Age relationships between greenstone belts and “granites” in the Rhodesian Archaean craton

Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced2780 ± 30 m.y. ago.The initial⁸⁷Sr/⁸⁶Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement.     

Ergs

Ergs (areas of aeolian sand deposits) invariably have bed forms usually with a 3-order hierarchy of ripples, dunes and draas (large sandy bed forms). Ergs may occur in any relatively vegetation-free area with an adequate sand supply. The virtual absence of ergs from highland areas is attributed largely to acceleration and divergence of the winds which makes the sandflow undersaturated. Ergs with thick sand deposits and draa development in the Sahara, Arabia and Asia are contrasted with thinner dune ergs in Australia which lack draa development. Sand thickness, proportion of sand-cover, bed-form height, spacing and orientation vary in a significant way with position within the Algerian ergs. This may partly be due to regional variations in the wind regime but more probably to regional grain-size variations which, because they determine the threshold velocity of sand movement, control the effective wind regime in an area. Grain-size variations can be interpreted from aerial photographs to a large extent because of their effect on the size and shape of the bed form. A study of the northern Algerian ergs shows that regional patterns of sandflow may vary according to the grain size considered and that there is no simple relation between wind directions and the draa trends in any area even although the latter are maintained consistently for large distances.     

Antimatter bodies and closure of the Universe

One of the hypotheses to account for the cosmic gamma-ray bursts invokes collisions between antimatter bodies of asteroidal mass and stars. This paper points out that, should this idea be correct, the numbers suggest that the antimatter is cosmological, and that alocally measurable ratio might be used to determine the effective gain in mean mass density relevant to the possible closure of the Universe. The effective mass gain factor would have a minimum value of 2 and could be much larger.     

Volcanism on a Proterozoic continental margin in northwestern Queensland

The oldest rocks exposed in northwestern Queensland are metamorphosed calc-alkaline volcanics (Leichhardt Metamorphics), which are intruded by elongate tonalitic to granitic batholiths (Kalkadoon Granite). These rocks are overlain by a less metamorphosed sequence containing basic lavas (Magna Lynn Metabasalt) overlain by extensive ignimbritic rhyolite and dacite (Argylla Formation). Sequences of basalt and psammite overlie the rhyolite unconformably and are overlain in turn unconformably by psammitic, pelitic, carbonate and possibly evaporitic sediments and minor volcanics. Younger granites intrude these rocks.The mineral assemblages of the Leichhardt Metamorphics, Magna Lynn Metabasalt and Argylla Formation indicate greenschist and lower amphibolite facies of metamorphism. The rocks contain no glass and some are obviously recyrstallized; however, phenocrysts, lithic fragments, spherulites, amygdales and flow-top breccias are still recognizable.Sixty-nine of the least deformed volcanic and sub-volcanic rocks were analysed for major elements and up to twenty trace elements. Element dispersion in these analyses indicated that metasomatism was probably of limited extent. The Magna Lynn Metabasalt is similar to low-potassium tholeiite, as it has less than 0.5% potash, high normative hypersthene, some normative quartz and typical Ti/Zr/Y ratios. The acid volcanics have calc-alkaline affinities although andesite is not common and the alumina content is relatively low. They have high K/Na ratios and their trace elements (especially Ba, Sr, Rb, Zr and Ce) are similar to Andean volcanics. The tholeiitic sequences that overlie the calc-alkaline volcanics in northwestern Queensland resemble the basaltic sequences of western U.S.A. that also overlie calc-alkaline volcanics.The predominantly calc-alkaline volcanics of northwestern Queensland are believed to have formed at a continental margin similar to that in the Andean region. The younger tholeiitic lavas and minor continental acid volcanism possibly accompanied crustal tension. Later regional metamorphism and intrusion of large granite batholiths stabilized the region.     

RECOVERY AND CLASSIFICATION OF THIRTY NEW METEORITES FROM ROOSEVELT COUNTY,NEW MEXICO

We report the discovery and classification of 30 new meteorites found in or close to Roosevelt County, New Mexico, including two H3 chondrites and a ureilite; the others are equilibrated ordinary chondrites. Over 160 meteorites representing at least 100 different falls have been recovered from this region, mostly from wind blowout areas. As in Antarctica, small specimens predominate and irons, achondrites and C and E chondrites are rare. Paired specimens are also very difficult to identify.     

Seismic isolation of cable‐stayed bridges for near‐field ground motions

This study examines the efficacy of using seismic isolation to favorably influence the seismic response of cable‐stayed bridges subjected to near‐field earthquake ground motions. In near‐field earthquake ground motions, large amplitude spectral accelerations can occur at long periods where many cable‐stayed bridges have significant structural response modes. This combination of factors can result in large tower accelerations and base shears. In this study, lead–rubber bearing seismic isolators were modeled for three cable‐stayed bridges, and three cases of isolation were examined for each bridge. The nine isolated bridge configurations, plus three non‐isolated configurations as references, were subjected to near‐field earthquake ground motions using three‐dimensional time‐history analyses. Introduction of a small amount of isolation is shown to be very beneficial in reducing seismic accelerations and forces while at the same time producing only a modest increase in the structural displacements. There is a low marginal benefit to continue to increase the amount of isolation by further lengthening the period of the structure because structural forces and accelerations reduce at a diminishing rate whereas structural displacements increase substantially. In virtually all cases the base shears in the isolated bridges were reduced by at least 50several instances by up to 80individual near‐field records showed large variability from one record to the next, with coefficients of variation about the mean as large as 50assessing the characteristics of near‐field ground motion for use in isolation design of cable‐stayed bridges. Copyright © 2003 John Wiley & Sons, Ltd.     

Nanoscale effects of strontium on calcite growth: An in situ AFM study in the absence of vital effects

This experimental study presents in situ measurements of step migration rates for layer growth of calcite at various levels of superaturation and fluid Sr concentrations. Our results show that Sr has complex behavior as an impurity. At low concentrations, Sr promotes faster growth. This effect may be associated with slight shifts in calcite solubility when Sr is incorporated or may be due to as yet uncharacterized kinetic effects. At higher concentrations, Sr stops step advancement by pinning kink-sites or step edges. The threshold concentration of Sr needed to halt growth is positively correlated with supersaturation.Addition of Sr to the calcite growth system leads to significant changes in hillock morphology. Hillocks become elongate perpendicular to the projection of the c-glide plane, in contrast to the changes previously reported for Mg. Step edges also become scalloped, and the boundary between the obtuse-stepped flanks disappears and is replaced by a new step direction with edges parallel to [010].Incorporation of Sr was measured at two supersaturation levels and identical fluid [Sr]. The results indicate a strong positive correlation between fluid supersaturation and crystal Sr content. Further, Sr is strongly fractionated between obtuse- and acute-stepped flanks by a factor of approximately two. The sensitivity of Sr uptake to supersaturation may explain apparently contradictory results in the literature regarding whether Sr uptake in the calcite produced by one-celled marine organisms is controlled by temperature. In addition, Sr contents of natural calcite samples may be good indicators of the levels of supersaturation at which the crystals grew.Results of this investigation demonstrate the importance of understanding impurity-specific interactions with calcite growth surfaces at the microscopic scale. Despite similar chemical behavior in some systems, Mg and Sr clearly have very different effects on calcite growth. If Sr and other impurities are to be used as robust indicators of growth conditions in natural calcite samples, well grounded understanding of the mechanisms of recording trace element signatures in calcite is an essential step toward correctly deciphering paleoenvironmental signals from fossil calcite compositions.     

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.