L.H. Ahrens

Observations on the Tn-U relationship in zircons from granitic rocks and from kimberlites

Abstract Existing data for thorium and uranium in zircons from granitic rocks are discussed, and new data for thorium and uranium in zircons from kimberlites are presented. The new data show that thorium and uranium are at very low levels (from 1.8 to 7.2 ppm for thorium, and from 7.0 to 28.0 ppm for uranium) in kimberlitic zircons, but that the Th:U ratio is just as low (from 0.23 to 0.70) as for granitic zircons. The Th:U ratio in zircons is then discussed by comparison with the ratio in igneous rocks, and possible reasons for the low ratio in zircons are suggested.

SOME OBSERVATIONS ON THE URANIUM AND THORIUM DISTRIBUTIONS IN ACCESSORY ZIRCON FROM GRANITIC ROCKS

Abstract The uranium and thorium distributions in seventy two specimens of accessory zircon from granitic rocks have been examined. Frequency distributions of both elements are positively skewed and the distributions are clearly not normal; both distributions approximate lognormality. The dispersion of the thorium concentration is distinctly greater than that of uranium; ionic radii differences may be the cause. The ratio Th U is also positively skewed and whereas the average ratio is 0.47, the modal (most frequent) value is ~0.35. Though not well-developed, there appears to be a distinct correlation between uranium and thorium in the zircon under consideration.

Observations on the Fe-Si-Mg relationship in chondrites

Abstract The Fe-Si-Mg relationship in chondrites, as represented on a triangular diagram and Si-Mg fractionation are considered. The possibility of utilizing some of these relationships for chondrite classification and assignment are discussed. Though Si-Mg fractionation between enstatite-chondrites, carbonaceous chondrites (including the olivine-pigeonite group) and the common chondrites (olivine-bronzite and olivine-hypersthene) is now well established, it has not been possible to ascertain whether fractionation between the olivine-bronzite and olivine-hypersthene chondrites has taken place; present evidence indicates that there might be slight fractionation. References

The determination of trace elements in silicate rocks by a combined spectrochemical-anion exchange technique

Abstract The paper describes an attempt to combine anionic exchange enrichment techniques with a spectrochemical method of analysis for the purpose of determining several rare elements in silicate rocks, soils and meteorites. The investigation has for the most part been confined to several elements which form chloro-complexes in hydrochloric acid; notably Ag, Pb, Sn, Bi, Cd, Zn, Tl, Mo and In. These may be absorbed on a basic anion exchanger: after elution and evaporation, the residues are analysed quantitatively by a d.c. arc method. The ability to determine several trace elements in a single general operation is an advantage of the procedure. The combined procedure has been applied to the determination of Cd, Zn, Bi, Tl and Sn in a suite of nineteen rocks including granite G-l and diabase W-l, and specimens of gneiss and sea-mud from the Antarctic.

Further observations on the composition of manganese nodules, with particular reference to some of the rarer elements

Abstract Mn, Fe, Ca, Co, Ni, Cu, Zn, Cd, Sn, Tl, Pb and Bi have been estimated in thirty-two nodules from the Pacific, Atlantic and Indian oceans. Various features about the composition of manganese nodules are discussed: element abundances, degrees of enrichment, inter-element relationships (notably between Ni and Cu, and between Zn and Cd), regional variations and some aspects of statistical distribution.

Some investigations on the composition of manganese nodules, with particular reference to certain trace elements

Abstract Using spectrochemical techniques Fe, Si, Mg, Co, Ni, Cu, V, Mo, Ti and Tl have been estimated in nineteen manganese nodules, eight from the Atlantic ocean, seven from the Pacific ocean and four from the Indian ocean. Though data on more samples are required before firm and detailed conclusions can be made about the distribution of elements in manganese nodules, several distinct features appear when the data on the nineteen samples are examined. 1. (i) Certain elements appear to enrich more strongly than others. For example, relative to igneous rocks Mo is much more strongly enriched than V. For several elements (Ni, Cu and particularly Co and Tl) the degree of enrichment in two Fe-low nodules is far smaller than in the other nodules. 2. (ii) The magnitude of dispersion of concentration appears to vary considerably for different elements; thus, whereas variation of concentration of V is relatively small, that of Ni, Cu, Co and Tl is far larger. 3. (iii) The statistical nature of the distribution of Fe in manganese nodules appears to be characteristic and different from that of the other elements studied so far. 4. (iv) Of the possible inter-element relationships examined that of Ni-Cu appears to be the most strongly developed.

