvin age of 3 x 107 years. It is now taken for granted that the main source of stellar energy comes from nuclear reactions. The fusion of four protons (hydrogen nuclei) to an alpha-particle (helium nucleus) is associated with the release of energy Q, where Q k, 26 MeV. The total available energy is thus... 

Neretnieks, I., 1981. Age dating of groundwater in fissured rock Influence of water volume in micropores. Water Resources Res., v. 17, no. 2, pp. 421 122. 

Age determinations of rocks from the Transantarctic Mountains were included in several compilations of isotopic dates of Antarctic rocks. These compilations include, but are not necessarily limited to Webb (1962), Picciotto and Coppez (1962,1964a, b), Angino and Turner (1963), Ravich and Krylov (1964), Webb and Warren (1965), Craddock (1969b), Stuiver and Braziunas (1985), and others. Stuiver and Braziunas (1985) listed not only radiogenic-isotope dates, but also carbon-14 dates that record enviromnental processes during the most recent past extending to about 30,0(X) years before the present (BP). 

RADIOMETRIC DATING comprises all methods of age determination of rocks and minerals based on nuclear decay of naturally occurring radioactive isotopes. The 84 elements found in nature are represented by 269 stable isotopes and 70 radioactive isotopes. Eighteen of the radioactive isotopes have long half-lives and have survived... 

Other isotopes can be used to determine the age of samples. The age of rocks, for example, has been determined from the ratio of the number of radioactive atoms to the number of stable gfPb atoms produced by radioactive decay. For rocks that do not contain uranium, dating is accomplished by comparing the ratio of radioactive fgK to the stable fgAr. Another example is the dating of sediments collected from lakes by measuring the amount of g Pb present. 

It is estimated that the earth s age is in the neighborhood of 4 to 7 billion years. These estimates are basically derived from carbon-14, potassium-40, uranium-235, and uranium-238 dating of earth rocks and meteorites. The meteorites give important data as to the age of our solar system. Geologic time is felt to be represented by the presence of rock intervals in the geologic column (layers of rock formations in vertical depth) or by the absence of equivalent rocks in correlative columns in adjacent locations [25,26]. The two basic factors that are used to determine geologic time are ... 

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

It seems clear by comparing Fig. 1.159 with Table 1.26 that the ages of hydrothermal mineralization and alterations determined by K-Ar age dating are consistent with those of sedimentary rocks affected by hydrothermal activity in the Oga. Hydrothermal activities were intense at ca. 14-13 Ma, 12.6 Ma, 10.5 Ma, and 8.2 Ma. 

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