Geochronology(redirected from geochronologically)
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the study of the chronological sequence of the formation and age of the rocks that make up the earth’s crust. A distinction is made between relative and absolute (or nuclear) geochronology. Relative geochronology involves the determination of the relative age of rocks, giving an idea of which deposits in the earth’s crust are younger and which are older, without estimating the length of time that has elapsed since their formation. Absolute geochronology establishes the so-called absolute age of rocks—that is, an age expressed in units of time, usually in millions of years. (In recent years, the term “absolute age” has frequently been replaced by the name “isotopic,” or “radiological,” age.)
Relative geochronology. The principle of superposition (the so-called Stensen [Steno] law) is commonly applied for the determination of the relative age of laminated sedimentary and pyroclastic rock, as well as volcanic rock (lavas). According to this principle, each overlying stratum (in an undisturbed sequence of stratification of laminated rock) is younger than the underlying stratum. The relative age of intrusive rock and other nonlaminated geological formations is determined by juxtaposing them to layers (strata) of laminated rock. The layer separation of a geological section—that is, the establishment of the sequence of deposition of its constituent rocks—composes the stratigraphy of a given region. The paleontological method, which is based on the study of the fossilized remains of extinct flora and fauna (leaf impressions, seashells, and so on) buried in rock beds, is used for the stratigraphic comparison of territories (regions, countries, or continents) distant from one another and the establishment within them of strata of similar age. The comparison of fossils from various beds made it possible to establish the irreversible evolutionary process of the organic world and to distinguish a series of stages in the geological history of the earth, each with its own distinctive complex of animal and plant life. Hence, the similarity of flora and fauna in sedimentary rock beds can be an indication that these beds were formed at the same time—that is, that they are of the same age. This method of determining the relative age of rocks was used for the first time in the 19th century by W. Smith in England and G. Cuvier in France. At that time the method had no dependable theoretical basis. Cuvier explained the differences in the composition of fossil complexes found in rock beds by the extinction of organisms as a result of sudden geological catastrophes and by the subsequent appearance of new complexes. The followers of Cuvier, including the French geologist and paleontologist A. d’Orbigny, presumed that the change in the organic world of the earth after each catastrophe was associated with the “creative acts of the deity.” The studies of C. Lyell on the slow natural transformations of the face of the earth and the classic works of C. Darwin and V. O. Kovalevskii on the evolutionary development of the organic world provided a materialistic basis for the paleontological method.
A general sequence of accumulation of the layers of the earth’s crust, which became known as the stratigraphic scale, was established as the result of the labors of several generations of geologists. The upper part of the scale (the Phanerozoic eon) was compiled with great accuracy by means of the paleontological method. For the underlying section of the scale (the Precambrian period), which corresponds to an enormously thick layer of rock, the paleontological method is of limited use because of the poor preservation or absence of fossils. Therefore the differentiation of the lower, or Precambrian, part of the stratigraphic scale is less detailed. According to the extent of rock metamorphism and other features, the Precambrian period is divided into the Archean (or Archeozoic) and Proterozoic periods. The upper, or Phanerozoic, part of the scale is divided into three groups (or eras): Paleozoic, Mesozoic, and Cenozoic. Each group is divided into systems (a total of 12 systems in the Phanerozoic; see Table 1). Each system is subdivided into two or three series; the latter are divided into stages, which are in turn divided into zones. The systems, as well as many stages, can be traced on all continents, but most zones have only local significance. The eon is the largest subdivision of the scale; it incorporates several groups (for example, the Paleozoic, Mesozoic, and Cenozoic groups are united in the Phanerozoic eon, or the Phanerozoic). The stratigraphic scale serves as the basis for the creation of its counterpart, the geochronological scale, which reflects the sequence of the time periods during which certain rock strata were formed. The geochronological scale has specific subdivisions that correspond to each subdivision of the stratigraphic scale. Thus, the time during which the rock of any system was deposited is known as a period. Series, stages, and substages (zones) correspond to the intervals of time known as epoch, age, and subage (time), respectively; eras correspond to groups. [The Russian term eonotema, the largest stratigraphic subdivision, is used to correspond to the chronological term “eon.”] There are two eons—the Precambrian, or Cryptozoic, and the Phanerozoic. The duration of the most ancient eon, the Precambrian, was approximately five-sixths of the entire geological history of the earth. Each of the periods of the Phanerozoic eon, with the exception of the last (the Anthropogenic, or Quaternary), spans time intervals that are approximately equal in magnitude. The Anthropogenic system, which corresponds to the period of man’s existence, is much shorter. Unlike other periods, the Anthropogenic is differentiated according to the fauna of land mammals, which evolves much faster than marine fauna (fundamental changes did not occur in the structure of marine fauna during the Anthropogenic), and on the basis of the study of glacial deposits that typify the epochs of overall cooling. Some researchers regard as incompetent the separation of Anthropogenic deposits into a distinct system and consider it to be the final stage of the preceding Neocene period.
|Table 1. Geochronological scale of the Phanerozoic|
|Beginning (million years ago)||Duration (million years)|
|1 From 600,000 to 3.5 million years, according to various data|
|Cenozoic (lasted 67 million years)|
|Mesozoic (lasted 163 million years)|
|Paleozoic (lasted 340 million years)|
The subdivisions of the stratigraphic scale, which were differentiated by means of the paleontological method, and their corresponding subdivisions of geologic time, which were incorporated into a single geochronological scale, were approved at the Second International Geological Congress in Bologna in 1881 and have since become generally accepted throughout the world. In the future, several changes and more precise definitions will be introduced into the original scheme of the geochronology of the earth owing to the improvement of paleontological research methods and the accumulation of new data.
