ON THE CHANGE IN THE RADIOACTIVITY OF ACCESSORY SPHENE: EVIDENCE FROM ROCKS OF THE ELEKMONAR MULTIPHASE GRANITOID MASSIF (GORNY ALTAI)

A. I. Bazhenov

(Presented by Professor A. M. Kuzmin)

The geochemistry of rare and dispersed elements (including radioactive ones) has recently been attracting the attention of an ever-growing number of geochemists, petrographers, and mineralogists. This circumstance is connected, in particular, with the fact that these elements are being used in attempts to solve numerous important theoretical problems in various fields of geology. A rather large number of works have been devoted to the distribution of uranium and thorium in granitoid rocks; these concern the radioactivity of rocks from individual regions, as well as the concentration of these elements in accessory and rock-forming minerals of granitoids and the forms in which they enter the lattices of concentrating minerals. It should be noted, however, that researchers differ in their views as to which minerals concentrate uranium and thorium in granitoid rocks. According to the data of Beer and Demay [2], almost all uranium occurs in accessory minerals; in the opinion of other researchers [1, 3, 4, 5], a major role is assigned to the nonisomorphic form of its entry into the crystal lattices of rock-forming minerals¹. L. L. Leonova and L. V. Tauson [2] showed that, in the rocks of even a single granitoid batholith, the role of uranium concentrators is played both by rock-forming minerals, containing 40–50%, and by accessories—60–50% of this element. In the different rocks of the Susamyr batholith (Central Tien Shan) described by L. L. Leonova and L. V. Tauson, accessory sphene may concentrate from 6.7 to 37% of the total amount of uranium. For the rocks of each intrusive phase, the uranium content in the accessories is a fairly constant value, on the basis of which the authors conclude that, over large areas, the average composition of the granitoids of a given phase and the conditions of its formation were fairly constant. It is therefore no accident that, in works on the geochemistry of uranium and thorium, as

as well as for the geochemistry of other trace elements, quantitative values are given on the assumption that the contents of the latter in the rocks of a particular phase represent a constant quantity.

In the present work, some conclusions are set forth that were obtained by the author as a result of studying the geochemistry and petrography of the multiphase Elekmonar granitoid massif, located in the basin of the middle course of the Katun River. This intrusive body is excellently exposed to a depth of about 800 m by the valleys of the right tributaries of the Katun River—the Kuyum, Elekmonar, Kuba, and others. In the structure of the pluton, rocks of four intrusive phases are clearly distinguished, their composition changing from basic to acidic: 1. Gabbro and pyroxenites. 2. Diorites (hornblende, biotite-hornblende diorites, leucodiorite). 3. Granodiorites (in endocontact areas porphyroid, porphyritic, and fine-grained varieties also develop). 4. Granites; subordinate amounts of granodiorites and alaskites appear. A characterization of the dike complex is not given here.

The rocks of the different phases are distributed unequally; the areas occupied by them increase in the direction of the younger formations.

Within the dioritoid rocks of the second phase there sometimes occur areas containing schlieren inclusions (not more than 0.15 × 0.25 m in size) with a pegmatoid structure.

The material composition of the intrusive formations in horizontal sections of the massif remains constant over considerable distances; only in the endocontact areas do hybrid varieties develop, caused by processes of assimilation and contamination of the enclosing rocks. Such constancy of the quantitative-mineralogical composition of the rocks is probably due to the stability of the thermodynamic conditions under which the intrusive body formed.

The study of vertical sections of the intrusive massif in areas composed of granodiorites of the third phase and especially granites of the fourth phase shows a regular evolution of the rocks toward basification with depth. This is manifested in an increased role of rock-forming minerals with relatively higher specific gravity, as well as of femic silicates. In connection with this, the basicity of the plagioclases increases, the iron content in biotites increases, and the amount of quartz and potassium feldspar decreases. In the alteration of the rocks there is a tendency for granodiorite to pass with depth into quartz diorite, and granite into plagiogranite or granodiorite. The evolution of the quantitative-mineralogical composition of granite of the fourth phase with depth is shown in Table 1.

