Conductivity and Absolute Age of K-Feldspathized Rocks of Intrusive Massifs on the Eastern Slope of the Kuznetsk Alatau
Abstract Generated abstract
This paper examines potassium-feldspathized intrusive rocks from massifs on the eastern slope of the Kuznetsk Alatau using temperature-dependent specific electrical conductivity as a basis for estimating absolute age and interpreting metasomatic alteration. Rock plates from potassium-rich samples were heated in vacuum while conductivity was measured, and ages were inferred from calibrated conductivity relationships with corrections for potassium, silicon, and iron. The results distinguish older unaltered quartz diorites, such as the Solgon massif at about 700 million years, from younger conductivity ages associated with aplites, pegmatites, monzonites, and potassium metasomatism, including notably low values for the Bel’skii massif. The authors argue that conductivity behavior reflects lattice changes linked to potassium-40 decay, argon loss, polymorphic transformations, and formation of quartz and anorthite, making the method potentially useful for reconstructing postmagmatic and metamorphic stages.
Full Text
CONDUCTIVITY AND ABSOLUTE AGE OF K-FELDSPATHIZED ROCKS OF INTRUSIVE MASSIFS ON THE EASTERN SLOPE OF THE KUZNETSK ALATAU
V. N. Salnikov, A. F. Korobeynikov
(Presented by the scientific seminar of the Department of FGP, Research Institute of RF)
The temperature dependence of the electrical conductivity of rocks of the Kommunar ore field was studied in order to assess the role of accumulation of decay products of \(K^{40}\) in the crystal lattice of potassium-bearing minerals, which provides information on age by the potassium-argon method and by the method of specific electrical conductivity. A mechanism is considered for a possible change in the chemical composition of rock-forming minerals containing potassium as a result of the release from bonds in the crystal lattice of some of the ions \(Si^{+4}\), \(Al^{+3}\), and the formation of new energetically favorable compounds. By the electrical-conductivity method, the absolute age of rocks of the Solon, Bel, Andat, and Sluchaininsk massifs was determined.
Disturbance of radiogenic equilibrium in minerals and rocks is caused by many primary and secondary processes, and, depending on the nature of these processes, accumulations of age information arise. In the overwhelming majority of cases, rocks do not bear traces of visible alteration, and if alterations are noted, they are insignificant [3]. The age of rocks that have undergone superimposed geological processes reflects the time at which the metamorphic process took place. Therefore, the discrepancy between age values and geological data, owing to the “rejuvenation” of rocks that bear no visible traces of alteration, is also associated with the effect of metamorphic processes. The influence of thermometamorphism on the change in the chemical composition of minerals over the course of geological time can be reproduced in simplified form under laboratory conditions by using a vacuum furnace; the physicochemical transformations can be recorded by measuring the values of the electrical conductivity of a compound during heating and cooling [1, 4].
