D’Orbigny: A non-igneous angritic achondrite?     

G. Kurat  M.E. Varela  G. Weckwerth  H.W. Weber  E. Wäsch 《Geochimica et cosmochimica acta》2004,68(8):1901-1921

D’Orbigny is the sixth and by far the largest angrite known. Its bulk chemical and mineral chemical compositions, rare gas abundances and oxygen and rare gas isotope compositions fit the compositional ranges known from other angrites. It is, however, peculiar with respect to three features: the abundance of hollow shells, the presence of abundant open druses and the abundant presence of glasses.The shape, structure and texture of D’Orbigny and its mineral and bulk chemical compositions indicate an unusual genesis under changing redox conditions. In our view, data and observations are incompatible with an igneous origin of this rock but are suggestive of a complex growth and metasomatism scenario. The sequence of events apparently began with the formation of spheres of a phase which later vanished and therefore is unknown but could have been CaS. On top of these spheres (sizes from < 1-30 mm) olivine-anorthite intergrowths precipitated forming compact shells and fluffy protrusions. Aggregation of these objects plus occasional large plates made of the same intergrowths led to formation of a highly porous object with abundant large open space between the olivine-anorthite intergrowths. The aggregate also included previously formed olivines, olivinite rocks and Al-spinels. The latter carry highly porous decomposition rims of Cr-enriched Al-spinel and record mildly oxidizing conditions prevailing very early in D’Orbigny’s history. Conditions changed (with falling T?) and became oxidizing causing the phase(s) that constituted the spheres to become unstable. Their breakdown liberated large amounts of Ca and trace elements which at least in part re-precipitated by reacting with Si and Mg from the vapor to form augites that grew into the open space thus forming augite druses. Also, some of the preexisting olivine was converted into augite, which is very rich in refractory lithophile trace elements (abundances ∼ 10 × CI). Augites grew mainly under oxidizing conditions leading to atomic Fe/(Fe+Mg) ratios of about 0.44. Finally, conditions became highly oxidizing and strongly mobilized Ca from a source that apparently became unstable. The high partial pressures of Ca and Fe (and also Ti) led to precipitation of Ca-olivine and kirschsteinite (∼Fo1La20 and ∼Fo1La33, respectively) and of titaniferous aluminous hedenbergite—atomic Fe/(Fe+Mg) ∼ 0.97. Ulvöspinel and sulfides were also precipitated. Because the original phase(s) forming the early spheres vanished during these oxidizing events, the shells remained empty.In this scenario, D’Orbigny provides us with a record of changing conditions ranging from extremely reducing to highly oxidizing and with a record of the formation of an achondritic rock from a chondritic source. Angrites bear many similarities with CAIs, texturally, mineralogically and chemically. Possibly, they can be seen as CAIs, which grew larger than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. They reproduce most of the textures and structures of CAIs: crystallized liquids (Asuka 881371, LEW 87051), metasomatic granoblastic rocks (LEW 86010, Angra dos Reis?) and aggregates (D’Orbigny). In addition, all angrites record metasomatic alterations, subsolidus processing after formation, also similar to what is recorded by most CAIs. Obviously, they missed the alkali metasomatic event recorded by many CAIs but they record a siderophile—lithophile element separation event that is not recorded by CAIs.  相似文献   

Recurring middle Pleistocene outburst floods in east-central Alaska     

Duane G Froese  Derald G SmithJohn A Westgate  Thomas A AgerShari J Preece  Amanjit SandhuRandolph J Enkin  Florence Weber 《Quaternary Research》2003,60(1):50-62

Recurring glacial outburst floods from the Yukon-Tanana Upland are inferred from sediments exposed along the Yukon River near the mouth of Charley River in east-central Alaska. Deposits range from imbricate gravel and granules indicating flow locally extending up the Yukon valley, to more distal sediments consisting of at least 10 couplets of planar sands, granules, and climbing ripples with up-valley paleocurrent indicators overlain by massive silt. An interglacial organic silt, occurring within the sequence, indicates at least two flood events are associated with an earlier glaciation, and at least three flood events are associated with a later glaciation which postdates the organic silt. A minimum age for the floods is provided by a glass fission track age of 560,000 ± 80,000 yr on the GI tephra, which occurs 8 m above the flood beds. A maximum age of 780,000 yr for the floods is based on normal magnetic polarity of the sediments. These age constraints allow us to correlate the flood events to the early-middle Pleistocene. And further, the outburst floods indicate extensive glaciation of the Yukon-Tanana Upland during the early-middle Pleistocene, likely representing the most extensive Pleistocene glaciation of the area.  相似文献   

