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Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Storø), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO2 in combination with addition of Al2O3, MgO, K2O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO2. The juxtaposition of relatively silica- and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO2 allowing corundum to stabilize in the former rocks. Furthermore, addition of Al2O3 via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of ultramafic rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface. 相似文献
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Juan Miguel Guotana Tomoaki Morishita Ikuya Nishio Akihiro Tamura Tomoyuki Mizukami Kenichiro Tani Yumiko Harigane Kristoffer Szilas D.Graham Pearson 《地学前缘(英文版)》2022,(1):297-309
Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt(ISB),hosted in the Itsaq Gneiss Complex of southwestern Greenland.These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record.Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages.Ti-rich humite-group minerals such as titanian chondrodite(Ti-Chn)and titanian clinohumite(Ti-Chu)often occur as accessory phases in the metamorphosed ultramafic rocks.The Ti-rich humite minerals are associated with metamorphic olivine.The host olivine is highly forsteritic(Fo96-98)with variable Mn O and Ni O contents.The concentrations of the rare-earth elements(REE)and high-field strength elements(HFSE)of the metamorphic olivine are higher than typical mantle olivine.The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting.The close association of olivine,antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between$1.3–2.6 GPa within the antigorite stability field(<700°C).The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels. 相似文献
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DNA adducts in gills and digestive gland, as well as polycyclic aromatic hydrocarbon (PAH) tissue levels were analysed in blue mussels (Mytilus spp.) from Nordic coastal areas (Iceland, Norway and Sweden) with diffuse or point sources of PAHs of various origins. Both DNA adduct and PAH tissue levels were generally low, indicating low PAH exposure to the mussels in the areas studied. DNA adducts were found to be higher in gills than in digestive gland of the mussels at all sites studied. Elevated DNA adduct levels in gills were found at 6 sites out of 18 compared to reference sites in respective coastal zones. Adduct levels ranged from 0.5 to 10 nmol adducts/mol normal nucleotides, being highest in mussels from Reykjavík harbour, Iceland (intertidal mussels), and from Fiskaatangen, Norway (subtidal mussels). Total PAH tissue levels in the mussels ranged between 40 and 11,670 ng/g dry wt., and were significantly correlated with DNA adduct levels (r(2)=0.73, p<0.001). PAH ratio values indicated that the PAHs were in most cases of pyrolytic origin, but with petrogenic input near harbours and an oil refinery. 相似文献
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Shear-wave splitting beneath the Snake River Plain suggests a mantle upwelling beneath eastern Nevada, USA 总被引：1，自引：0，他引：1
Kristoffer T. Walker Götz H.R. Bokelmann Simon L. Klemperer 《Earth and Planetary Science Letters》2004,222(2):529-542
The Snake River Plain (SRP), a 90-km-wide topographic depression in southern Idaho, is a topographically anomalous feature in the western U.S. Previous seismic studies focused on the northeastern SRP to study its relationship with the Yellowstone hotspot. We present new teleseismic shear-wave splitting data from six broadband seismic stations deployed along the axis of the SRP from June 2000 to September 2001. We also analyze splitting at HLID, a permanent station of the National Seismic Network located ∼100 km north of the plain. Splitting of individual teleseismic phases is consistent at all stations within 2σ errors, and we favor the interpretation of anisotropy with a single horizontal fast axis, although a dipping-axis interpretation is statistically permitted at two of the stations. Our station fast directions, as well as shear-wave splitting data from numerous other stations throughout the Basin and Range, are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume-like upwelling material beneath eastern Nevada into a southwestward flowing asthenosphere (with respect to a fixed hotspot reference frame). This parabolic asthenospheric flow (PAF) model for the Great Basin is attractive because it explains the observed high elevations, high mantle buoyancy, low-velocity anomaly beneath eastern Nevada, high heat flow, and depleted geochemistry of some erupted basalts. The lack of Pliocene-Recent major volcanism in eastern Nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy. Our splitting station delay times vary in a way not predicted by the PAF model, and can be explained by: a zone of aligned magma-filled lenses and/or partially molten dikes beneath the SRP lithosphere, a depleted olivine-rich residuum underneath the sides of the eastern SRP, and/or the effect of lateral lower crustal flow from beneath the eSRP toward its adjacent flanks. 相似文献
