Radioactivity of Rocks

radioactivity determined by the content of radioactive elements in rocks. These elements include the members of the radioactive decay series and and the radioactive isotope . The content of other radioactive isotopes does not significantly affect the total radioactivity of rocks because the rate of radioactive decay of these isotopes is extremely low. The average content of both isotopes of uranium in the earth’s crust (to a depth of 16 km) is about 2.5 × 10^–4 percent by weight; the content of thorium is 1.3 × 10^–3 percent and that of the radioactive isotope of potassium is 0.029 percent. Furthermore, decay products of radioactive elements are present in rocks and sometimes migrate to surrounding rock and form streams of underground gases (He, Ar). Radon, which is of radiogenic origin, accumulates in soils.

Among igneous rocks, persilicic types have the greatest radioactivity (U—3.5 × 10^–4; Th—1.8 × 10^–3), and ultrabasic types the least (U—3 × 10^–7; Th—5 × 10^–7). In crystalline rocks, radioactive elements are constituents of such accessory minerals as allanite, zircon, monazite, apatite, and sphene and are present in the form of oxides not chemically bound to any specific minerals.

The content of radioactive elements in sedimentary rocks (U—3.2 × 10^–4; Th—1.1 × 10^–3) is determined by their origin. The maximum concentrations in organogenic deposits result from the presence of carbon of organic origin and of phosphates and other substances that are important precipitating agents of uranium. On the other hand, chemogenic deposits, such as gypsum and rock salt, are distinguished by a low radioactivity.

In soils, the ratio of Th to U is significantly higher than in bedrock. This condition derives from the accumulation of Th in indestructible rock fragments and from the ready migration of U.

In young bathypelagic deposits, accumulations of ionium —an isotope of Th and a member of the radioactive decay series—are observed that are tens of times greater than its equilibrium content in uranium. These accumulations of ionium result from its chemical nature; ionium precipitates from water, whereas U remains in solution.

The crystalline rocks of the moon (basalt, anorthosite) are markedly poor in radioactive elements (U—0.24 × 10^–4; Th—1.14 × 10^–4). In the rocks of Venus, the ratios of U (2.2 × 10^–4) and Th (6.5 × 10^–4) are close to those of the earth. Stony meteorites contain U—1.5 × 10–6 and Th—4 × 10^–6.

The Irish geologist J. Joly in 1905 was the first to point out that the radioactivity of rocks is important as a source of the earth’s heat energy. Calculations have shown that if the concentration of radioactive elements in the earth’s volume were the same as that in the surface layer, then the total amount of heat generated as a result of radioactive decay would exceed by a factor of several tens the amount of heat lost by the earth through irradiation into space. The conclusion is therefore drawn that all the radioactive elements are concentrated in the upper part of the earth’s crust. This hypothesis was partially confirmed in the 1970’s with the measurement of the concentrations of U and Th (10^–6 percent) in rock samples of the earth’s mantle taken from beneath the ocean floor.

In the years 1923–27, the Norwegian scientist V. M. Gold-schmidt showed that the content of radioactive elements located mainly in the upper (granitic) shell of the earth is related to the chemical behavior of silicates, that is, to the isomorphous intrusion of U and Th into the structure of silicates. The extraction of the silicate crust of the earth from the mantle according to the principle of zone fusion inevitably leads to the enrichment of the crust with U, Th, and alkaline-earth elements.

In the initial stage of the earth’s development, the emission of radiogenic heat, according to the calculations of the Soviet geophysicist E. A. Liubimova, was five times greater than in the present period. This difference is related to the greater radioactivity of rocks in the past as a consequence of the higher content of radioactive elements (mainly and ) and also, probably, to transuranium elements that have since disappeared entirely.