Kornerupine in Mg-Al-rich gneisses from Namaqualand,South Africa: mineralogy and evidence for late-metamorphic fluid activity

Three kornerupine occurrences are reported in distinctive SiO₂-poor, MgO- and Al₂O₃-rich paragneisses from the Namaqualand Metamorphic Complex in South Africa. Kornerupine coexists stably with phlogopite, cordierite, orthopyroxene, gedrite, sapphirine, sillimanite and plagioclase and, in sapphirine-free rocks, with spinel and corundum. Tourmaline of a texturally older generation than kornerupine is commonly present in the same samples.Ten analysed kornerupines show a variation in total Fe as FeO from 1.8 to 10.9 weight per cent. B₂O₃ contents are estimated from x-ray data and a few spectrochemical analyses to range from 0.9 to 3.5 weight per cent. There is a strong inverse correlation between B³⁺ and Al³⁺. Total iron content has a strong and systematic effect on refractive index, colour and dispersion. Fe and Mg are systematically partitioned with the other minerals, and Mg/(Mg+Fe) ratios increase as follows: spinel 相似文献   

Sedimentology and kinematics of a large, retrogressive growth-fault system in Upper Carboniferous deltaic sediments, western Ireland

Growth faulting is a common feature of many deltaic environments and is vital in determining local sediment dispersal and accumulation, and hence in controlling the resultant sedimentary facies distribution and architecture. Growth faults occur on a range of scales, from a few centimetres to hundreds of metres, with the largest growth faults frequently being under‐represented in outcrops that are often smaller than the scale of feature under investigation. This paper presents data from the exceptionally large outcrops of the Cliffs of Moher, western Ireland, where a growth‐fault complex affects strata up to 60 m in thickness and extends laterally for ≈ 3 km. Study of this Namurian (Upper Carboniferous) growth‐fault system enables the relationship between growth faulting and sedimentation to be detailed and permits reconstruction of the kinematic history of faulting. Growth faulting was initiated with the onset of sandstone deposition on a succession of silty mudstones that overlie a thin, marine shale. The decollement horizon developed at the top of the marine shale contact for the first nine faults, by which time aggradation in the hangingwall exceeded 60 m in thickness. After this time, failure planes developed at higher stratigraphic levels and were associated with smaller scale faults. The fault complex shows a dominantly landward retrogressive movement, in which only one fault was largely active at any one time. There is no evidence of compressional features at the base of the growth faults, thus suggesting open‐ended slides, and the faults display both disintegrative and non‐disintegrative structure. Thin‐bedded, distal mouth bar facies dominate the hangingwall stratigraphy and, in the final stages of growth‐fault movement, erosion of the crests of rollover structures resulted in the highest strata being restricted to the proximity of the fault. These upper erosion surfaces on the fault scarp developed erosive chutes that were cut parallel to flow and are downlapped by the distal hangingwall strata of younger growth faults.     

A study of volcanogenic exhalation mineralization

This paper considers volcanogenic exhalation mineralization using data from 35 years of observation of fumarole activity during an earlier phase of the posteruptive activity of the Second Cone, which is one of the New Tolbachik volcanoes that were formed during the eruption at the North Vent of the Great Tolbachik Fissure Eruption (Kamchatka, 1975–1976). We describe the main types of mineral associations, identify the key mineral species and the secondary and accessory minerals, as well as the sequence of mineral generation. We provide a summary of minerals and compounds that have been identified in ejecta of fumaroles on the Second Cone.     

Ice flow direction during late Paleozoic glaciation in western Paraná Basin, Brazil

The recent discovery of ice-striated surfaces associated with the late Paleozoic Aquidauana Formation suggests that glaciers coming from southwest Africa reached westernmost parts of the Paraná Basin in central Brazil. Abrasion features were developed by glaciers moving from SSE towards NNW, mainly on an unconsolidated bed. These records expand to about 1,050,000 km², the coverage of the late Paleozoic glaciation in the region of the Paraná Basin in Western Gondwana.

Resumen

A recente descoberta de superfícies estriadas associadas à Formação Aquidauana, de idade permocarbonífera, sugere que as geleiras provenientes do sudoeste da África alcançaram as porções ocidentais da Bacia do Paraná, na região central do Brasil. As feições de abrasão foram geradas pelo deslocamento de geleiras de SSE para NNW, principalmente sobre substrato inconsolidado. Estes novos registros evidenciam que a glaciação neopaleozóica cobriu uma área de pelo menos de 1.050.000 km² na região ocupada pela Bacia do Paraná no Gondwana Ocidental.     

