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Abstract— The Brunflo fossil meteorite was found in the 1950s in mid‐Ordovician marine limestone in the Gärde quarry in Jämtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L‐chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule‐size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent‐body breakup. Chromite maximum diameters and well‐defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Gärde quarry is relatively rich (about 0.45 grain kg−1) in sediment‐dispersed extraterrestrial chromite grains (>63 μm) with chemical composition similar to those in L chondrites and the limestone (1–3 grains kg−1) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell. 相似文献
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The autocorrelation technique for estimating grain-size from digital images of sand beds has been extended and validated for use on coarse sand (0·7 mm) and gravel (up to ∼20 mm). A number of aspects of the technique have been explored and some potential improvements suggested. Autocorrelation is just one suitable statistical method sensitive to the grain-size of sediment in digital images; four additional techniques are presented and their relative merits discussed. A collective suite of techniques applicable to the general problem of grain-size estimation from digital images of sediment might broaden the applicability to more sedimentary environments, as well as improve its accuracy. These techniques are compared using a large data set from a gravel barrier beach in southern England. Based on over 180 samples, mean grain-size of sieved and imaged sediments correspond to within between 8% and 16%. Some theoretical aspects of the spatial arrangement of image intensity in digital images of natural sediments are addressed, including the fractal nature of sediments in images, which has potential implications for derivation of grain-size distributions from images of sand-sized material through segmentation and thresholding. The methods outlined in this contribution may also find application in further uncovering the geometric structure of sediment beds, as well as in the simulation of sedimentation processes. 相似文献
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Partial Melting and Assimilation of Dolomitic Xenoliths by Mafic Magma: the Ioko-Dovyren Intrusion (North Baikal Region, Russia) 总被引：5，自引：0，他引：5
WENZEL THOMAS; BAUMGARTNER LUKAS P.; BRUGMANN GERHARD E.; KONNIKOV EDUARD G.; KISLOV EVGENIY V. 《Journal of Petrology》2002,43(11):2049-2074
A petrological study was carried out on Mg-skarn-bearing dunitecumulates that are part of the Neo-Proterozoic Ioko-Dovyrenintrusion (North Baikal region, Russia). Skarn xenoliths containbrucite pseudomorphs after periclase, forsterite and Cr-poorspinel. Fine-grained forsterite–spinel skarns occur withthe brucite skarns or as isolated schlieren. Field relationshipsreveal that the Mg-skarns formed from silica-poor dolomiticxenoliths by interaction with the mafic magma of the Ioko-Dovyrenintrusion. Rapid heating of dolomitic xenoliths by the maficmagma caused the decomposition of dolomite into calcite + periclase,releasing much CO2. Further heating quantitatively melted thecalcite. A periclase-rich restite was left behind after extractionof the low-density, low-viscosity calcite melt. The extractedcalcite melt mixed with the surrounding mafic melt. This resultedin crystallization of olivine with CaO contents up to 1·67wt %. A local decrease in the silica concentration stabilizedCaAl2SiO6-rich clinopyroxene. Brucite/periclase-free forsterite–spinelskarns probably originated by crystallization from the maficmelt close to the xenoliths at elevated fO2. The high fO2 wascaused by CO2-rich fluids released during the decompositionof the xenoliths. The above case study provides the first evidencefor partial melting of dolomite xenoliths during incorporationby a mafic magma. KEY WORDS: dunite; dolomite assimilation; partial melting 相似文献
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SELVERSTONE JANE; SPEAR FRANK S.; FRANZ GERHARD; MORTEANI GIULIO 《Journal of Petrology》1984,25(2):501-531
The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO2-Al2O3-TiO2-Fe2O3-MgO-FeO-MnO-CaO-Na2O-K2O-H2O-CO2.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop. 相似文献
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GERHARD EINSELE 《Sedimentology》1977,24(5):639-655
In growing sedimentary sequences range and velocity of vertically ascending pore water (advection) can be determined quantitatively by using a simple graphical method. During continuing deposition and maintenance of compaction equilibrium, the pore water from deeper layers cannot reach the sediment-water interface. Range and velocity of advective flow decrease from top to bottom of a sequence and depend on the thickness affected by compaction. The velocity of pore water flow as well as the transport of solutes by advective flux are proportional to the sedimentation rate. Compaction flow can also result from under-con-solidation and diagenetic reduction of pore space. At the sediment-water interface, a direct contact between the ascending pore water and the overlying water body is feasible only under special conditions (interrupted sedimentation over under-consolidated material, erosion). These results are demonstrated on geometric models of sedimentary sequences, on experimentally deposited clay, and on selected examples of the Deep Sea Drilling Project. 相似文献
