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131.
Some 1800 km2 of Archaean terrain have been mapped (the Eastern Geotraverse of the Botswana Geological Survey, Geodynamics Project) including a part of the schist belt/granitic terrain of the Rhodesian Craton, and part of the Limpopo Mobile Belt in the south. The cratonic area includes the whole or part of four schist belts (greenstone belts) now referred to as ‘schist relics’ as they are shown to be mega-xenoliths surviving regional deformation and granitisation.The schist relics display a typical greenstone belt composition with basal ultra-mafic schist, extrusive meta-basalts, serpentinites, and meta-dacites, -andesites, -rhyolites and volcanoclastic rocks appearing up the succession. These are termed the Volcanic Group. They also contain marble, graphitic phyllite, meta-greywacke, banded ironstone and aluminous schist.The schist relics overlie thick sequences of granitised clastic sediments with intercalated volcanic and sedimentary layers which are regarded as an integral, lower part of the succession. The total sequence, including the Volcanic Group and underlying rocks is at least 30 km thick.The schist relics originally formed a more continuous and lithostratigraphically equivalent succession, which may have included some of the Rhodesian schist belts. Such a succession would be of the order of size of an island arc system in length and width.An early stage of regional folding (F1) is recognized, from preserved fabrics and structural analysis, which is tentatively proposed as nappe folding on a regional scale. This was accompanied by low-grade metamorphism. This first stage of deformation determined the regional pattern as it is still seen.Tonalite/monzonite plutons were emplaced within the Volcanic Group and probably protected the relics during regional granitisation.Regional granitisation, which isolated the schist relics, was accompanied by pyroxene hornfels metamorphism. This was followed by a regional, penetrative deformation (F2) with amphibolite facies (Barrovian type) metamorphism. This is the most prominent style of deformation which, although most intensely developed in the Limpopo Mobile Belt, is imposed throughout the whole area. The Limpopo Belt cuts across the area in the south as a large ductile shear zone and affects rocks largely similar to those of the craton, although anorthosites are not known outside it.The Limpopo Belt has its own style of deformation — interference folds, and several stages of folding related to transcurrent movement, and intense cataclasis.Metamorphism is similar throughout Limpopo Mobile Belt and Craton, each event being recognized in both domains and being of similar grades.In many respects the Botswana Eastern Geotraverse area resembles other Archaean terrains and analogies can be drawn. In attempting to find a geotectonic model into which it can be placed there is a considerable range of choices, and not enough constraints existing either within the imperfectly preserved geological record, nor within the expanding and elastic framework of Global tectonic theory.No direct evidence for subduction zones or plate boundaries has been found, nor is this surprising considering the subsequent history. On the other hand we see nothing in the rather detailed body of evidence that has been accumulated by mapping to preclude a place within Plate theory as outlined for the Phanerozoic. It allows a moderately uniformitarian interpretation rather than one supposing unique conditions preculiar to the Archaean, such as the concept of isolated volcanic depositaries, or of unique and peculiar crustal conditions. 相似文献
132.
Roger H. Mitchell 《Lithos》1977,10(1):29-37
Magnesium ilmenite from discrete nodules and lamellar intergrowths with pyroxene from the Kao, Sekameng, Frank Smith, and Monastery kimberlites has been analysed for Ti, Fe, Mg, Nb, Zr, Cr, Cu, Zn, Mn, Co, and Ni. Each kimberlite contains discrete ilmenites which exhibit a wide compositional range within the ilmenite-geikeilite series. Lamellar ilmenites from Frank Smith and Monastery differ in composition but both show a limited range in composition which lies within the compositional range shown by discrete ilmenites from these pipes. The ilmenites are enriched in Nb, Zr, Cr, Co, and Ni and depleted in Mn, Cu, and Zn relative to Mg-poor ilmenites from basic intrusions. Nb, Zr, and Ni correlate with Fe, Ti and Mg variations but the abundances of the other trace elements are independent of major element variation. R-mode factor analysis is interpreted to imply that the geochemistry cannot be interpreted in terms of a differentiation hypothesis in which trace elements are removed from or concentrated in residua. Factor scores and major element abundances indicate that each pipe is characterized by a particular suite of discrete ilmenite nodules, which are considered to be phenocrysts in a proto-kimberlite magma. Lamellar ilmenite-pyroxene intergrowths are unlikely to have had a eutectic origin, and show no simple relationship to the discrete ilmenites. 相似文献
133.
