John A. S. Adams

Computer-derived geochemical balances and element abundances

Abstract The classical geochemical equation or balance assumes that all sediments were ultimately derived from igneous rocks. Despite unproven assumptions and many uncertain data, the geochemical equation does provide a number of internal checks on elemental and isotopic abundance data. Utilizing high speed digital computers, a new approach to geochemical balancing has been developed. The input into the system consists of 1. (a) physical estimates (for example, volume) of six sedimentary and marine domains of the earths crust; and 2. (b) minimum and maximum geochemical abundance data (derived from forty-four published lists) for sixtyfive elements in eight lithologic categories. Through the use of an optimization-iterative technique, the system generates as output 1. (a) total mass of weathered igneous rock, 2. (b) mass distribution of the elements within the geologic and marine domains, 3. (c) a list of those elements that could not be balanced and 4. (d) elemental abundance tables derived automatically from the optimized model. Although some aspects of the model are artificial, the computations do lead to an internally consistent estimate of abundances in the crust and major sedimentary rock types. The computations also indicate that 2 × 10 18 tons of igneous rock have weathered to form the sedimentary rocks of the earths crust and ions dissolved in the ocean. The following elements cannot be brought into balance, probably due to important volcanic contributions and/or inadequate data: boron, sulfur, chlorine, manganese, arsenic, selenium, bromine, molybdenum, iodine and lead. The input into the system is left open for future revision.

THE RELATIONSHIP BETWEEN THE PETROLOGY AND THE THORIUM AND URANIUM CONTENTS OF SOME GRANITIC ROCKS

Abstract Thorium and uranium contents of granitic rocks are intimately related to modal compositions and general petrologic features. Correlations are quite distinct between thorium content and common indices of general petrogenetic evolution, such as amount of dark minerals, percentage of anorthite in plagioclase, and ratio of potassium feldspar to plagioclase. Thorium content increases regularly toward the more acidic rocks, and the increase is most pronounced in the most highly alkalic samples. Uranium content generally shows little, if any, relationship to modal composition or other petrologic features, and the increase in abundance of uranium toward the more acidic rocks is irregular. The greater petrogenetic control of thorium than of uranium content may be explained on the basis of oxidation and repeated loss of uranium from magmas during the later stages of their differentiation. Such an explanation assumes that magmas are originally derived from a relatively homogeneous source; remobilization, however, of different types of sedimentary or other rocks might provide granitic magmas of widely different initial thorium and uranium contents. The possibility that thorium is added hydrothermally to granites is partly supported by unusually high abundance of thorium in some red, porphyritic, allanite-bearing rocks, but the general petrologic control of thorium abundances argues against major secondary addition of material.

The distribution of thorium and uranium in a Pennsylvanian weathering profile

Abstract Eleven samples representing a pre-Pennsylvanian weathering profile on the Boulder Creek granodiorite near Boulder, Colorado, have been analysed for thorium and uranium by γ-ray spectrometric and chemical methods. In an effort to determine the possible sites of thorium and uranium in these samples, a study of their leachability in hot 2 N hydrochloric acid was undertaken. The fresh granodiorite was found to contain 9·3 p.p.m. thorium and 2·5 p.p.m. uranium. The first stages of weathering resulted in an apparent removal of 25 per cent of the thorium and 60 per cent of the uranium present in the original granodiorite. The leaching study of the fresh granodiorite demonstrated that as much as 90 per cent of the thorium and 60 per cent of the uranium could be removed by an acid leach solution. This seems to indicate that most of the thorium and uranium in the fresh rock is situated in acid soluble minerals or in interstitial materials. After the initial drop of the concentration in the lowest part of the weathered mantle, the total uranium and thorium content of the weathered rock increases by a factor of at least 4 in the uppermost, most-weathered rock material. Leaching studies of the weathered rock indicate that uranium is present largely in the primary resistates, such as zircon, xenotime, and apatite, and thorium occurs mainly in or on clays or in the secondary resistates—minerals formed during weathering.

Hydrologic investigations of the groundwaters of central Texas using U-234U-238 disequilibrium

Abstract U -234 U -238 disequilibrium can be used as a natural tracer in the groundwaters of central Texas. Utilizing the analytical methods of isotope dilution and alpha-particle spectrometry in monitoring variations in the uranium isotopic activity ratios and concentrations of natural waters, it is possible to evaluate the hydrogeology of the Trinity aquifer in terms of (a) the general groundwater circulation pattern, (b) the differentiation of water bodies upon the basis of their uranium signatures, and (c) the determination of absolute flow rates beyond the age limits of other techniques.