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The collection of time series data is an essential component in the investigation of earth surface processes. Spectral analysis of these time series can provide an invaluable insight into the behaviour of geophysical processes. Spectral analysis of a single time series produces an auto-spectrum which provides a representation of the amount variance of the time series as a function of frequency. Prior to spectral analysis, the time series should be plotted to identify the presence of any trends in the mean or the variance of the series, and to identify anomalies in the data which should be corrected. To satisfy the assumption of stationarity, any trend (in either the mean or variance) should be removed from the time series. Consequently, the probability density function of the time series should be plotted and compared with the Gaussian distribution. The final stage in preparing the time series for spectral analysis is to apply a taper to reduce spectral leakage and distortion of the auto-spectrum. Following the calculation of the periodogram, spectral estimates should be combined to reduce the variability associated with the estimates and thereby ensure that the autospectrum is more representative. Finally, confidence limits should be constructed around the spectral density function so that statistically significant spectral peaks (or troughs) can be identified. 相似文献
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Possible seismic origin of molar tooth structures in Neoproterozoic carbonate ramp deposits, north China 总被引：21，自引：0，他引：21
The Xinmincun Formation forms the uppermost unit of a thick Neoproterozoic section which accumulated near the east margin of the North China Block and is overlain by two thin nearshore to continental formations below fossiliferous Lower Cambrian sediments. Although tectonically deformed, sedimentary structures are preserved undeformed on cleavage-parallel surfaces, and an 80 m section has been reconstructed by correlation across minor folds and faults in the Golden Stone beach area, 50 km NE of the city of Dalian, southern Liaoning province. The measured section shows 65 m of storm-dominated deposits, consisting of alternations of micrites and sharp-based graded intraclastic grainstone beds (tempestites), some with rudaceous, commonly erosional or guttered, bases. The top 15 m of section shows three alternations of similar subtidal lithofacies with partly to completely dolomitized peritidal deposits (laminated, sometimes fenestral and desiccated, micrite beds, and intraclastic rudite and grainstone beds). Tempestite beds become thinner and less abundant upwards towards a muddy upper shoreface zone. This environment was characterized in part by the occurrence of micrite with thin or streaky lamination (probable storm-resuspended sediment), interbedded tempestites, numerous erosion surfaces, and evidence of liquidization and sediment slumping into hollows. Molar tooth structures are widespread in micrite beds of the Xinmincun Formation and are present in lesser abundance in tempestites and liquefied channel-fills. Originally the structures were sub-vertical cracks, 1–20 cm long, tapering upwards and downwards. Subsequently they became filled with microspar cement and buckled rigidly during compaction of surrounding sediment during burial. Evidence of repeated episodes of cracking, presence of brecciated cracks and localization of cracking within beds, together with variable degree of development and variation of preferential alignment in plan indicates a mechanical origin. Crack generation may have been by seismic surface waves generated by movement along faults defining either the basin's margins or its internal structure, or possibly by wave action during storms. A seismic origin for molar tooth structure is consistent with other Neoproterozoic occurrences. Their preferential occurrence in Precambrian deposits arises from the relative rigidity of micritic sediment at this time related to lack of bioturbation, incipient cementation and possibly microbial binding. 相似文献
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Short-term morphological change and sediment dynamics in the intertidal zone of a macrotidal beach 总被引：1，自引：0，他引：1
This paper describes the morphological and sedimentological evolution of a macrotidal beach over a 20 day period under varying hydrodynamic conditions (significant breaker heights of 0·3–2 m and tidal ranges of 2–5 m). During the field campaign, an intertidal bar developed around the mid‐tide level, migrated onshore, welded to the upper beach and was then flattened under energetic wave conditions. The bar had a wave breakpoint origin and its formation was triggered by a reduction in tidal range, causing more stationary water‐level conditions, rather than an increase in wave height. Most of the onshore bar migration took place while the bar was positioned in the inner to mid‐surf zone position, such that the bar moved away from the breakpoint and exhibited ‘divergent’ behaviour. The depth of disturbance over individual tidal cycles was 10–20% of the breaker height. Such values are more typical of steep reflective beaches, than gently sloping, dissipative beaches, and are considered to reflect the maximum height of wave‐generated ripples. The grain size distribution of surficial sediments did not vary consistently across the beach profile and temporal changes in the sedimentology were mostly unrelated to the morphological response. The lack of clear links between beach morphology and sedimentology may be in part due to shortcomings in the sampling methodology, which ignored the vertical variability in the sediment size characteristics across the active layer. 相似文献