Megan Gawith Roger Street Richard Westaway Anna Steynor 《Global Environmental Change》2009,19(1):113-121
The UKCIP02 climate change scenarios have become the standard reference for climate change in the UK since their release in 2002. This paper describes and reflects on the ways in which they have been applied. It then identifies some strengths, weaknesses and barriers to their application, and extracts key lessons that may inform the development and provision of future climate change scenarios.Analysis of the application of UKCIP02 shows that the scenarios have been used primarily as a communication device, as well as for scientific research and to inform policy and decision-making on climate change. They have played a critical role in raising awareness on climate change and in engaging organisations in the need to adapt. Their presentation in an accessible style, and their availability in a variety of formats, greatly facilitated their uptake. However, analysis has also revealed weaknesses which served as barriers to their uptake. Some of these, such as file format and accessibility issues, were readily overcome through technical solutions. Others, such as the issue of how to use uncertain information in decision-making, have only been partially addressed and remain an outstanding challenge for future scenario packages. Two key lessons have emerged which may benefit the provision of future climate scenarios in the UK and elsewhere. First, it is not enough to simply make climate change scenarios available. Their provision must be accompanied by ongoing guidance and support to ensure widespread and appropriate uptake. Second, on-going dialogue between those providing scenarios and the communities using them is fundamental to constructively meet the challenges associated with delivering credible scenarios that balance user requirements and expectations with what the science can deliver. 相似文献
134.
Hyunjin Jeong Martin Bureau Sukyoung K. Yi Davor Krajnovi Roger L. Davies 《Monthly notices of the Royal Astronomical Society》2007,376(3):1021-1032
We present Galaxy Evolution Explorer ( GALEX ) far-ultraviolet (FUV) and near-ultraviolet (NUV) imaging of the nearby early-type galaxy NGC 2974, along with complementary ground-based optical imaging. In the ultraviolet, the galaxy reveals a central spheroid-like component and a newly discovered complete outer ring of radius 6.2 kpc, with suggestions of another partial ring at an even larger radius. Blue FUV–NUV and UV-optical colours are observed in the centre of the galaxy and from the outer ring outwards, suggesting young stellar populations (≲1 Gyr) and recent star formation in both locations. This is supported by a simple stellar population model which assumes two bursts of star formation, allowing us to constrain the age, mass fraction and surface mass density of the young component pixel by pixel. Overall, the mass fraction of the young component appears to be just under 1 per cent (lower limit, uncorrected for dust extinction). The additional presence of a nuclear and an inner ring (radii 1.4 and 2.9 kpc, respectively), as traced by [O iii ] emission, suggests ring formation through resonances. All three rings are consistent with a single pattern speed of 78 ± 6 km s−1 kpc−1 , typical of S0 galaxies and only marginally slower than expected for a fast bar if traced by a small observed surface brightness plateau. This thus suggests that star formation and morphological evolution in NGC 2974 at the present epoch are primarily driven by a rotating asymmetry (probably a large-scale bar), despite the standard classification of NGC 2974 as an E4 elliptical. 相似文献
135.
136.
137.
Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite 下载免费PDF全文
Hugues Leroux Damien Jacob Maya Marinova Roger H. Hewins Brigitte Zanda Sylvain Pont Jean‐Pierre Lorand Munir Humayun 《Meteoritics & planetary science》2016,51(5):932-945
Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near‐) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium‐ and coarse‐grained (a few tens to hundreds of micrometers) Ca‐rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high‐temperature (magmatic) origin as revealed by the well‐developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia. 相似文献
138.
Veronika S. HEBER Roger C. WIENS Amy J. G. JUREWICZ Nadia VOGEL Daniel B. REISENFELD Heinrich BAUR Kevin D. McKEEGAN Rainer WIELER Donald S. BURNETT 《Meteoritics & planetary science》2011,46(4):493-512
Abstract– We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the “gold cross” mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code “stopping and range of ions in matter.” The 20Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass‐dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions. 相似文献
139.
