SULFUR ISOTOPIC COMPOSITION IN METEORITES AND TERRESTRIAL OBJECTS

For a long time D. I. Mendeleev’s periodic system of the elements was based on chemical atomic weights. This was made possible by the fact that the atomic weights of various elements, within the limits of the accuracy attainable by chemical analysis, remained practically unchanged, irrespective of the substances from which these elements were separated and of the parts of the globe in which these substances were obtained. Before the discovery of isotopy and of the relationship between nuclear charge and the position of an atom in the periodic

system, the constancy of atomic weights seemed a natural necessity, while anomalies in the positions of certain elements (argon—potassium, etc.) remained unexplained.

The discovery of isotopes posed before science the problem of the constancy of atomic weights. Systematic investigations of the isotopic composition of elements by means of the mass-spectrometric method, whose accuracy far exceeds that of chemical methods, showed that, even if fluctuations in the percentage content of isotopes are sometimes observed, they are small. Similar investigations were carried out for the isotopes of H, B, C, O, Si, Cl, Ar, K, Fe, Ni, Sr, Sb, Pb. The question of the reasons for the constancy of the isotopic composition of elements is of very great interest, since it is directly connected with the question of the origin of the elements and the formation of the Earth.

This question acquired special interest when, alongside terrestrial objects, the atomic weights and isotopic composition of elements in meteorites arriving on Earth from cosmic space began to be studied. Careful chemical analyses established that the atomic weights of terrestrial and meteoritic elements C, O, Si, Cl, K, Fe, Ni, Cu, Ga are identical[^2]. Recent analyses of the isotopic composition of several elements (C, O, Fe, Cu, Ga) also did not reveal any difference within the limits of the accuracy of the mass-spectrometric method. However, the number of works comparing the isotopic composition of terrestrial and meteoritic elements, and still more the number of elements investigated, is extremely small. Therefore further investigations in this field are very important.

Recently, at the Institute of Geochemistry and Analytical Chemistry named after V. I. Vernadsky, A. Trofimov carried out an investigation of the isotopic composition of sulfur in meteorites and terrestrial objects[^3]. The determination of the isotopic composition of terrestrial sulfur, made by Nier in 1938[^4], showed that it has four stable isotopes of the following percentage abundance: S³² (95.1%), S³³ (0.74%), S³⁴ (4.2%), S³⁶ (0.016%). Since then no investigations of the isotopic composition of sulfur have been carried out.

In A. Trofimov’s work, four samples of sulfur extracted from two stony (Saratov and Grossliebenthal) and two iron (Augustinovka and Sikhote-Alin) meteorites were studied. For comparison, four samples of terrestrial sulfur were also taken: water of the Greenland Sea; volcanic elemental sulfur obtained from the Klyuchevskaya Sopka; pyrite and reactive sulfuric acid. The analysis was made with the aid of a sector mass spectrometer (sector angle 90°), constructed according to the type of the mass spectrometers of Nier[^5] and Hipple[^6]. The concentration of isotopes was determined by measuring ion currents with a tube amplifier, whose limiting sensitivity was \(\sim 1 \cdot 10^{-15}\) a over 1 mm of the scale of the mirror galvanometer. Control of the ion beams was effected by changing the accelerating field; at the same time, unlike Nier’s work, errors due to the “volt effect” were determined and corrections for it were introduced into the final results. To carry out the analysis, samples of sulfur dioxide gas \(SO_2\) were prepared from all the specimens. Upon ionization of the gas in a source of ions produced by electrons with an energy of \(\sim 60\) eV, \(SO_2^+\) ions arise and, in smaller quantity, \(SO^+\) and \(S^+\) ions. Each of them forms a group of peaks corresponding to all possible combinations of sulfur and oxygen isotopes. However, the \(SO_2^+\) ions, as well as \(S^+\), fell into a region of the mass range in which, in the instrument after introduction of the gas, an irremovable background appeared, sharply and irregularly distorting the picture of isotope distribution. In the mass region of \(SO^+\) ions, enclosed between 48 (\(S^{32}O^{16}\)) and 52 (\(S^{34}O^{18}\)), the vacuum background was absent even during prolonged continuous operation of the instrument, and the picture of the peaks

was well reproduced in repeated measurements. Therefore, determination of the isotopic composition in all samples was carried out only on the \(SO^{+}\) ions. Two series of independent measurements were performed at different times. The determination of the ratios \(S^{34}:S^{32}\) was made with an accuracy of up to 1%, and of the ratios \(S^{33}:S^{32}\), up to 3%. With respect to the relatively rare isotope \(S^{36}\), it was only possible to show that the order of its concentration was the same in all samples. The averaged measurement results, compiled with allowance for the influence of the different oxygen isotopes and corrections for the “volt-effect,” are given in the table.

It is seen from the table, first, that the isotopic composition of sulfur is the same in all four meteorites, and that the average isotopic composition of terrestrial and meteoritic sulfur is completely identical, as is the case for the other elements investigated up to now. Secondly, it turned out that the ratio obtained for sea water differs appreciably from all the others, indicating enrichment of it with the heavy isotope \(S^{34}\). This enrichment amounts to about 1%, i.e., it lies almost within the limits of possible measurement error; however, it is apparently quite real, since it was observed in all measurements. The fact that sea water is enriched specifically in the heavy isotope also confirms the possibility of the existence of some process responsible for this enrichment. Finally, since the degree of accuracy of the measurements described is twice as high as Nier’s and, in addition, the number of measurements of the isotopic composition is considerably larger here, the average ratio of the sulfur isotopes, calculated on the basis of all the data in the table and equal to \(S^{32}:S^{33}:S^{34} = 100:0.80:4.56\), is more accurate than that given by Nier and included in all isotope tables, \(100:0.78:4.4\).

V. L.

CITED LITERATURE

1. F. W. Aston, Mass Spectra and Isotopes, IL (1948).
2. C. Baxter and F. Hilton, Journ. Am. Chem. Soc. 115, 694 (1923).
3. A. Trofimov, DAN SSSR, LXVI, No. 2, 181 (1949).
4. A. Nier, Phys. Rev. 53, 282 (1938).
5. A. Nier, Rev. Sc. Instr. 11, 212 (1940).
6. J. Hipple, Journ. Appl. Phys. 13, 551 (1942).