A rock sample in the form of a plate \(20 \times 20 \times 3\) mm was heated linearly at a rate of \(10^\circ\) per minute from 20 to \(1100^\circ\)C, followed by cooling. During heating and cooling, the specific electrical conductivity was measured at a constantly applied voltage of 20–50 V. Electrical conductivity was studied in 47 samples with an elevated potassium content in order to determine the absolute age of intrusive massifs and the role of potassium metasomatism. From each sample, three plates were cut, and their specific electrical conductivity was measured as a function of temperature. The value of electrical conductivity was taken as the arithmetic mean, which at \(970^\circ\)C in vacuum, for rocks of identical composition selected from different points, varies by 2–15%. When
Table 1
Absolute age of rocks by electrical conductivity
| Sample no. | Rock name | Sampling site | $\mathrm{ohm}^{-1}\mathrm{mm}^{-1}$ with corrections for Fe$^{+2}$, Si$^{+4}$, K$^{+1}$ | $\tau$, million years |
|---|---|---|---|---|
| 538 | Pegmatite | Pegmatite stock, Sorsk deposit | $2.5 \cdot 10^{-8}$ | 506 |
| 523-P | Pegmatite veins among limestones | Right bank of the Poputnaya River, Ivanovka | $4.2 \cdot 10^{-8}$ | 300 |
| 508-P | Pegmatite veins among diorites | Trench 282, Poputnaya Ivanovka | $2.5 \cdot 10^{-8}$ | $510 \pm 5$ |
| B-1 | Plagioclase–K-feldspar rock from the marginal part of the massif | Sluchaininskii massif | $2.6 \cdot 10^{-8}$ | $495 \pm 5$ |
| B-16 | Pegmatite from a K-feldspar veinlet | Andat massif, “Maiskaya” vein | $3.8 \cdot 10^{-8}$ | 340 |
| B-15 | Quartz–K-feldspar veinlet | Andat massif, “Maiskaya” vein | $3.1 \cdot 10^{-8}$ | 380 |
| 501-S | Quartz diorite | Solgon massif | $2.0 \cdot 10^{-8}$ | 700 |
| 501-S$_1$ | K-feldspathized diorite (monzonite) | Solgon massif | $3.8 \cdot 10^{-8}$ | $315 \pm 5$ |
| 503-S | Aplite | Solgon massif | $4.0 \cdot 10^{-8}$ | $305 \pm 5$ |
| 504-S | Aplite | Solgon massif | $3.5 \cdot 10^{-8}$ | 340 |
| 020-19 MGZ | Pegmatite veinlet | Solgon massif | $4.1 \cdot 10^{-8}$ | $295 \pm 5$ |
| 505-S | K-feldspathized diorite (monzonite) | Solgon massif | $3.1 \cdot 10^{-8}$ | 380 |
| 339б | Aplite | Bel’skii massif | $4.2 \cdot 10^{-8}$ | 300 |
| 2-69-1 | K-feldspathized diorite | Bel’skii massif, southern part | $5.9 \cdot 10^{-8}$ | $240 \pm 10$ |
| 2-69-2 | Monzonite | Bel’skii massif, southern part | $6.3 \cdot 10^{-8}$ | $230 \pm 10$ |
| 1-69-2 | Aplite with relicts of hornblende | Bel’skii massif, eastern part, headwaters of the Kamennyi River | $9.4 \cdot 10^{-8}$ | $180 \pm 10$ |
| 1-69-1 | Monzonite | Same place | $9.0 \cdot 10^{-8}$ | $190 \pm 10$ |
| 3-69-1 | Monzonite | Summit of Bel’skii Golets | $6.6 \cdot 10^{-8}$ | $210 \pm 10$ |
| 5-69-1 | Monzonite | Northwestern part of the Bel’skii massif | $1.05 \cdot 10^{-7}$ | $170 \pm 10$ |
| 550 | Sodium aplite (pegmatoid) | Kedrovskii massif | $2.2 \cdot 10^{-8}$ | 610 |
| 546 | Porphyritic granite | Summit of Izyk-Chula | $4.1 \cdot 10^{-8}$ | $295 \pm 5$ |
| 549 | Syenite-diorite (K-feldspathized diorite) | Malosobakinskii massif | $3.4 \cdot 10^{-8}$ | 380 |
at atmospheric pressure and a temperature of \(20^\circ\mathrm{C}\), electrical conductivity depends on the moisture content and the amount of other mechanical impurities; therefore the difference in its values from sample to sample reaches 1–2 orders of magnitude.
The absolute age was determined from the curve of the dependence of electrical conductivity on time, constructed from standard samples. The value of electrical conductivity was taken with corrections for \(K^{+1}\), \(Si^{+4}\), \(Fe^{+2}\). Despite the preliminary processing of the data, an interesting regularity was obtained when comparing the absolute ages of intrusive massifs. The obtained age of the quartz diorites of the Solgonskii massif is 700 million years, while that of the K-feldspathized diorites and aplites is 315–380 and 305–540 million years, respectively. The pegmatite veins of the Andatskii massif have an age of 340–380 million years. A plagioclase K-feldspathized rock from the Sluchaininskii massif yielded an age of 450 million years. Pegmatite veins among diorites (ditch 282, Poputnaya Ivanovka) have an age of 510 million years, and among limestones, 300 million years. Low age values were obtained for rocks of the Bel’skii massif: from 180 to 240 million years for monzonites and 300 million years for aplites. The absolute age of unaltered rocks, determined by the electrical-conductivity method, is close to the value given in the literature. The age within the range 180–380 million years probably characterizes the stages of formation of aplite bodies, potassium metasomatism, and the formation of Devonian pegmatites in the enclosing strata. From Table 1 it is evident that rocks subjected to alkaline metasomatism differ sharply in electrical conductivity from the original intrusive rocks.