Dale P. Cruikshank Allan W. Meyer Roger N. Clark Katrin Stephan Scott A. Sandford Gianrico Filacchione Philip D. Nicholson Thomas B. McCord J. Brad Dalton Dennis L. Matson 《Icarus》2010,206(2):561-572
Several of the icy satellites of Saturn show the spectroscopic signature of the asymmetric stretching mode of C-O in carbon dioxide (CO2) at or near the nominal solid-phase laboratory wavelength of 4.2675 μm (2343.3 cm−1), discovered with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft. We report here on an analysis of the variation in wavelength and width of the CO2 absorption band in the spectra of Phoebe, Iapetus, Hyperion, and Dione. Comparisons are made to laboratory spectra of pure CO2, CO2 clathrates, ternary mixtures of CO2 with other volatiles, implanted and adsorbed CO2 in non-volatile materials, and ab initio theoretical calculations of CO2 * nH2O. At the wavelength resolution of VIMS, the CO2 on Phoebe is indistinguishable from pure CO2 ice (each molecule’s nearby neighbors are also CO2) or type II clathrate of CO2 in H2O. In contrast, the CO2 band on Iapetus, Hyperion, and Dione is shifted to shorter wavelengths (typically ∼4.255 μm (∼2350.2 cm−1)) and broadened. These wavelengths are characteristic of complexes of CO2 with different near-neighbor molecules that are encountered in other volatile mixtures such as with H2O and CH3OH, and non-volatile host materials like silicates, some clays, and zeolites. We suggest that Phoebe’s CO2 is native to the body as part of the initial inventory of condensates and now exposed on the surface, while CO2 on the other three satellites results at least in part from particle or UV irradiation of native H2O plus a source of C, implantation or accretion from external sources, or redistribution of native CO2 from the interior.The analysis presented here depends on an accurate VIMS wavelength scale. In preparation for this work, the baseline wavelength calibration for the Cassini VIMS was found to be distorted around 4.3 μm, apparently as a consequence of telluric CO2 gas absorption in the pre-launch calibration. The effect can be reproduced by convolving a sequence of model detector response profiles with a deep atmospheric CO2 absorption profile, producing distorted detector profile shapes and shifted central positions. In a laboratory blackbody spectrum used for radiance calibration, close examination of the CO2 absorption profile shows a similar deviation from that expected from a model. These modeled effects appear to be sufficient to explain the distortion in the existing wavelength calibration now in use. A modification to the wavelength calibration for 13 adjacent bands is provided. The affected channels span about 0.2 μm centered on 4.28 μm. The maximum wavelength change is about 10 nm toward longer wavelength. This adjustment has implications for interpretation of some of the spectral features observed in the affected wavelength interval, such as from CO2, as discussed in this paper. 相似文献
140.
Enhancing the relevance of Shared Socioeconomic Pathways for climate change impacts,adaptation and vulnerability research 总被引:1,自引:1,他引:0
Bas J. van Ruijven Marc A. Levy Arun Agrawal Frank Biermann Joern Birkmann Timothy R. Carter Kristie L. Ebi Matthias Garschagen Bryan Jones Roger Jones Eric Kemp-Benedict Marcel Kok Kasper Kok Maria Carmen Lemos Paul L. Lucas Ben Orlove Shonali Pachauri Tom M. Parris Anand Patwardhan Arthur Petersen Benjamin L. Preston Jesse Ribot Dale S. Rothman Vanessa J. Schweizer 《Climatic change》2014,122(3):481-494
This paper discusses the role and relevance of the shared socioeconomic pathways (SSPs) and the new scenarios that combine SSPs with representative concentration pathways (RCPs) for climate change impacts, adaptation, and vulnerability (IAV) research. It first provides an overview of uses of social–environmental scenarios in IAV studies and identifies the main shortcomings of earlier such scenarios. Second, the paper elaborates on two aspects of the SSPs and new scenarios that would improve their usefulness for IAV studies compared to earlier scenario sets: (i) enhancing their applicability while retaining coherence across spatial scales, and (ii) adding indicators of importance for projecting vulnerability. The paper therefore presents an agenda for future research, recommending that SSPs incorporate not only the standard variables of population and gross domestic product, but also indicators such as income distribution, spatial population, human health and governance. 相似文献