Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden,and new evidence for the L‐chondrite parent body breakup event     

Philipp R. Heck  Birger Schmitz  Heinrich Baur  Rainer Wieler 《Meteoritics & planetary science》2008,43(3):517-528

Abstract— We present noble gas analyses of sediment‐dispersed extraterrestrial chromite grains recovered from ?470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment‐dispersed grains and the meteorite Gullhögen 001 were generated in the L‐chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment‐dispersed chromite grains have partly been retained during ?470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment‐dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting‐Robertson (P‐R) drag. The fossil meteorite Gullhögen 001 has a low cosmic‐ray exposure age of ?0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L‐chondrite parent body breakup.  相似文献   

The effect of copper-treated net pens on farmed salmon (Salmo salar) and other marine organisms and sediments     

Solberg CB  Saethre L  Julshamn K 《Marine pollution bulletin》2002,45(1-12):126-132

Samples of Atlantic salmon (Salmo salar), saithe (Pollacius virens), blue mussel (Mytilus edulis), brown seaweed (Ascophyllum nodosum) and sediment were collected from six different fish farms. Five of the farms used net pens treated with copper-containing coatings, whereas one farm did not use copper-containing coating (this was used as a reference location). Samples of muscle, liver and gills of Atlantic salmon and saithe, blue mussel and brown seaweed were freeze dried, homogenised, wet digested and analysed for copper by flame atomic absorption spectrometry. The results showed no significant differences in copper concentrations among the samples from the different locations. The copper contents of some of the samples appeared to be in the upper part of the normal concentration range. From a nutritional point of view, the use of copper-coatings on net pens did not affect the quality of the seafood products either within, or around the net pen.  相似文献   

Monazite begets monazite: evidence for dissolution of detrital monazite and reprecipitation of syntectonic monazite during low-grade regional metamorphism   总被引：2，自引：0，他引：2  

Birger Rasmussen  Janet R. Muhling 《Contributions to Mineralogy and Petrology》2007,154(6):675-689

Back-scattered electron (BSE) imaging and X-ray element mapping of monazite in low-grade metasedimentary rocks from the Paleoproterozoic Stirling Range Formation, southwestern Australia, reveal the presence of distinct, high-Th cores surrounded by low-Th, inclusion-rich rims. Previous geochronology has shown that the monazite cores are older than 1.9 Ga and overlap with the ages of detrital zircon grains (∼3.5–2.0 Ga), consistent with a detrital origin. Many cores have scalloped and embayed surfaces indicating partial dissolution of former detrital grains. Textural evidence links the growth of the monazite rims (∼1.2 Ga) to deformation and regional metamorphism during the Mesoproterozoic Albany-Fraser orogeny. These results indicate that high-Th detrital monazite is unstable under low-grade metamorphic conditions (<400°C) and was partially or completely dissolved. Dissolution was followed by near-instantaneous reprecipitation and the formation of low-Th monazite and ThSiO₄. This reaction is likely to operate in other low-grade metasedimentary rocks, resulting in the progressive replacement of detrital monazite by metamorphic monazite during regional prograde metamorphism.  相似文献   

Stratigraphic evaluation of a Holocene clay-slide in Northern Norway     

Louise Hansen  Raymond Eilertsen  Inger-Lise Solberg  Kåre Rokoengen 《Landslides》2007,4(3):233-244

An almost 6,000 years old slide in marine clay at Fossmoen, Northern Norway is studied to: characterize the scar, the slide deposits and the slide event; evaluate the role that stratigraphic variations played for failure; and view the slide event within long-term landscape development. A geological model for the area is based on drilling, outcrops and ground-penetrating radar with emphasis on the stratigraphic variations of fjord deposits. The slide’s age implies that the deposits were sensitive already shortly after emergence above sea level, and layers are still sensitive. River incision was probably responsible for the initial slide, whereas stratigraphy and groundwater movement controlled the location and shape of the scar. Laminated, inclined and discontinuous bedding are suggested as playing different roles for ground-water flow and pore pressures, adding to existing models on the development of soft and sensitive layers prone to sliding.  相似文献   

Attenuation of seismic waves and the universal rheological model of the Earth’s mantle     

B. I. Birger 《Izvestiya Physics of the Solid Earth》2007,43(8):635-641

Analysis of results of laboratory studies on creep of mantle rocks, data on seismic wave attenuation in the mantle, and rheological micromechanisms shows that the universal, i.e., relevant to all time scales, rheological model of the mantle can be represented as four rheological elements connected in series. These elements account for elasticity, diffusion rheology, high temperature dislocation rheology, and low temperature dislocation rheology. The diffusion rheology element is described in terms of a Newtonian viscous fluid. The high temperature dislocation rheology element is described by the rheological model previously proposed by the author. This model is a combination of a power-law non-Newtonian fluid model for stationary flows and the linear hereditary Andrade model for flows associated with small strains. The low temperature dislocation rheology element is described by the linear hereditary Lomnitz model.  相似文献   

