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81.
Christopher R. J. Charles Pierre-Yves F. Robin Donald W. Davis Phil J. A. McCausland 《Meteoritics & planetary science》2018,53(5):935-951
The approximately spherical shapes of chondrules has long been attributed to surface tension acting on ~1 mm melt droplets that formed and cooled in the microgravity field of the solar nebula. However, chondrule shapes commonly depart significantly from spherical. In this study, 109 chondrules in a sample of CR2 chondrite NWA 801 were imaged by X-ray computed tomography and best-fitted to ellipsoids. The analysis confirms that many chondrules are indeed not spherical, and also that the chondrules’ collective shape fabric records a definite 13% compaction in the host meteorite. Dehydration of phyllosilicates within chondrules may account for that strain. However, retro-deforming all chondrules shows that a large majority were already far from spherical prior to accretion. Possible models for these initial shapes include prior deformation of individual chondrules in earlier hosts, and, as suggested by previous authors, rotation of chondrules as they were solidifying, and/or “streaming” of molten chondrules by their differential velocities with their gaseous hosts after melting. More in situ 3-D work such as this study on a variety of unequilibrated chondrites, combined with detailed structural petrography, should help further constrain these models and refine our understanding of chondrite formation. 相似文献
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Summary Although female street prostitution is widely acknowledged to occur in specific spaces, characteristically referred to as 'red-light' districts, geographers have been remarkably reticent in examining the nature and location of these spaces. Against a backdrop of mounting debate surrounding the legal status of prostitution in England and Wales, this paper considers the importance of such areas in the regulation and containment of street prostitution, and speculates as to how changing social and legal attitudes might result in very different geographies of prostitution. 相似文献
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Mike Walker Sigfus Johnsen Sune Olander Rasmussen Trevor Popp Jørgen‐Peder Steffensen Phil Gibbard Wim Hoek John Lowe John Andrews Svante Björck Les C. Cwynar Konrad Hughen Peter Kershaw Bernd Kromer Thomas Litt David J. Lowe Takeshi Nakagawa Rewi Newnham Jakob Schwander 《第四纪科学杂志》2009,24(1):3-17
The Greenland ice core from NorthGRIP (NGRIP) contains a proxy climate record across the Pleistocene–Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision. This climatic event is most clearly reflected in an abrupt shift in deuterium excess values, accompanied by more gradual changes in δ18O, dust concentration, a range of chemical species, and annual layer thickness. A timescale based on multi‐parameter annual layer counting provides an age of 11 700 calendar yr b2 k (before AD 2000) for the base of the Holocene, with a maximum counting error of 99 yr. A proposal that an archived core from this unique sequence should constitute the Global Stratotype Section and Point (GSSP) for the base of the Holocene Series/Epoch (Quaternary System/Period) has been ratified by the International Union of Geological Sciences. Five auxiliary stratotypes for the Pleistocene–Holocene boundary have also been recognised. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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The Rotoiti (~120 km3) and Earthquake Flat (~10 km3) eruptions occurred in close succession from the Okataina Volcanic Centre at ~50 ka. While accessory mineral geochronology
points to long periods of crystallization prior to eruption (104–105 years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production
and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic
system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An37–74, ~40–227 ppm MgO, 45–227 ppm TiO2, 416–910 ppm Sr and 168–1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10–30%) in Sr, MgO
and XAn that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence
for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization
of modal assemblage. MI also display a wide range in XH2O that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied
by abrupt increases in Ti concentration (30–80 ppm) that reflect temperature increases ~50–110°C. Diffusion across these resorption
horizons is restricted to zones of <20 μm, suggesting most crystallization within the magma occurred in <2000 years. These
episodes are brief compared to the longevity (104–105 year) of the crystal mush zones. All textural and compositional features observed within the quartz and plagioclase crystals
are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system
with silicic melt. These periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed
the caldera-forming Rotoiti eruption. 相似文献
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Geochemical and statistical approach for evaluation of heavy metal pollution in core sediments in southeast coast of India 总被引:6,自引:2,他引:4
B. R. R. Seshan M.Phil. U. Natesan Ph.D. K. Deepthi M.Sc. 《International Journal of Environmental Science and Technology》2010,7(2):291-306
Industrialization coupled with urbanizaton has led to stress in the Buckingham Canal which runs parallel to Bay of Bengal at a distance of around 1 km from the coastline. 4 sediment cores were collected along Ennore — Pulicat stretch to determine acid leachable trace metal concentration. Core samples were collected using gravity corer. The cores were sliced horizontally at 2.5 cm to determine the grain size, sediment composition, pH, organic matter, calcium carbonate, acid leachable trace metals; cadmium, chromium, copper, lead, zinc. The trace metals were extracted using acid mixture containing hydro fluoric acid, nitric acid and sulphuric acid and analysed by atomic emission spectrophotometer. In an attempt to infer anthropogenic input from geogenic input, several approaches including comparison with sediment quality guidelines — ecotoxicological sense of heavy metal contamination and classification by quantitative indexes such as geoaccumalation index, anthropogenic factor, enrichment factor, contamination factor and degree and pollution load index was attempted. Grain size analysis and sediment composition of core samples shows Ennore is sandy in nature having a neutral pH. Organic matter enrichment is observed to a higher extent in core 3. Core 2 at a depth of 5 cm shows organic matter of 9.4 %. calcium carbonate is totally absent at the surface sediments in core 2. Cores collected within the canal showed a higher heavy metal concentration than the cores collected from Pulicat lagoon and 2 km into the Ennore Sea. The trace metal concentration for cadmium, lead and zinc in Ennore does not pose a threat to the sediment dwelling fauna whereas chromium and copper are likely to pose a threat. Quantitative indexes place Ennore under moderately polluted. Ennore is likely to face a serious threat of metal pollution with the present deposition rates unless stringent pollution control norms are adopted. 相似文献