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Cylindrical sediment traps were deployed at various depths in the anoxic water of Framvaren for two periods of one year (1981–1982 and 1983–1984). The traps were emptied three times during 1981–1982 and five times during 1983–1984. The vertical fluxes of total suspended material, organic carbon and nitrogen were calculated on a daily and annual basis. The average annual sediment flux 20 m above the bottom was approximately 60 g m−2 y−1 and the flux of organic carbon was 20 g m−2 y−1. On the basis of an average C/N ratio of 8 and a constant carbon flux below a depth of 20 m, it is concluded that little mineralization of the organic matter takes place in the anoxic water column. Assuming a primary production of the order to 50–100 g m−2 y−1, 22–24% of that reaches the anoxic water masses. Further breakdown of organic matter takes place in the surface sediments. 相似文献
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Three-dimensional modelling of an isolated carbonate buildup (Triassic, Dolomites, Italy) 总被引：1，自引：0，他引：1
Wolfgang Blendinger Peter Brack† Arne Kristoffer Norborg‡ Erik Wulff-Pedersen§ 《Sedimentology》2004,51(2):297-314
The Middle Triassic carbonate buildups of the Dolomites show facies similarities with mud mounds but display apparent architectural elements of flat‐topped carbonate platforms. In order to test whether the facies similarities to mounds are also reflected in the internal buildup architecture, a three‐dimensional modelling study of the Middle Triassic Monte Cernera buildup has been carried out. The Cernera buildup exhibits apparent geometries suggesting a mounded platform in the lower and uppermost part of the buildup, separated by an interval with apparent platform geometry and a retrogradational platform interior, which is difficult to explain with a flat top platform model. For this purpose, a number of three‐dimensional models were constructed using the three‐dimensional modelling programme petrel TM. Key geological horizons were constructed based on outcrop measurements, intermediate horizons were calculated in the modelling program, and the intersections of the modelled layers with a digital topography surface were displayed and compared with outcrop photographs. The models were refined stepwise until a best fit with the actual bedding architecture was achieved. The best fit model shows that the mounded geometries in the lower and uppermost part of the buildup are real architectural elements. The intermediate platform stage, about 1·5 km across, had probably retained a mounded top with a relief of up to 50 m, which is difficult to distinguish from an absolutely flat top, but necessary to explain the retrogradational platform interior. The present study shows that Monte Cernera was dominated by mounded geometries at all stages of platform development. The mounded geometry plus facies data suggest that the platform is a deep‐water accumulation, below the zone of intense wave energy, but within the photic zone. The Cernera represents a tropical buildup type, which did not have the capacity to grow into continuously wave‐swept environments because of the small size and the absence of a wave‐resistant energy barrier. Such buildup types are probably common after major crises in earth history, when reef organisms were virtually absent. 相似文献
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Kristoffer Szilas Vincent J. Van Hinsberg Alex F.M. Kisters J. Elis Hoffmann Brian F. Windley Thomas F. Kokfelt Anders Scherstén Robert Frei Minik T. Rosing Carsten Münker 《Gondwana Research》2013,23(2):436-451
The Tartoq Group, located in SW Greenland, consists of supracrustal rocks of mainly tholeiitic basaltic composition, including pillow lavas, sills/dykes and gabbros, as well as ultramafic rocks. Metamorphic grade ranges from greenschist facies to granulite facies. The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean TTG orthogneisses. The supracrustal rocks form part of a SE vergent fold and thrust belt consistent with the imbrication of TTG gneisses and supracrustal rocks along a convergent margin. LA-ICP-MS U–Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 ± 4 Ma for the Tartoq Group. This age is consistent with MC-ICP-MS Lu–Hf and Sm–Nd isotopic whole-rock data for mafic samples from different blocks of the Tartoq Group, which yield errorchron ages of 3189 ± 65 Ma and 3068 ± 220 Ma, respectively. The mafic supracrustal rocks of the Tartoq Group have chondrite-normalized REE patterns with LaCN/SmCN of 0.67–1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26–1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4 indicative of an arc affinity when compared to rocks from modern settings. The similar geochemistry of the different lithological units, together with their coeval formation, as evident from trace element geochemical trends, supports a co-magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust. Accordingly, we propose that the Tartoq Group represents remnants of Mesoarchaean oceanic crust, which formed in a suprasubduction zone geodynamic environment. 