Selenium concentration in food and blood of residents of Abeokuta Metropolis,Southwestern Nigeria

Selenium like any other element can both be essential and toxic depending upon the concentration at which it occurs in human diet. This paper assessed the level of this trace element in twenty different (20) types of locally produced food items around Abeokuta town and the blood obtained from the antecubital fossa vein of eighty-eight (88) healthy adults resident in Abeokuta.     

Reaching critical mass? Theory, politics, and the culture of debate in children's geographies

A small but growing number of voices have begun to raise questions about the current direction of children's geographies as a subfield and its status within the wider discipline. This article intervenes in these emerging discussions to examine the status of debate itself within children's geographies. I argue that children's geographies over the past decade has operated primarily in a consensus-based mode, with a number of potential tensions and differences between practitioners masked as a result. I develop the example of notions of children's competent social agency, a core theoretical assumption that is rarely interrogated in much depth. In closing, I pose questions regarding several contemporary political issues concerning children's agency about which geographers have had surprisingly little to say. I suggest that explicitly addressing some of these vexing issues would contribute to a richer state of debate within children's geographies.     

Ice-sheet growth and high-latitudes sea-surface temperature

The response of the LLN 2-D climate model to the insolation and CO₂ forcings during the Eemian interglacial is compared to reconstructions obtained from deep-sea cores drilled in the Norwegian Sea and in the North Atlantic. Both reconstructions and modeling results show a decrease of sea-surface temperature (SST) in the higher latitudes (70–75 °N zonal belt for the model and the Norwegian Sea for the proxy records), associated with a more moderate cooling at lower latitudes (50–55 °N and North Atlantic), at the middle of isotopic substage 5e, several millenia before the beginning of continental ice-sheet growth. Such a comparison between the simulated SST and ice volume of the Northern Hemisphere has been extended to the whole last glacial-interglacial cycle. The influence of the insolation forcing on SST and the shortcomings of the model due to its zonal character are discussed. Received: 6 July 1995/Accepted: 19 December 1995     

Scope for a small circumsolar annular gravitational contribution to the Pioneer anomaly without affecting planetary orbits

All proposed gravitational explanations of the Pioneer anomaly must crucially face the Equivalence Principle. Thus, if Pioneers 10 and 11 were influenced by anomalous gravitational effects in regions containing other Solar System bodies, then those bodies should likewise be influenced, irrespective of their shape, composition or mass. Although the lack of any observed influence upon planetary orbits severely constrains such explanations, here we aim to construct by computer modeling, hypothetical gravitating annuli having no gravitational impact on planetary orbits from Mercury to Neptune. One model has a central zone, free of radial gravitation in the annular plane, and an ‘onset’ beyond Saturn’s orbit, where sunward annular gravitation increases to match the Pioneer anomaly data. Sharp nulls are included so that Uranus and Neptune escape this influence. Such models can be proportionately reduced in mass: a 1 % contribution to the anomaly requires an annulus of approximately 1 Earth mass. It is thus possible to comply with the JPL assessment of newly recovered data attributing 80 %, or more, of the anomaly to spacecraft heat, which appears to allow small contributions from other causes. Following the possibility of an increasing Kuiper belt density at great ranges, another model makes an outward small anomalous gravitation in the TNO region, tallying with an observed slight indication of such an effect, suggesting that New Horizons may slightly accelerate in this region.     

Thermodynamics of minerals of variable composition: Andradite-grossularite and pistacite-clinozoisite solid solutions

Exchange-mineral equilibria with Al and Fe³⁺ aqueous chloride solutions (aq.), Andr + AlCl _aq ³ = FeCl _aq ³ + Gros, (1) Psc + AlCl _aq ³ = FeCl _aq ³ + Czo, (2) were studied under the following experimental conditions: 500°;C and 580°;C and 1 and 2 kbar, respectively, with an overall concentration of metals in the aqueous solutions of about 0.5 M and pH 3. The mixing functions of the components in garnet and epidote were calculated from the experimental data. Thermodynamic treatment of experimental evidence for reaction (1) led to the conclusion that, within the accuracy of experiment, garnet in the andradite-grossularite series was an ideal solid solution. However, epidote solid solution markedly departed from the ideal, as was shown by concentration and pressure-temperature (PT) dependencies of Gibbs's molar excess energies and by mixing-volume concentration dependence.