John J. W. Rogers

Radiometric determination of thorium, uranium and potassium in basalts and in two magmatic differentiation series

Abstract Thorium, uranium and potassium contents of basalts and other basic rocks have been measured by gamma-ray spectrometry. The ratios of Th/K, U/K and Th/U are constant within one order of magnitude in a wide variety of rock types. The averages for basalts are: Th/K × 104 = 2.8, U/K × 104 = 0.60 and Th/U = 4.8. Some tendency exists, however, for increase in these ratios with igneous differentiation, as shown by investigations of the Duluth and Southern California intrusive sequences. The Th/U ratio is particularly low in tholeiites (1·6) from the orogenically active Japanese area and is also lower in the basic rocks of the Duluth and Southern California sequences than in most basalts. Apparently the process which leads to the formation of magma in orogenic areas causes removal of thorium, potassium and to a lesser extent, uranium from source materials before intrusion or eruption. Close relationship between potassium content and Th/U ratio in basic rocks from widely separated areas, however, suggests that the subcrust in which basic magmas originate is compositionally uniform and differences between the compositions of primary basalt magmas are probably the result of local processes.

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.

A worldwide comparison of alkali olivine basalts and their differentiation trends

Abstract Worldwide alkali olivine basalts (AOB) and their differentiation series have been subdivided into continental, oceanic, or island-arc assemblages according to the inferred crustal environment at their time and place of eruption. No systematic differences have been found in major element composition of the AOBs from these three different environments. As plotted on (Na 2 O + K 2 O) vs. SiO 2 and AMF diagrams, AOB differentiation trends also show no differences between environments. Thus, AOB appears to be a primary magma generated at sufficient depth in the mantle that its major element content is unaffected by chemical or thermal differences between mantle regions underlying continents, ocean basins, or island arcs. The major element chemistry of AOB is also apparently unaffected by passage through different types of crust.

Variation of thorium and uranium in selected granitic rocks

Abstract A selected suite of granitic rocks representing both individual differentiation sequences and a broad sampling of granites from North America have been analysed for thorium, uranium, and potassium by γ-ray spectrometry. In the White Mountain and Oliverian series of New Hampshire both the thorium and uranium contents and the Th/U ratio tend to increase with igneous evolution. In the Southern California batholith the thorium and uranium contents increase during differentiation, but the Th/U ratio is constant. The general tendency for the Th/U ratio to increase with petrogenetic evolution is shown by a comparison of the Th/U ratio with the ratio of potassium feldspar to plagioclase for a broad sampling of granitic rocks. This increase is probably caused by oxidation during magmatic differentiation

Late cenozoic alkali-olivine basalts of the Basin-Range Province, USA

Petrographic and chemical analyses demonstrate that late Cenozoic mafic lavas from the Basin-Range Province, western United States, are predominantly alkali-olivine basalts. Associated with these lavas are lesser volumes of basaltic andesite which appear to be differentiates from the more primitive alkali basalts. Late Cenozoic basalts from adjacent regions (Columbia River Plateau, Snake River Plain, Yellowstone area, High Cascades and Sierra Nevada) are predominantly tholeiitic. This apparent petrologic provincialism is supported by complementary variations in heat flow, seismic velocities, crustal thickness, magnetic anomalies and geologic setting.Alkali-olivine basalts from Japan and eastern Australia are analogous to those from the Basin-Range province both in composition and tectonic environment. It is suggested that these lavas are the products of a unique environment characterized by high heat flow and a thin crust.Recent melting experiments on peridotites and basalts and measurements of heat flow allow limits to be placed on the depth of origin of Basin-Range alkali-olivine basalt magmas. It is proposed that these lavas are produced by partial melting (less than 20%) of peridotitic mantle material at depths between 40 and 60 km in response to an elevated geothermal gradient. The basaltic andesites may be derived from hydrous alkali basalt magma by fractionation at depths of 30 to 40 km.

