Isotope Methods in Geology

methods of studying geological processes based on the investigation of the content and ratios of radioactive, radiogenic, and stable isotopes of chemical elements in rocks, minerals, natural waters, gases, and organic matter.

The methods of absolute geochronology are developed best and are extensively used to determine the absolute age of rocks and minerals according to the ratio of radioactive isotopes and daughter products of their decay, for example, ²³⁵U–²⁰⁷Pb; 238^U–²⁰⁶pb; ²³²Th–²⁰⁸pb; ⁸⁷Rb–⁸⁷Sr; ⁴⁰K–⁴⁰Ar The ages of earth and lunar rocks and of meteorites have been determined by the methods of absolute geochronology. The radiation age of meteorites (the time they have been exposed to cosmic radiation) is judged by the isotopic content of inert gases (Ar, Xe, and numerous others). The isotopic composition of the inert gases of the earth and of meteorites carries a wealth of information about characteristics of the formation of the matter of the solar system. The content of ^UC (T_½ = 5,600 years) in fossils makes it possible to determine when they were buried; the ages of many archaeological finds have been determined using ¹⁴C. Differing amounts of ¹⁴C in the annual growth rings of trees may indicate differing intensity in their formation in the atmosphere of past geological periods, which is related to periods of change in the intensity of cosmic irradiation of the planet. The rate and time of accumulation of various marine bottom sediments in the bot-tomset beds of the oceans and seas are determined from the pairs ²³⁰Io-²³²Th an(²³⁰io-²³¹Ra, as well as from the absolute content of radioactive ¹⁴C and ¹⁰Be. The average length of accumulation of unconsolidated sediments in the ocean can be as much as 150 X 10⁶ years.

Variation in the content of stable isotopes plays a large part in geological research. Despite the slight differences in the physical and chemical properties of isotopes, the isotopes of some chemical elements in certain geological processes are fractionated (separated). The greatest fractionating effect is typical of the light elements, such as H, C, N, O, and S, because the relative difference in mass of the isotopes is greatest for them. Differences in the properties of the isotopes of heavy elements are small, and it is difficult to determine them with current measurement techniques. Measurements are made on mass spectrometers in relation to standards whose isotopic composition is universally accepted. The results of measurements are expressed in quantities designated as S, which show how much, in percent or parts per thousand, the content of the heavy isotope in the test sample is greater (+δ) or less (−δ) than in the standard.

One of the most common processes in the fractionation of stable isotopes is isotope exchange. The depth of separation of isotopes is determined by kinetic and thermodynamic factors. At high temperatures, fractionation is minimal, and at low temperatures, maximal. At normal temperatures, the most reduced compounds of C, S, and N contain more of the light isotope; their highly oxidated compounds contain more of the heavy isotope. (See Figure 1.)

Studying variations in the composition of stable isotopes makes it possible to solve one of the most important problems of geochemistry: reconstructing the history of atoms and the ways they have migrated in the course of geological processes. Thus, the release of ⁴He, ³He, and other isotopes of neutral gases during volcanic eruptions, especially in the vicinities of the midoceanic ridges, makes it possible to study deep-seated processes taking place in the earth’s mantle. The evaporation of water from the surface of the oceans and seas is accompanied by the separation of isotopes. The isotopic composition of hydrogen (¹H/²H) and oxygen (¹⁶O/¹⁸O) is lighter in water vapor than in sea water. Water vapor contains mainly 1H₂O, while the heavier water molecule (²H₂0) enriches ocean water. When water vapor condenses, the isotopes are again separated and the first drops of rain contain “heavier” water than subsequent ones. The “lightest” water crystallizes in the form of snow and ice in the polar regions, for example, in the antarctic, where the content of ²H in different layers of the snow and ice depends on the season of the year in which they have accumulated. Fresh waters are lighter than ocean waters, and their isotopic composition sometimes shows seasonal fluctuation.

Figure 1

With isotope exchange among different components, equilibrium is established during the reaction, for example: C¹⁶O₂ + H₂¹⁸O ⇄ H₂C¹⁶O₂¹⁸O ⇄ H₂¹⁶O + C¹⁶O¹⁸O. Thus, the formation of carbonates under conditions of thermodynamic equilibrium with the solution is accompanied by a shift in the isotopic composition of the oxygen. The magnitude of this shift depends on the temperature. For example, the greatest enrichment of calcium carbonate (CaCO₃) with the isotope ¹⁸O occurs when CaC0₃ is precipitated in cold water. The dependence of isotope fractionation on the temperature at which the reaction occurs serves as the basis of the paleothermometric method. Thus, studying the isotopic composition of the oxygen in the calcareous skeletons of fossil marine organisms enables us to determine the temperature of the ancient seas. The method is so sensitive that seasonal variations in temperature of the ancient seas are established on the basis of the growth rings in shells.

The isotopes of sulfur play an important role in the study of geological processes. The isotopic composition of sulfur in rocks and minerals of the earth varies greatly. Meteorite sulfur is taken as the standard isotopic composition. Variations in the ratios of the most common isotopes ³²S/³⁴S are usually measured. The basic process of isotope fractionation in sulfur involves a redistribution of isotopes between oxidized (sulfates) and reduced (sulfides) sulfur compounds. Isotope fractionation in geological processes could only have begun after the appearance of oxidized sulfur compounds, that is, after the appearance of free oxygen on the earth. Therefore, by studying the isotopic composition of sulfur in ancient deposits, it is possible to determine when the earth’s oxygen atmosphere formed. The reduction of sulfates is an important mechanism in separating sulfur isotopes. Under low temperature, reduction is usually accomplished by using sulfate-reducing bacteria. The hydrogen sulfide that forms is enriched by the light sulfur isotope, while the remaining sulfate becomes heavier. All the sulfur in sulfide compounds has undergone biogenic oxidation, as a result of which the isotopic composition of sulfur, for example, oceanic sulfates, has become heavier by several percent compared to meteorite sulfur. This quantity serves as an important planetary constant. The isotopic composition of the sulfur in deposits of sulfides of nonferrous heavy metals enables us to reconstruct the history of sulfur atoms to the time when they were fixed in the ores and to determine the source of the ore matter. In particular, the large part played in ore formation by sulfur, which had gone through the stage of sulfate reduction, is becoming evident. It has been determined that the matter of sedimentary rocks is frequently drawn into magmatic processes.

Two types of compounds are singled out by the isotope ratios of carbon ¹²C/¹³C. One type has a higher content of heavy carbon (δ¹³C ~ 0 +), for example, the carbon of sedimentary carbonate deposits, and the other has more of the light carbon (δ¹³C ~ − 20, –40%0), for example, the carbon in petroleum, combustible gases, and modern organisms. During the formation of diamonds and carbonatites in the earth’s mantle, there is fractionation of the isotopic composition of carbon. The isotopic composition of the carbon in diamonds and carbonatites differs from the carbon of, for example, carbonates and is uniform at different points on the earth. Studying the isotopic composition of carbon brings us closer to solving the problem of the origin of petroleum, gas, diamonds, and hydrocarbons in magmatic rocks and of graphite in ancient metamorphic strata.

The methods of isotopic research constitute a new, developing area of geology. Variations in the isotopic composition of B, Mg, Cu, Si, and several other elements have been found in recent years. Studying the geological significance of these variations is a task of the future.