Zircon growth in very low grade metasedimentary rocks: evidence for zirconium mobility at ~250°C     

Birger?RasmussenEmail author 《Contributions to Mineralogy and Petrology》2005,150(2):146-155

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 μm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of ∼250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.  相似文献   

Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen‐deficient atmosphere or hydrothermal alteration?   总被引：2，自引：0，他引：2  

Gavin L. England  Birger Rasmussen  Bryan Krapez  David I. Groves 《Sedimentology》2002,49(6):1133-1156

Petrographic and sulphur isotope studies support the long‐held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz‐pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar‐type, clast‐supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source‐rocks during erosion and sediment transport. A large range in textures reflects source‐rock lithologies, with known varieties linked to sedimentary‐hosted diagenetic pyrite, volcanic‐hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ³⁴S values (? 5·3 to + 6·7‰) than that of previously reported conventional bulk‐grain analyses (? 1 to + 4‰). Rounded pyrite from the Steyn Reef has significant variation in δ³⁴S values (? 4·7 to + 6·7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ³⁴S values (? 0·5 to + 2·5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand‐type Au‐U ore deposits, implies deposition of redox‐sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen‐poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au‐U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization.  相似文献   

Metal precipitation in the Cretaceous-Tertiary boundary clay at Stevns Klint,Denmark     

Birger Schmitz 《Geochimica et cosmochimica acta》1985,49(11):2361-2370

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Testing Sr/Sr as a paleosalinity indicator on mixed marine, brackish-water and terrestrial vertebrate skeletal apatite in late Paleocene-early Eocene near-coastal sediments, Mississippi     

Birger Schmitz  Stephen L. Ingram  David T. Dockery  III  Gran berg 《Chemical Geology》1997,140(3-4):275-287

Mixed marine, brackish-water and terrestrial vertebrate skeletal apatite is found in the late Paleocene-early Eocene Tuscahoma and Bashi Formations of Mississippi. The co-Occurrence in these near-coastal marine deposits of skeletal apatite with different provenances offers a unique opportunity for unraveling the effects of diagenesis on apatite ⁸⁷Sr/⁸⁶Sr composition, with bearings on the use of this isotope pair as paleosalinity indicator. The results show that the Sr isotopes of all originally poorly ordered phases of skeletal apatite have extensively re-equilibrated with pore water with significantly lower ⁸⁷Sr/⁸⁶Sr ratios than late Paleocene-early Eocene seawater. For example, marine shark and ray teeth from the Bashi Formation all have clearly nonmarine ⁸⁷Sr/⁸⁶Sr ratios, 0.7073-0.7075, compared with a coeval seawater ratio in the range 0.7077-0.7078. However, separated shark tooth enamel, with a high original crystallinity, appears to have retained all or most of its in vivo Sr isotopic signature until the present. Here we also show that ⁸⁷Sr/⁸⁶Sr results on recent vertebrate skeletal material from different environments are in good agreement with expected values for respective habitat.

Earlier analyses of calcite material from the Bashi Formation indicated the existence of a shift in seawater ⁸⁷Sr/⁸⁶Sr to a lower value at the Paleocene-Eocene boundary. The strong influence of pore water with low, nonmarine ⁸⁷Sr/⁸⁶Sr on the apatite in the Bashi Formation suggests that the proposed isotopic shift may be an artifact related to diagenetic processes.  相似文献   

Rheology of the Earth and a thermoconvective mechanism for sedimentary basin formation   总被引：1，自引：0，他引：1  

Boris I. Birger 《Geophysical Journal International》1998,134(1):1-12

A power-law non-Newtonian fluid is usually assumed to model slow flows in the mantle and, in particular, convective flows. However, the power-law fluid has no memory, in contrast to a real material. A new non-linear integral (having a memory) model is proposed to describe the rheology of rocks. The model is consistent with the theory of simple fluids with fading memory and with laboratory studies of rock creep. The proposed model reduces to the power-law fluid model for stationary flows and to the Andrade model for flows associated with small strains. Stationary convection beneath continents has been studied by Fleitout & Yuen (1984 ), who used the power-law fluid model and obtained the cold immobile boundary layer (continental lithosphere). In a stability analysis of this layer, the Andrade model must be used. The analysis shows that the lithosphere is overstable (the period of oscillation is about 200  Ma). In the present study, it is suggested that these thermoconvective oscillations of the lithosphere are a mechanism for sedimentary basin formation. The vertical crustal movement in sedimentary basins can be considered as a slow subsidence on which small-amplitude oscillations are superimposed. The longest period of oscillatory crustal movement is of the same order of magnitude as the period of convective oscillation of the lithosphere found in the stability analysis. Taking into account the difference between depositional and erosional transport rates we can explain the permanent subsidence as well as the oscillations.  相似文献