Let us consider the mechanism of changes in the electrical conductivity of rocks over geological time. Usually, in dielectrics, in the region of intrinsic conductivity, the dependence of specific electrical conductivity on the absolute temperature \(T\) is described by the equation:
\[ \sigma = A e^{-\frac{E_w}{kT}}, \]
where
\(A\) is a constant depending on the concentration of charge carriers and only slightly dependent on temperature;
\(E_w\) is the activation energy of the electrical-conductivity process (the energy necessary for the formation and drift of charge carriers in the direction of the field);
\(k\) is the Boltzmann constant.
The graph of this function in the coordinate system \(\lg \sigma = f\left(\frac{1}{T}\right)\) has a linear character under the condition \(E_w = \mathrm{const}\) and \(A = \mathrm{const}\). For our objects, which are complex heterogeneous systems, the temperature dependences of electrical conductivity are sufficiently complicated. On the curves, inclined and horizontal segments are observed, caused by polymorphic transformations of minerals, and intervals with maxima and minima of electrical conductivity, associated with endothermic or exothermic processes (DTA curves, Fig. 1).
If \(E = \mathrm{const}\), \(A = \mathrm{const}\), and during polymorphic transformations the \(T_1\) of the sample is also \(\mathrm{const}\), then the number of current carriers will be constant; therefore, a change in electrical conductivity at that moment should not be observed. But since the heat capacity of the sample is greater than the heat capacity of the electrode on which the sample is located, and the thermocouple does not directly measure the temperature of the sample but measures the temperature of the electrode, on the graph of the dependence \(\lg \sigma = f\left(\frac{1}{T}\right)\) a series of horizontal straight segments is obtained, characterizing the time and the initial temperature of polymorphic transformations of the minerals that make up the rocks.
Fig. 1. Temperature dependence of conductivity for rocks with DTA curves. Curves of the dependence of \(\sigma\) on \(f\left(\frac{1}{T}\right)\) (1, 2, 3) and thermograms (a, b) (1; a) — sample 1-69-2, 2 — sample 3396, (3; b) — sample 1-69-1.
If \(E_w = const\), \(A = const\), and \(T_2\) of the sample decreases as a result of an endothermic effect, then under our experimental conditions, on the curve of the dependence \(\lg \sigma = f\left(\frac{1}{T}\right)\), we obtain a decrease in electrical conductivity with time. Polarization often occurs in samples containing mica and potassium feldspar, owing to the weakening of the primary bond of some of the ions \(K^+\), \(Si^{+4}\), \(O^{-2}\), or to charging as a result of expansion of the crystal during polymorphic transformations (piezoelectric effect). Therefore the electrical conductivity is recorded by the instrument according to the residual current, which does not correspond to the true value. This discrepancy is caused by the influence of the electromotive force of polarization \((P)\), directed oppositely to the external field [Fig. 1, curve 1].
At a temperature of 970–1040°C, complete liberation of \(Si^{+4}\) ions from the silica–oxygen tetrahedra occurs, and their combination with oxygen ions, which leads to the formation of quartz and an anorthite molecule in the structure of K-feldspar and mica.
One of the causes of changes in the chemical composition of rocks is the \(\beta\)-decay of \(K^{40}\) and its replacement in the crystal lattice by potassium-
Ca-containing minerals with \(Ca^{40}\), which leads to the accumulation of positive vacancies that subsequently form nodal complexes with hydroxyl and fluorine ions, or else the neutrality of the crystal is preserved through the reduction of trivalent iron, isomorphously entering the crystal lattice. Hence the number of nodal vacancy complexes in the mineral lattice will depend on time and potassium content, while the amount of preserved information on the absolute age of the object will depend on the history of the sample [5].