For the purpose of investigating the regularities in the distribution of accessory minerals and rare and dispersed elements in the rocks of the Elekmonar massif, samples weighing 3 kg (1 dm³) were taken in horizontal and vertical sections of the latter; these were crushed to a particle size of 0.1 mm, after which triple washing was carried out until a gray heavy-mineral concentrate was obtained. Further separation of the latter was performed by the usual method. In the horizontal direction samples were taken every 1 km, in vertical sections every 200 m, and in one section every 40 m.

Among the accessories in all rocks of the intrusive body described, sphene is constant and one of the most abundant minerals. In the present communication no data are given on the distribution of sphene, since this question is considered separately. Here it may only be noted that the concentration of sphene is controlled by the general trend of change in the chemistry of the magmatic formations of the pluton.

The radioactivity of accessory sphene was determined by V. A. Yankovsky under the following conditions: each sample was measured for 10 minutes; the volume of the cup was \(0.04\ \mathrm{cm}^3\); the packing density was \(\rho = 1.8\); a correction for the nonlinear dependence of the pulse-counting rate on the sample weight (for several samples from granodiorite) was introduced from a curve constructed from 4 standard samples weighing 100, 200, 300, and 400 mg.

Table 1

Change in the quantitative mineralogical composition of the granite of the younger phase of the Elekmonar massif in the vertical direction

Fig. 1. Radioactivity of accessory sphene (in arbitrary units) in rocks of different intrusive phases and in endocontact formations of the Elekmonar granitoid massif.
I — gabbro of the first phase. II — diorite of the second phase. 1 — diorite from pegmatitic schlieren. III — granodiorite of the third phase. 2 — granodiorite from the contact with granite of the fourth phase. IV — granite of the fourth phase. 3 — granite from the contact with granodiorite of the third phase. 4 — granite from the contact with hornfels.

The average radioactivity of sphene from rocks of different intrusive phases is characterized (Fig. 1) by the following values (in arbitrary units): gabbro—0.33; diorite—0.5; diorite from pegmatitic schlieren—0.73; granodiorite—1.20; granite—1.0. Thus, a definite increase in radioactivity is observed in the products of later magmatic phases.

The radioactivity of sphene from the endocontact zones of the intrusive body, irrespective of the composition of the contacting formations, is in all cases lower than the radioactivity of the corresponding rocks (Fig. 1): sphene from endocontact granodiorite (third phase) has a radioactivity of only 0.07 arbitrary units; from granite in contact with granodiorite—0.4 arbitrary units; and in the case of contact with hornfels—0.86 arbitrary units.

In horizontal sections of the intrusive body, the radioactivity of the material described is characterized by fairly stable values, specific to the rocks of each intrusive phase.

Fig. 2. Change in the radioactivity of accessory sphene (in arbitrary units) from granite of the fourth intrusive phase in the vertical section of the Elekmonar massif.
The points show the radioactivity of sphene from individual samples.

In the vertical direction, the radioactivity of sphene from granite changes in an entirely regular manner (Fig. 2), with the radioactivity in the upper horizons of the intrusive body increasing sharply (almost 50-fold). It should be noted that the change in radioactivity vertically is not linear: moving from bottom to top, over the first 200 m there is a direct relationship—on average, an increase of 1.4 arbitrary units for every 100 m of rise; in the upper part of the intrusive body there is a sharp increase of this value to 7.5 arbitrary units per 100 m of rise. It must be especially emphasized that the study of the material composition of mag—

magmatic formations of the Elekmonar massif gives no grounds whatever for supposing that at a depth of about 200–300 m there occurs a sharp change in chemism. On the contrary, all the available material concerning changes in the chemical composition of the rock-forming and accessory minerals and their properties convincingly indicates an additive character in the evolution of the latter.