相似文献
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Artificial neural networks for removal of couplings in airborne transient electromagnetic data 
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下载免费PDF全文 Kristoffer K. Andersen Casper Kirkegaard Nikolaj Foged Anders V. Christiansen Esben Auken 《Geophysical Prospecting》2016,64(3):741-752
Modern airborne transient electromagnetic surveys typically produce datasets of thousands of line kilometres, requiring careful data processing in order to extract as much and as reliable information as possible. When surveys are flown in populated areas, data processing becomes particularly time consuming since the acquired data are contaminated by couplings to man‐made conductors (power lines, fences, pipes, etc.). Coupled soundings must be removed from the dataset prior to inversion, and this is a process that is difficult to automate. The signature of couplings can be both subtle and difficult to describe in mathematical terms, rendering removal of couplings mostly an expensive manual task for an experienced geophysicist. Here, we try to automate the process of removing couplings by means of an artificial neural network. We train an artificial neural network to recognize coupled soundings in manually processed reference data, and we use this network to identify couplings in other data. The approach provides a significant reduction in the time required for data processing since one can directly apply the network to the raw data. We describe the neural network put to use and present the inputs and normalizations required for maximizing its effectiveness. We further demonstrate and assess the training state and performance of the network before finally comparing inversions based on unprocessed data, manually processed data, and artificial neural network automatically processed data. The results show that a well‐trained network can produce high‐quality processing of airborne transient electromagnetic data, which is either ready for inversion or in need of minimal manual processing. We conclude that the use of artificial neural network scan significantly reduce the processing time and its costs by as much as 50%. 相似文献
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Kristoffer T. Nielsen Per Moldrup Sren Thorndahl Jesper E. Nielsen Lene B. Duus Sren H. Rasmussen Mads Uggerby Michael R. Rasmussen 《水文研究》2019,33(26):3364-3377
Rainfall simulators can enhance our understanding of the hydrologic processes affecting the total runoff to urban drainage systems. This knowledge can be used to improve urban drainage designs. In this study, a rainfall simulator is developed to simulate rainfall on urban green surfaces. The rainfall simulator is controlled by a microcomputer programmed to replicate the temporal variations in rainfall intensity of both historical and synthetic rainfall events with constant rainfall intensity on an area of 1 m2. The performance of the rainfall simulator is tested under laboratory conditions with regard to spatial uniformity of the rainfall, the kinetic energy of the raindrops, and the ability to replicate historical and synthetic rainfall events with temporally varying intensity. The rainfall simulator is applied in the field to evaluate its functionality under field conditions and the influence of wind on simulated rainfall. Finally, a field study is carried out on the relationship between runoff, soil volumetric water content, and surface slope. Performance and field tests show that the simulated rainfall has a uniform spatial distribution, whereas the kinetic energy of the raindrops is slightly higher than that of other comparable rainfall simulators. The rainfall simulator performs best in low wind speed conditions. The simulator performs well in replicating historical and synthetic rainfall events by matching both intensity variations and accumulated rainfall depth. The field study shows good correlation between rainfall, runoff, infiltration, soil water content, and surface slope. 相似文献
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