Calculations show that, when heated to \(1040^\circ C\), \(1\ cm^3\) of K-feldspar having an age of three billion years has the energy of its crystal lattice lowered by 304 J owing to the formation of anorthite molecules. Consequently, the replacement of \(K^{40}\) by \(Ca^{40}\) as a result of \(\beta\)-decay would have to lead to the formation of a new molecule in the mineral structure if the chemical bonds in the silicon–oxygen tetrahedra of potassium feldspar or muscovite were disrupted. These bonds may be broken when the mineral melts, or partially weakened when the compound is heated, when the energy \(E\omega\) necessary for the formation and drift of the intrinsic ion of the crystal lattice is supplied. The weakening of bonds leads to increased migration of argon and to its loss by minerals in the zone of metamorphism, while the partial formation of quartz and anorthite according to the proposed scheme does not lead to the destruction of the information that gives the value of the absolute age by the electrical-conductivity method.
From the foregoing the following conclusions may be drawn:
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By determining the absolute age of rocks by the electrical-conductivity method, one can record stages in the development of intrusive massifs over geological time.
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The Bel’skii intrusive massif, which has the lowest age values, probably, in addition to potassium metasomatism in the Devonian, underwent secondary reheating at the boundary between the Paleozoic and the Mesozoic, which led to the formation of granophyres.
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By modeling the process of metamorphism under laboratory conditions, one can explain those minor changes in minerals and rocks that lead to a discrepancy between age values and geological data.
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By quantitatively determining the value of the polarization and endothermic effect in the temperature interval \(870—1040^\circ C\), one can judge the intensity of metamorphism, as a result of which part of the liberated silicon, aluminum, and oxygen ions formed quartz and anorthite molecules, while the remaining part of the ions can be liberated with the formation of the corresponding compounds when the sample is heated in the cell.
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Within mobile geosynclinal zones with active magmatic and tectonic activity, argon loss and the formation of quartz and anorthite occur simultaneously in rocks, since both processes are caused by weakening of the bonds of the crystal lattice of minerals; however, argon loss leads to lowered values when determining absolute age by the potassium–argon method, whereas partial formation of quartz and anorthite has no substantial effect on the age indications obtained by the electrical-conductivity method.
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The accuracy of determining absolute age by the electrical-conductivity method will be affected by the metasomatic introduction of individual components with ions \(Si^{+4}\), \(Al^{+3}\), \(K^{+1}\), \(Fe^{+2}\), as well as by mechanical destruction of the crystal lattice, which promotes cation exchange in silicon–oxygen tetrahedra.
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The formation, over geological time, of a molecule of more basic plagioclase, the liberation of \(Si^{+4}\), and its combination with \(O_2\) with the formation of—
... with the release of $SiO_2$ gives us yet another mechanism for the separation of magma into acid and basic components as a result of the radioactive decay of $K^{40}$. From this point of view, one can explain the formation of the granophyres of the Belsk massif and the phenomena of metamorphic silicification.
- Further improvement of the method of electrical conductivity of rocks will make it possible to apply it to elucidating the relative (and possibly absolute) sequence of postmagmatic processes.
REFERENCES
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L. G. Berg. Introduction to Thermography. Moscow, 1961.
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W. Bragg, G. Claringbull. The Crystalline Structure of Minerals. Moscow, 1967.
-
V. M. Klyarovskii. Determination of the Age of Geological Formations. Novosibirsk, 1960.
-
E. K. Zavadovskaya, M. N. Treskina. Measurement of the electrical conductivity of solid dielectrics over a broad temperature interval. “Zavodskaya laboratoriya,” 1961, No. 5.
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V. N. Salnikov, A. F. Korobeinikov. Determination of the absolute age of rocks by the electrical-conductivity method. In: Problems of the Geology of Siberia. Tomsk, 1971.