Conclusions

1. In the magmatic formations of the Elekmonar intrusive massif, rocks of four phases are distinguished (in order of formation, from early to younger): 1. Gabbro and pyroxenites. 2. Diorites. 3. Granodiorites. 4. Granites.

2. The material composition of the rocks of the massif described remains fairly constant in the horizontal direction. In vertical sections a regular change in the rocks with depth is established, in the direction of transition of granodiorite of the third phase into diorites, and of granites of the fourth phase into plagiogranite or granodiorite. In the endocontact portions of the intrusive body, hybrid formations develop.

3. Sphene, being one of the most widespread minerals in all varieties of rocks of the Elekmonar massif, is distributed unevenly in vertical sections, whereas in horizontal sections only slight deviations from the mean values are observed.

4. Sphene in the rocks of the Elekmonar massif possesses noticeable radioactivity, and the magnitude of the latter (in arbitrary units) is fairly constant for the rocks of each phase. A regular increase is observed in its radioactivity in the rocks of later phases, which indicates accumulation of radioactive elements in the products of later magmatic formations.

Sphene from hybridized rocks of endocontact portions always has a lower radioactivity than the mineral from rocks of the corresponding phase. The radioactivity of sphene under these conditions depends little on the composition of the contacting formations.

1. In the vertical direction a regular increase in radioactivity is observed in the upper part of the intrusive body, reaching a maximum in its apical region. The radioactivity of sphene changes (unlike the character of the change in the material composition of the rocks and the radioactivity of other accessory minerals) nonadditively. As a result, in the upper part of the intrusive massif a sharp increase in radioactivity is recorded, whereas in deeper portions a successive decrease in this property is observed. On the graph the radioactivity curve has a distinct inflection; it should be noted here that, for example, the radioactivity of accessory zircon is in direct dependence on depth. The appearance of such an inflection on the radioactivity curve of sphene from the granites of the Elekmonar massif is, apparently, due to the fact that here the erosional section is small and the apical part of the intrusive body is exposed. Therefore, it is highly probable that this inflection corresponds to the boundary of the latter. This assumption should subsequently be checked on sphene from rocks of other intrusive massifs situated under analogous conditions (a small depth of erosional section and preservation of their apical parts). If it proves valid, then sphene may be used as a convenient indicator for fixing the apical part of an intrusive massif and, consequently, the depth of the erosional section of the latter.

1. The change in the radioactivity of sphene in the rocks of the Elekmonar granitoid massif is consistent with the conclusions of L. V. Tauson [4] and other investigators concerning the concentration of radioactive elements (chiefly uranium) in the products of later (younger) intrusive phases.

2. In studying the regularities in the distribution of radioactive elements in rock-forming and accessory minerals of intrusive formations, hypsometric referencing of the sampling points is necessary; in its absence, interpretation of the results of geochemical studies is not only made considerably more difficult, but may also lead to erroneous conclusions.

REFERENCES

1. Larsen E. S., Phair D. et al. Uranium in magmatic differentiation. In: Geology of Atomic Raw Materials, 1956.

2. Leonova L. L., Tauson L. V. Distribution of uranium among minerals of the Caledonian granitoids of the Susamyr batholith (Central Tien Shan). Geochemistry, No. 7, 1958.

3. Nierberg J. Uranium in igneous rocks of the USA. In: Geology of Atomic Raw Materials, 1956.

4. Tauson L. V. On the geochemistry of uranium in the granitoids of the Chernovinsky massif (Gorny Altai). Geochemistry, No. 3, 1956.

5. Hurley P. M. Distribution of radioactivity in granites and possible relation to helium age measurement. Bull. of the Geol. Soc. of America, No. 1, 1950.

1. In the case of nonisomorphic entry of a chemical element, the latter occurs in the form of predominantly molecular dispersion and does not occupy a definite (regular) position in the spatial lattice of the host mineral. The impurity element is confined to cleavage cracks, the boundaries of crystal-growth zones, or is distributed relatively uniformly in the concentrating mineral [4]. ↩