The distribution of thorium, uranium, and potassium in the Mancos shale

Abstract Over 135 samples of the upper Cretaceous Mancos shale have been analysed for uranium, thorium, and potassium by chemical and radiometric means. These samples were collected from 16 localities in Colorado, Utah, Arizona and New Mexico. The average concentrations found in the Mancos shale samples are 10·2 p.p.m. thorium, 3·7 p.p.m. uranium, and 1·9 per cent potassium as metal. The average Th/U ratio is 3·1. For the most part, the variations in the thorium, uranium and potassium concentrations are gradual and take place over large distances. Thorium, Th/U ratio and potassium tend to decrease and uranium tends to increase with distance from the upper Cretaceous shoreline. The K/Th ratio in the shale, including sandy intertongues, shows remarkedly little variation on a regional basis. Laboratory studies indicate that the uranium in the Mancos shale is present largely in the fine-grained primary resistate minerals, and the thorium occurs in the fine-grained secondary resistates or fixed in or on clays.

K-Ar dating of Italian plio-pleistocene volcanic rocks

Abstract K-Ar ages were determined on whole rocks and mineral separates from southern Italy volcanics which yielded ages from 6.0 to 0.2 m.y. Most dates of different separates from the same rock are consistent and also coherent from a stratigraphic point of view. In two cases the K-Ar ages are largely inconsistent. The age pattern indicates contemporaneous beginning of activity for different volcanoes, and stresses once again the variation of construction times of volcanic cones.

Thorium, uranium and potassium in some sandstones

Abstract Thorium, uranium, and potassium concentrations have been determined in nineteen sands and sandstones by γ-ray spectrometry, fluorometric uranium analysis, and α-counting. The samples were selected so that both common and extreme thorium and uranium ratios would be represented. The average and nearly uniform values found in orthoquartzitic, clay-free sands were potassium (as metal) 0.64 ± 0.04 percent; thorium 1.7 ± 0.1 p.p.m.; and uranium 0.45 ± 0.05 p.p.m., giving a Th/U ratio of 3.8 ± 0.8. The Th/U ratio found is very near to that of average igneous rocks and may represent an independent determination of the average crustal Th/U ratio. Heavy mineral separations on some of the ordinary sands indicate that very little of the thorium, uranium, and potassium in a common sand is associated with the heavy detrital grains. Neglecting a very minor feldspar content, the radioactivity of the common sandstones studied is contained almost entirely in the quartz, where it appears to be largely related to microscopic and submicroscopic inclusions. The heavy detrital minerals do not begin to affect the Th/U ratio or concentration ratios until they occur in large quantities as in placer sands. Within experimental error, the modern beach sands studied were in radioactive equilibrium. These preliminary data provide some independent and experimental comparisons for the average thorium and uranium content of sandstones calculated from geochemical balances. The data are also helpful in constructing models of the changes in thorium and uranium ratios that might be observed in moving from a beach line through shallow water sediments to deep-water sediments.

LOGNORMALITY OF THORIUM CONCENTRATIONS IN THE CONWAY GRANITE

Abstract The frequency distribution of thorium concentrations in the Conway granite of New Hampshire has been investigated by measurements with a portable gamma-ray spectrometer. A total of 737 determinations has been made, including out-crops within the main central batholith, a set of blocks at one quarry, two 600-ft. cores from the central mass, and one 500-ft. core from a separate body of granite. A lognormal curve appears to be the best description of the frequency distribution of these sets of data. This lognormality fits the mathematical model used by the writers, which predicts a lognormal distribution of trace element concentrations from homogeneous samples of igneous rocks. The model also predicts an arithmetically normal distribution for most common elements and a distribution skewed opposite to lognormality for elements which constitute more than half of the sample studied. The lognormal distribution of thorium concentrations in the Conway granite correlates well with the primary nature of the thorium mineralizations.

The radioactivity of the Ivory Coast tektites and the formation of the Bosumtwi Crater (Ghana)

Abstract Further evidence for the impact origin of the Ivory Coast tektites and the Bosumtwi Crater is presented: uranium, thorium and potassium abundances show very similar patterns in tektites and crater rocks.

Concentration changes of thorium, uranium and other metals in hydrothermally altered Conway granite, New Hampshire

Abstract Concentrations of Th, U, K, Mg, Ca, Mn, Fe, Zn, Co and Cu are significantly different in two sections of a 1000 foot core sample of Conway granite. The differences, attributable to the action of weak hydrothermal fluids, indicate that the fluids, though weak, caused effective redistribution of these elements.