9.5 Radioactive Rocks

Strontium isotopes can be used to determine the ages of rocks. Radioactive rubidium 87 decays into strontium 87, with a half-life of 4.9 billion years. Geologists can use the ratio of Sr-87 to naturally occurring... 

Figure 8 The variables used to calculate the background rate in the fiducial volume from PMT and external rock radioactivity.

Radioactivity is also used to diagnose and treat many other conditions, including cancer, thyroid disease, abnormal kidney and bladder function, and heart disease. These examples from medicine are just a few of the many applications of radioactivity. Naturally occurring radioactivity allows us to estimate the age of fossils and rocks. Radioactivity also led to the discovery of nuclear fission, used for electricity generation and nuclear weapons. In this chapter, we explore radioactivity—how it was discovered, what it is, and how it is used. 

By measuring the levels of certain radioactive elements in fossils or rocks, radioactivity can be used to date objects. The age of Earth is estimated to be 4.5 billion years based on the ratio of uranium to lead in the oldest rocks (8.10-8.12). High levels of radioactivity can kill human life. Lower levels can be used in therapeutic fashion to either diagnose or treat disease (8.13). 

As noted above, the radioactivity produced by a radioisotope is described in terms of its half-life. The persistency of a radiation hazard therefore is a function of this and also the physical form in which the isotope is distributed. Radioisotopes can exist in gas, liquid or sold form. The radioactive gas Radon is namrally released at low levels into the atmosphere from granitic rocks. Radioactive isotopes dispersed as a dust can create a highly persistent contamination whose hazard then depends... 

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. 

Thus, the ratios of lead isotopes 204,206,207 and 208 can vary markedly depending on the source of the lead. One use of these ratios lies in determination of the ages of rocks from the abundances of the various isotopes and the half-lives of their precursor radioactive isotopes. 

From the radioactive decay constants and measurement of the amount of argon in a rock sample, the length of time since formation of the rock can be estimated. Essentially, the dating method requires fusion of a rock sample under high vacuum to release the argon gas that has collected through radioactive decay of potassium. The amount of argon is determined mass spectrometrically,... 

Isolation of radioactive wastes for long periods to allow adequate decay is sought by the use of multiple barriers. These include the waste form itself, the primary containers made of resistant materials, overpacks as secondary layers, buffer materials, concrete vaults, and finally the host rock or sod. Barriers limit water access to the waste and minimize contamination of water suppHes. The length of time wastes must remain secure is dependent on the regulatory limit of the maximum radiation exposure of individuals in the vicinity of the disposal site. 

Underground chambers are also constructed in frozen earth (see subsection Xow-Temperature and Cryogenic Storage ). Underground tunnel or tank storage is often the most practical way of storing hazardous or radioactive materials. A cover of 30 m (100 ft) of rock or dense earth can exert a pressure of about 690 kPa (100 Ibf in"). 

Radon (Rn) is a naturally occurring radioactive gas. Radon enters buildings from underlying soil and rocks as soil gas is drawn into buildings. 

For example, consider the chemical composition of a very old crystal of pitchblende, U308. We may presume that this crystal was formed at a time when chemical conditions for its formation were favorable. For example, it may have precipitated from molten rock during cooling. The resulting crystals tend to exclude impurities. Yet, careful analysis shows that every deposit of pitchblende contains a small amount of lead. This lead has accumulated in the crystal, beginning at the moment the pure crystal was formed, due to the radioactive decay of the uranium. 

In order to understand the impact of pollution on Earth, we must realize that the planet itself is not stagnant, but continually moving material around the system naturally. Any human (anthropogenic) redistribution in the elements is superimposed on these continuous natural events. Energy from the sun and radioactive decay from the Earth s interior drive these processes, which are often cyclic in nature. As a result, almost all of the rocks composing the continents have been processed at least once through a chemical and physical cycle involving... 

If the amount of a radioactive nuclide in a rock sample is N, the sum of this amount plus the amount of its product nuclide is A/q. For argon dating, Nq is the sum of potassium-40 and argon-40 present in a sample of rock. Assuming that Ar gas escapes from molten rock but is trapped when the rock cools and solidifies, the lifetime obtained by substituting these values into Equation is the time since the rock solidified. Such analyses show that the oldest rock samples on Earth are 3.8 X 10 years old. 

Figure 23. Measured ( °Th/ Th) ratios in basalts from Piton de la Fournaise (Reunion Island) as a function of their eraption ages deduced from mineral isochrons. These ratios decrease with increasing emption ages as a result of post-eraptive radioactive decay. The curve shows the theoretical evolution by radioactive decay for a rock with a Th/U ratio of 3.95 and a ( °Th/ Th) ratio of 0.93, similar to the values measured in presently erapted lavas. An approximate age can thus be obtained from the measured ( °Th/ Th) ratio of an old sample. Part of the dispersion around the theoretical curve are due to small source heterogeneities (slightly variable ( °Th/ rh) and Th/U ratios), also evidenced by Sr/ Sr ratios (Condomines et al. 1988, and unpublished results).

Capaldi G, Cortini M, Gasparini P, Pece R (1976) Short lived radioactive diseqttihbria in freshly erupted volcanic rocks and their implications for the pre-emption history of a magma. J Geophys Res 81 350-... 

ChemiakDJ (2002) Ba diffusion in feldspar. Geochim Cosmochim Acta 66 1641-1650 Cherdyntsev W, Kishtsina Gl, Kuptsov VM, Kuzmina YA, Zverev VL (1967) Radioactivity and absolute age of yoimg volcanic rocks. Geokhimiya 7 755-762... 

Rosholt NJ (1967) Open system model for uranium-series dating of Pleistocene samples. In Radioactive Dating and Methods of low-level Counting. 1. A. E. A. Proc Ser Publ, SM-87/50, p 299-311 Rosholt JN, Antal PS (1962) Evaluation of the Pa /U-Th °/U method for dating Pleistocene carbonate rocks. US Geol Survey Prof Paper 450-E 108-lll... 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

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