Relationships between porosity, median size, and sorting coefficients of synthetic sands

ABSTRACT The relationship between porosity, median size, and sorting coefficients has been investigated by studying synthetic sands with lognormal size distributions and various median sizes and sorting coefficients. Poorly sorted sands are considerably less porous than well sorted ones, and porosities show an inverse linear relationship to sorting coefficient except in very well sorted samples in which porosity increases more rapidly than sorting. Porosity is independent of median size in well sorted sands but appears to decrease slightly with increasing median size in poorly sorted sands. The relationship between median size and porosity probably results from the fact that larger grains have a higher sphericity and tend to pack more closely together than smaller, more irregularly shaped grai s.

Origin and differentiation of Triassic dolerite magmas, North Carolina, USA

Ten dolerite dikes intruded into Triassic fault troughs in the Piedmont area of North Carolina have been analyzed for the contents of major elements plus selected trace elements. The average composition of the initial magma, as indicated by four chill margins for major elements and three for trace elements, is: SiO2, 48.6%; Al2O3, 16.9%; TiO2, 0.57%; Fe2O3, 3.30%; FeO, 6.72%; MgO, 10.59; CaO, 10.42%; Na2O, 2.03%; K2O, 0.20%; MnO, 0.20%; Rb, 2.6 ppm; Sr, 133 ppm; Zr, 46 ppm; Th, 0.4 ppm; and U, below detection limit of approximately 0.1 ppm. One large dike (BP) exhibits a Palisades-type of differentiation by crystal settling of olivine, and the comparatively thick JY dike shows development of micropegmatite toward the center; the smaller dikes, however, are relatively homogeneous across their width. Study of the relationship between SiO2 content and the ratio FeO+Fe2O3/MgO+ FeO+Fe2O3 indicates that most dikes crystallize under conditions of decreasing oxygen pressure, but the differentiation trend of the JY, RD, and RS dikes indicates either constant or increasing oxygen pressure during their evolution.Statistical comparison of the composition of the initial dolerite magmas with a variety of basalt types around the world suggests that the North Carolina dolerites are far more similar to oceanic or oceanic margin tholeiites than to continental tholeiites. The North Carolina rocks are distinctly different from plateau basalts but are similar to the chill zones of the Precambrian Bushveld and Stillwater lopoliths. The comparatively low contents of Th, U, and Sr, plus the relatively high K/Rb ratio all support the possibility that the magmas for the North Carolina dolerites evolved in a dominantly oceanic environment. It seems distinctly possible that continental-type crust and mantle did not exist in the Appalachian Piedmont area in Triassic time, even after major orogeny and the concurrent formation of granitic intrusions.

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.

Paleozoic and Lower Mesozoic Volcanism and Continental Growth in the Western United States

Compositions of Paleozoic and Lower Mesozoic volcanic rocks of western Nevada and adjoining areas reflect the tectonic condition of their time and place of eruption. Despite structural complication and metamorphic overprinting, a general change can be observed from continental, cratonic, volcanic assemblages in the east to oceanic assemblages in the west. In Paleozoic (pre-Sonorria) time, an ensimatic basin was present west of the craton margin and east of correlative island-arc assemblages. In early Mesozoic time, continental assemblages passed westward and northwestward into continental margin assemblages in far western Nevada and into island-arc assemblages farther west. These arc assemblages may have formed on a crust that was compositionally intermediate between oceanic and continental crust. The North American continent grew westward by accretion of one or more island arcs in late Paleozoic–early Mesozoic (Sonoma) time and by general segregation of sial along the margin. Crustal thickening occurred during and probably following the Sonoma orogeny. The sialic material must have been newly formed from the mantle and represented significant westward continental growth in the past 300 m.y.

Sizes of Naturally Abraded Materials

ABSTRACT Sand and silt are formed by two different abrasional mechanisms. Silt grains generally represent chips broken off of larger grains, and large grains are rarely abraded down to the fine sand or silt range. The sand and silt thus represent two different populations, each with a characteristic size distribution curve, and with little overlap between the two. The break between the populations probably occurs at a size of approximately 0.25 mm and accounts for the commonly observed deficiency of sedimnet within 0.05 to 0.25 mm range.