Absolute geochronology. In the early 20th century P. Curie in France and E. Rutherford in England proposed the use of the radioactive decay of chemical elements to determine the absolute age of rocks and minerals. The principle assumed by these scientists as the basis of absolute age determinations has been in use since that time. Age is measured by the products of radioactive decay contained in minerals. The decay of radioactive elements takes place at a constant rate. Atoms of stable elements, which are no longer decaying and whose amount increases in proportion to the age of the mineral, appear as the result of radioactive decay. In this case, the assumption that the rate of radioactive decay in the history of the earth has always remained constant is taken to be sufficiently well founded. Different elements decay at different rates. The decay of such elements as uranium, thorium, and potassium takes place very slowly, over a period of several billion years. For example, any quantity of uranium (238 U) decays by one-half over a period of 4.51 x 109 years; the half-life of thorium (232Th) is 1.41 x 1010 years. These long-lived elements are the ones that are generally used to determine the absolute age of rocks and minerals.
In 1907, on the initiative of E. Rutherford, B. Boltwood (in Canada) determined the age of a number of radioactive minerals by their lead accumulation. In the USSR radiological research was pioneered by V. I. Vernadskii. His work was carried on by V. G. Khlopin, I. E. Starik, and E. K. Gerling. The Commission on the Determination of the Absolute Age of Geological Formations was established in 1937.
The figures obtained as a result of the first absolute age determinations of rocks allowed the English geologist A. Holmes to propose in 1938 the first geochronological scale of the Phanerozoic. This scale was repeatedly revised and made more precise. It is reproduced in Table 1, based on the most recent data (G. D. Afanas’ev, 1968).
Because of the absence of remains of skeletal fauna, the geochronological scale of the Precambrian (see Table 2) was constructed for the most part according to the data from numerous absolute age determinations of magmatic rocks on various continents, which made it possible to establish the synchronism of the large-scale tectonomagmatic cycles that are the basis for the division of the Precambrian.
|Table 2. Geochronological scale of the Precambrian|
|Beginning (million years ago)||Duration (million years)|
Each of the Precambrian subdivisions as used in the USSR—the Archean and Proterozoic—greatly exceeds the individual groups of the Phanerozoic in terms of duration. The Proterozoic is subdivided into three parts—lower, middle, and upper. The upper part was introduced into geochronology under the name Riphean, which many geologists regard as a subdivision corresponding to the group.
The most ancient rocks found on earth have an age of approximately 3,500 million years and mark the beginning of the Archean. There have been no reliable discoveries of rocks that originated in the time interval between 3,500 and 4,500 million years ago (the probable age of the earth).
Methods of absolute age determination. The accumulation of the products of radioactive decay over a period of time, which is assumed as the basis of absolute age determinations, is expressed by the formula D = P (eλt - 1), where D is the number of atoms of a nonradioactive substance that are formed over a period of time t, P is the number of radioactive element atoms at the present time, e is the natural logarithm base, and λ is the decay constant that indicates what part of the radioactive element atoms disintegrates per unit of time (year, day, minute, and so on) in relation to the original quantity. The rate of decay is sometimes expressed as the half-life period (T½)—the time in which any quantity of a substance decays by one-half. The ratio D/P is the function of the age (t) of the mineral. Thus, D/P = eλt - 1. The age of a mineral specimen (t) can be calculated from this according to the formula t = (1/λ)ln (1 + D/P).
The true age can be determined in this case if the ratio D/P changes only because of radioactive decay—that is, if the mineral is a closed system.
The principal types of radioactive decay used for age determination are as follows:
The following methods of nuclear geochronology are distinguished depending on the products of decay: lead (uranium-thorium-lead), helium, argon (argon-potassium), calcium, strontium (strontium-rubidium), and osmium. The most widely used are the lead, argon, and strontium methods.
The lead method is based on analyses of radiogenic lead in minerals (uraninite, monazite, zircon, and orthite). It is the most reliable method, since the problem of the age of the uranium-thorium mineral is solved by three independent equations:
Pb, U, and Th denote the lead, uranium, and thorium isotope content in the minerals; λ1, λ2, and λ3 are the decay constants of 238U, 235U, and 232Th isotopes.
If equation (2) is divided by equation (1), the following equation is produced:
This equation yields the age values closest to actuality, since it depends very little on possible losses of uranium and lead by the mineral during its geological life. It permits age estimation by just the single measured ratio 207Pb/206Pb, since the ratio 235U/238U equals 137.7 at the present time and is virtually identical in all minerals and rocks. The concurrence of the age values derived by all four equations indicates good preservation of the mineral being studied, high accuracy of the analyses conducted, and good reliability of the estimated absolute age. The isotope composition of lead is measured by a mass spectrometer.