Lognormal-type distributions—III

Abstract Aspects of recent papers by Miller and Goldberg , and by Aubrey , are discussed, and further examples of lognormal-type distributions are given. There is now a large body of evidence to show that lognormal-type distributions are very common indeed. Attention is drawn to extreme positive skewness in a histogram of grain-size distribution of pyroxene from the Merensky Reef, Bushveld Igneous Complex.

Si-Mg fractionation in chondrites

Abstract An examination of the analytical data on chondrites indicates that the two most abundant lithophile elements, Si and Mg, are significantly fractionated in these meteorites. The SiO2/MgO average for all chondrites is 1.60, a value may be compared with the minimum magnitude of ~1.40 which is approximated by all carbonaceous chondrites (together with a few other chondrites) and the maximum magnitude of ~1.90 which approximated by all entstatite chondrites (plus a few other stones). The entstatite chondrites together with a few other Mg-deficient chondrites tend to form a seemingly well-defined upper boundary in a SiO2—Fe (total) diagram. Some of the Si-Mg-Fe relationships described in this paper may have a significant bearing on meteorite classification as they appear to be of a fundamental nature.

The uniformity of concentration of lithophile elements in chondrites—with particular reference to Cs

Abstract The paper is in two parts. In the first, the dispersion of concentration of the lithophile elements Li, Na, K, Rb, Mg, Ca, Sr, Ba, Al, Sc, Eu, Si, Ti, Zr, Th and U is examined in chondrites. Where twenty or more analyses have been made, relative deviations have been calculated; these range from 5 to 29 per cent. Some of the variation is clearly due to analytical error and it is concluded that the relative deviations of most, perhaps all, lithophile elements is near the lower limit (5 per cent) of the above range. Such small dispersion of concentration for a wide variety of elements, including Li, K and Mg which are sensitive to fractionation, indicates that chondrites have come from a source with an exceptionally uniform composition. It is concluded further that variation of concentration of a lithophile element in the silicate phase, as distinct from the meteorite as a whole, may be even smaller and that the ratio of one lithophile element to the other lithophile elements may turn out to be extremely small indeed. Analytical data of a very high order of excellence (relative deviation of 1–2 per cent) for all elements are evidently required to estimate such small variations precisely. The second part of the paper is concerned specifically with caesium. This element has been estimated spectrochemically in K—Rb—Cs fractions obtained by ion exchange techniques from six chondrites and seven specimens of basic rock. In the chondrites, the Cs content (ave. 0.12 p.p.m.) and the K/Cs ratio (ave. 7000) remains constant to within the precision of the method of analysis. The magnitude of the average agrees with that (0.09 p.p.m.) of Webster , Morgan and Smales but the small variation of the Cs concentration and of the K/Cs ratio contrasts with their observations as they report large variations (× 30 or more). The K/Cs ratio in the seven basic rocks is fairly uniform (in contrast with granites) and similar in general magnitude to that in the chondrites.

Lognormal-type distributions in igneous rocks—IV

Abstract The general statistical nature of the distribution of the oxides of the abundant elements (Si, Al, Na, K, Mg, Ca, FeII, FeIII, Mn, Ti and P) in 343 granitic rocks from Japan has been considered. The distribution of the most abundant constituent, SiO2, shows decided negative skewness; K2O might also be negatively skewed. With the exception of Na2O which appears to approach normality, all other constituents are positively skewed in varying degrees and in this respect apparently have the same tendency as many trace elements. The negative skewness of SiO2 bears out the comment by Aubrey that it is not possible for every abundant constituent of a specific material to have a positively skewed distribution. All published examples of element distributions in specific igneous rocks are summarized and classified into four categories: 1. (i) positive skew lognormal-type—by far the most common distribution (34 examples), 2. (ii) negative skew (one and possibly two examples) 3. (iii) normal or near normal (one example) and 4. (iv) uncertain assignment (five examples).