Climatology of Regional Flow Patterns around Basel     

R. O. Weber 《Theoretical and Applied Climatology》1998,59(1-2):13-27

Summary Recently a method has been developed to classify local-scale flow patterns from the wind measurements at a dense network of stations. It was found that in the area around Basel twelve characteristic near-surface flow patterns occur. However, as the dense network of stations was running for one year only during the MISTRAL field experiment, no reliable climatology of the flow patterns can be inferred from these data. Mainly the annual cycle is not well determined from a single year of observations. As there exist several routinely operated meteorological stations with longer data records in and near the area discussed, a method was searched to identify the local flow patterns from the wind observations at these few permanent stations. A linear discriminant analysis turned out to be the best method. Based on wind data from 11 stations which were simultaneously in operation during 1990–1995, a six-year climatology of the flow patterns could be obtained and compared to synoptic weather types. Although there are relations to existing synoptic classes there is no unique correspondence between them and the flow patterns. Received February 24, 1997 Revised June 9, 1997  相似文献   

天然变形方解石质岩石的低温塑性     

刘俊来 《地质学报》2002,76(3):372-372

对四套具不同特点的方解石质原岩断层构造岩应用光学显微镜、透射电子显微镜和阴极发光显微镜开展了系统研究。它们都表现出相似的变形显微构造特点与变形作用,宏观碎裂结构与微观糜棱状结构。碎屑颗粒或者变形的原岩颗粒具有发育的晶内变形显微构造(变形双晶、扭折和微破裂等),基质颗粒为弥散的极细粒物质。透射电镜下的微构造表现为反映脆性变形与晶质塑性变形共存的位错  相似文献   

Sweden     

Helmut Goerke  Gerhard Eder  Kurt Weber  Wolfgang Ernst 《Marine pollution bulletin》1979,10(5):127-132

Four invertebrate and one fish species (Cerastoderma edule, Mya arenaria, Arenicola marina, Crangon crangon, Solea solea) from a single area of the German Wadden sea were investigated for concentrations of ten organochlorine pollutants in 1976. PCB concentrations (wet tissue basis) were predominating with 10–200 ng g^?1, while p,p′-DDD, dieldrin, α-HCH, γ-HCH and p,p′-DDE generally ranged between 1 and 10 ng g^?1. Values of α-endosulfan were even lower; p,p′-DDT, endrin and heptachlor epoxide could not be detected. Residue levels in shrimps were close to the lower range of values which have been found in this species from other areas of the North Sea since 1974. The patterns of residue concentrations in the five aquatic species were remarkably different indicating species- and compound-specific bioaccumulation. Trophic level and lipid content were shown to be responsible for PCB concentrations increasing from bivalves to fish.  相似文献   

Age and degree of metamorphism and time of nappe emplacement along the southern margin of the Damara Orogen/Namibia (SW-Africa)     

Dr. H. Ahrendt  Dr. J. C. Hunziker  Prof. Dr. K. Weber 《International Journal of Earth Sciences》1978,67(2):719-742

Along the southern margin of the Damara orogen age and degree of metamorphism were determined by means of K/Ar dating and illite crystallinity. The investigations include the following units:

1. The southwestern-most part of the east-west striking branch of the Damara orogen.
2. The nappes of the Naukluft Mountains.
3. The Nama-Group from north of the Naukluft Mountains to the Fish River in the south (including the western part of the Dwyka-Formation).

The metamorphism of the Naukluft nappes as well as the underlying Nama beds corresponds to the higher part of the anchi-zone and lower epi-zone. Between the Naukluft nappes and the folded Nama rocks adjoining the southeastern front of the nappes there is an obvious step from higher down to lower metamorphism. Further to the southeast the metamorphism in the Nama beds decreases continuously down to diagenesis. K/Ar age determinations were carried out on the three units mentioned above and also on the basement underlying the Nama sequence. Muscovites of this basement gave an age of about 1160 m. y. Determinations on white micas of the southern Damara belt, the Naukluft Mountains and the northern Nama basin define two isochrons with ages of 495 and 530 m. y The age of 530 m. y. represents the peak of metamorphism and the age of syncrystalline deformation. The age of 495 m. y. can be interpreted as a cooling age of the higher metamorphic rocks or as a dating of rejuvenation caused by a second post-crystalline deformation in parts of the Naukluft and Damara rocks. This age of 495 m. y. was also found in the mylonite of the main thrust plane of the Naukluft nappes and it represents the time of emplacement of the Naukluft nappes.  相似文献   