However, different equations frequently yield different values for the age of the same mineral. To establish the facts in this case, a graph is constructed in 206Pb/238U: 207Pb/235U coordinates (see Figure 1). On this graph are plotted the curve OA (the concordia), which is calculated theoretically for different ages, and the straight line OB (the isochrone), on which lies the results of measurements for several analyzed minerals having the same age. The value at the intersection of the curve OA and the straight line OB is considered to be the actual age.
Since all radioactive minerals contain an admixture of ordinary lead in addition to the radiogenic lead, it is necessary to make a correction in the age calculation. To avoid this, an age determination method was proposed based on the measurement of the isotopic composition of lead in several minerals of the same rock for the purpose of constructing an isochrone for the results obtained. A graph in 207Pb/204Pb: 206Pb/204Pb coordinates is plotted. The data on the isotope composition of the lead in the minerals, if they are of the same age, lie on one straight line—the isochrone. The tangent of the angle of inclination of this straight line to the x-axis is the ratio 207Pb/206Pb, by which the age of a rock is determined according to the formula.
The age of common lead minerals can also be calculated if the isotopic composition of the lead is known. Ordinary lead consists of a mixture of the four isotopes 204Pb, 206Pb, 207Pb, and 208Pb, of which 204Pb is not associated with radioactive decay, and its proportion is arbitrarily taken to be 1. The rest of the isotopes are generated and gradually accumulate as a result of the radioactive decay of uranium and thorium; the gain rate of a given isotope is determined by the corresponding decay constant. Therefore, the lead of different epochs has a different isotopic composition: the lead of more ancient epochs contains a lower quantity of isotopes with weights of 206, 207, and 208, and in the lead of younger epochs the quantity of isotopes is higher relative to 204Pb. The age estimated according to the isotopic composition of lead ore has come to be known as the model age, since it is correct only for that kind of model (system) in which the Pb: U: Th ratio
changes in time because of radioactive decay alone. In reality, both concurrences of the model age with the true age for a number of deposits and significant deviations, which become more frequent in young geological formations, take place.
The argon method is based on the radiogenic accumulation of argon in potassium minerals. The argon method, which is more practicable because of the ease of production of the required material (potassium minerals) and the relative simplicity of processing, is highly popular. The absence of an inherent control (one equation) is a negative feature of this method. As much experimental research has shown, potassium minerals lose radiogenic argon comparatively easily. This applies to a lesser degree to micas and to a considerably greater degree to feldspars, which makes them ill-suited for age determination. An important positive feature of the argon-potassium method is its applicability for determining the age of sedimentary deposits with the mineral glauconite. Age determination experiments with immutable glauconites of both young (mid-Cenozoic) and old age have indicated that glauconite retains argon and potassium well, independent of time. In spite of its relatively weak resistance this mineral is made convenient by the fact that, even during small changes that cast doubt upon the usefulness of a given specimen, it immediately displays a change in color and chemical composition.
The strontium method, which is based on the radioactive decay of 87Rb and its transformation into 87Sr, has not become widespread in the USSR. The reason for this lies in the fact that, in rubidium-rich regions, rubidium may be introduced into minerals of a considerably later period of formation, which could result in serious distortions in the direction of decreasing age during the age determination of these minerals; on the other hand, in regions with intensive alkali metasomatosis, rubidium is readily lost from minerals, and then the age value according to the 87Sr/87Rb ratio becomes greatly exaggerated. Generally, in measuring age by the 87Sr/87Rb ratio, the mineral constituents of granite are isolated, and the 87Sr/86Sr and 87Rb/86Sr ratios are determined in each of them. The data from the analyses of the separate minerals of granite are arranged on a straight line, the isochrone, which is drawn upward and to the right on a graph in 87Sr/86Sr: 87Rb/86Sr coordinates. The tangent of the angle of inclination of the isochrone with the x-axis represents the value 87Sr/87Rb, which determines the age of a given rock.
The radiocarbon method assumed great importance for estimating the age of geological objects within a range of 60,000 years. This method is based on the fact that the nuclear reaction 14N + n = 14C + P occurs in the atmosphere of the earth under the influence of cosmic rays because of the abundance of nitrogen; moreover, 14C is radioactive and has a half-life of more than 5,700 years. An equilibrium between the synthesis and decay of this isotope became fixed in the atmosphere, and as a result, the 14C content in the air is constant. Plants and animals continually exchange carbon with the atmosphere during their lifetimes, and therefore the concentration of 14C in them is maintained at a constant level; the exchange with the atmosphere ceases in dead organisms, and their concentrations of 14C begins to decrease in accordance with the law of radioactive decay. It is possible to establish the age of organic remains by measuring the 14C content with the aid of highly sensitive radiation-measurement equipment. In this way, for example, the age of burial of a mammoth on the Taimyr Peninsula (11,000 years) was ascertained from bones and skin. This same method was used to help date the epochs of glaciation in Europe and North America and to determine the age of traces of ancient human cultures.
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