Termoluminescencja minerałów i skał w zastosowaniu do badań geologicznych

Jadwiga Pawłowska




Thermoluminescence, or light emitting caussed by heating of substances, belongs to a group of known physical phenomena related to the excitation state in the atom. This phenomenon appears in minerals characterized by ionic or polarized bonds that make a transition from ionic to covalent bonds. The process comprises about 75% of minerals and rocks that occur in natural state. Thermoluminescence properties are characteristic of these minerals, in which metastable energy levels are preserved (Fig. 1). Apparently constant quantum states of atoms appear in defects of crystal lattice, which play a role of traps far electrons. These are irreversible defects and their origin is related to the history of minerals; thus they are called biographic defects. To the most significant belong here Schottkye's and Frenkel's mechanical defects, and chemical defects (Fig. 2). There are various reasons of accumulation of electrons in the traps. To the main activation factors belong: natural radioactivity and quick changes in thermodynamic conditions. Part of electrons may fall into traps during the process of production of Frenkel's defects and of chemical defects. Instrument for thermoluminescence research consists of three main parts: electric heater with programmed increase in temperature, photomultiplier and recorder (Fig. 3). The course of thermoluminescence curve and the maximum position depend upon the velocity of temperature increase, the susceptibility of photomultiplier, and the amount of chemical admixtures in the minerals examined. The thermoluminescence method is applied to explain the thermal history of rocks, to examine contact metamorphism, and to make correlation of mineral parageneses, frequently also as an auxiliary method in absolute age determination. In the study there are used relative comparisons of intensity of thermoluminescence, or sums of light in the samples examined. The research of thermal history is based mainly on the fact that the heating up of rocks adjacent to a vein, dike, or intrusive body leads to the destruction of thermoluminescence, the degree of light emission depending upon the temperature gradient and the distance of a given point from the heat source. Thus, the intensity of the thermoluminescence examined under laboratory conditions will increase with the distance from the vein (Fig. 4). The sum of the accumulated light in the minerals examined is a basis to correlate the mineral parageneses. Here an exposure is applied using X-rays which regenerate the whole thermoluminescence ability, even after a complete light emission of a sample. The repeated light emission of the irradiated sample allows us to evaluate the difference in age of the minerals examined, and to draw conclusions as to the kind and intensity of the active geological processes. In absolute age determination, natural Earth’s radioactivity is used. As far as basic rocks and limestones axe concerned, the calculations are based on the determinations of α activity; in the case of acid rocks, where main role is played by the radioactivity caused by the potassium isotope K40 - on the determinations of β activity. The thermoluminescence research has been initiated in the Geological Institute in Warsaw.

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