A Method for Characterizing Rotation Rates in the Soft X-Ray Corona     

Weber  M.A.  Acton  L.W.  Alexander  D.  Kubo  S.  Hara  H. 《Solar physics》1999,189(2):271-288

Differential rotation rates of soft X-ray features in the solar corona are quantified by a method of harmonic filtering using the Lomb-Scargle periodogram. This approach leads reasonably to a quantitative discrimination between uncertainty estimates and spectral leakage of the fundamental rotation frequency due to the presence of multiple rotating tracers. Mean rotation rates as a function of latitude and year are calculated for the years 1992–1997 (roughly the declining phase of the last solar activity cycle). The corona is found to have a small but measurable latitudinal gradient in rotation rate. The presence of multiple features places a lower bound of 1–2% on the relative uncertainties with which a `mean' rotation rate can be measured. The results are compared with autocorrelation estimates and found to agree within 1.  相似文献   

Precise U-Pb zircon ages for the Molson dyke swarm and the Fox River sill: Constraints for Early Proterozoic crustal evolution in northeastern Manitoba,Canada     

L. M. Heaman  N. Machado  T. E. Krogh  W. Weber 《Contributions to Mineralogy and Petrology》1986,94(1):82-89

Precise U-Pb zircon ages have been obtained for samples from the Molson dyke swarm and the Fox River sill in NE Manitoba, Canada. The ages determined for the Cross Lake and Cuthbert Lake dykes are 1,883.7 _–1.5 ^+1.7 and 1,883±2 Ma, respectively, and are in excellent agreement with the 1,882.9 _–1.4 ^+1.5 Ma age obtained for the Fox River sill. These results support the contention that the emplacement of the Fox River sill and the Molson Dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities. The timing of Early Proterozoic mafic magmatism in the western Superior Province appears to be synchronous with igneous activity in other parts of the Circum-Superior Belt and in the Trans Hudson orogen to the west. The emplacement of the Molson dyke swarm at 1,883 Ma indicates a 700 Ma interval of quiescence between the final igneous activity that is recorded in the Archean basement and dyke intrusion. The presence of deformed equivalents of Molson dykes in the Thompson Nickel Belt indicates that the intense deformation in this belt occurred sometime after 1,883 Ma.  相似文献   

Die Laufzeitfunktion und ihre Interpretation in der Refraktionsseismik des einachsig inhomogenen Körpers     

Priv.-Doz. Dr. Max Weber 《Pure and Applied Geophysics》1959,43(1):167-181

Zusammenfassung Es wird das Interpretationsproblem der Refraktionsseismik in einem einachsig inhomogenen Körper allgemein behandelt und exakt gelöst.

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Summary The interpretation-problem of the refraction seismic in a uniaxial inhomogeneous medium is generally treated and exactly solved.

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Bringing Time Back In: A Study on the Influence of Travel Time Variations and Facility Opening Hours on Individual Accessibility   总被引：2，自引：0，他引：2  

Joe Weber  Mei‐Po Kwan 《The Professional geographer》2002,54(2):226-240

Although recent studies of individual accessibility have used detailed representations of urban street networks, unrealistic measures of travel time based on assumptions about constant travel speeds through the network were often used. Utilizing constant travel times does not allow for daily congestion and assumes that the effects of congestion are uniform throughout the city and affect all people equally. This research measures individual space‐time accessibility in order to show that the incorporation of locally specific travel times within a street network allows a significant increase in the ability to realistically evaluate individual accessibility within cities. The results show that the accessibility of individuals within cities is not homogenous, and neither does access to employment or shopping opportunities vary according to common expectations about urban form and human behavior. Instead, the role of distance in predicting accessibility variations within cities is quite limited. This article also shows that incorporating time into accessibility measures in the form of congestion and business hours leads to additional (and highly spatially uneven) reductions in accessibility, revealing that the temporal dimension is very important to accurately assessing individual accessibility.  相似文献