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  • 1
    Description / Table of Contents: Exactly 100 years before the publication of this volume, the first paper which calculated the half-life for the newly discovered radioactive substance U-X (now called 234Th), was published. Now, in this volume, the editors Bernard Bourdon, Gideon Henderson, Craig Lundstrom and Simon Turner have integrated a group of contributors who update our knowledge of U-series geochemistry, offer an opportunity for non-specialists to understand its basic principles, and give us a view of the future of this active field of research. In this volume, for the first time, all the methods for determining the uranium and thorium decay chain nuclides in Earth materials are discussed. It was prepared in advance of a two-day short course (April 3-4, 2003) on U-series geochemistry, jointly sponsored by GS and MSA and presented in Paris, France prior to the joint EGS/AGU/EUG meeting in Nice. The discovery of the 238U decay chain, of course, started with the seminal work of Marie Curie in identifying and separating 226Ra. Through the work of the Curies and others, all the members of the 238U decay chain were identified. An important milestone for geochronometrists was the discovery of 230Th (called Ionium) by Bertram Boltwood, the Yale scientist who also made the first age determinations on minerals using the U-Pb dating method (Boltwood in 1906 established the antiquity of rocks and even identified a mineral from Sri Lanka-then Ceylon as having an age of 2.1 billion years!) The application of the 238U decay chain to the dating of deep sea sediments was by Piggott and Urry in 1942 using the "Ionium" method of dating. Actually they measured 222Ra (itself through 222Rn) assuming secular equilibrium had been established between 230Th and 226Ra. Although 230Th was measured in deep sea sediments by Picciotto and Gilvain in 1954 using photographic emulsions, it was not until alpha spectrometry was developed in the late 1950's that 20Th was routinely measured in marine deposits. Alpha spectrometry and gamma spectrometry became the work horses for the study of the uranium and thorium decay chains in a variety of Earth materials. These ranged from 222Rn and its daughters in the atmosphere, to the uranium decay chain nuclides in the oceanic water column, and volcanic rocks and many other systems in which either chronometry or element partitioning, were explored. Much of what we learned about the 238U, 235U and 232Th decay chain nuclides as chronometers and process indicators we owe to these seminal studies based on the measurement of radioactivity. The discovery that mass spectrometry would soon usurp many of the tasks performed by radioactive counting was in itself serendipitous. It came about because a fundamental issue in cosmochemistry was at stake. Although variation in 235U/238U had been reported for meteorites the results were easily discredited as due to analytical difficulties. One set of results, however, was published by a credible laboratory long involved in quality measurements of high mass isotopes such as the lead isotopes. The purported discovery of 235U/238U variations in meteorites, if true, would have consequences in defining the early history of the formation of the elements and the development of inhomogeneity of uranium isotopes in the accumulation of the protoplanetary materials of the Solar System. Clearly the result was too important to escape the scrutiny of falsification implicit in the way we do science. The Lunatic Asylum at Caltech under the leadership of Jerry Wasserburg took on that task. Jerry Wasserburg and Jim Chen clearly established the constancy and Earth-likeness of 235U/238U in the samplable universe. In the hands of another member of the Lunatic Asylum, Larry Edwards, the methodology was transformed into a tool for the study of the 238U decay chain in marine systems. Thus the mass spectrometric techniques developed provided an approach to measuring the U and Th isotopes in geological materials as well as cosmic materials with the same refinement and accommodation for small sample size. Soon after this discovery the harnessing of the technique to the measurement of all the U isotopes and all the Th isotopes with great precision immediately opened up the entire field of uranium and thorium decay chain studies. This area of study was formerly the poaching ground for radioactive measurements alone but now became part of the wonderful world of mass spectrometric measurements. (The same transformation took place for radiocarbon from the various radioactive counting schemes to accelerator mass spectrometry.) No Earth material was protected from this assault. The refinement of dating corals, analyzing volcanic rocks for partitioning and chronometer studies and extensions far and wide into ground waters and ocean bottom dwelling organisms has been the consequence of this innovation. Although Ra isotopes, 210Pb and 210Po remain an active pursuit of those doing radioactive measurements, many of these nuclides have also become subject to the mass spectrometric approach. In this volume, for the first time, all the methods for determining the uranium and thorium decay chain nuclides in Earth materials are discussed. The range of problems solvable with this approach is remarkable-a fitting, tribute to the Curies and the early workers who discovered them for us to use.
    Pages: Online-Ressource (XX, 656 Seiten)
    ISBN: 0939950642
    Language: English
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  • 2
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers. Initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase-melt partition coefficient. Trace element diffusion will subsequently act to modify this initial profile and – given enough time – produce a profile that is in equilibrium with the anorthite variations within the crystal. We argue that the trace element partition coefficient Da/b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal-liquid partition coefficients, and that the equilibrium profile of the crystal can be calculated. The time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase. Strontium plagioclase-melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions. A one-dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir. The model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the Kameni Island dacites, Santorini, and from the 1979 Soufriere andesite, St. Vincent. Three out of eight plagioclase phenocrysts have Sr concentration profiles that are not in diffusive equilibrium. For these, the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase. Maximum crystal residence times range from 100 to 450 years. This is in good agreement with estimates from crystal size distribution and from Ra-Th disequilibrium studies for the Kameni Islands. For Soufriere, however, such short residence times are incompatible with U-Th mineral errorchron data that suggest residence times of 〉40 ka in a thermally buffered magma reservoir. To reconcile these apparently different ages, we invoke a more complicated magmatic history for Soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract New data are presented for lavas from the Kamchatka Peninsula and the Aleutian arc. Radiogenic isotopes are strikingly homogeneous in the Kamchatka lavas and although incompatible trace element ratios exhibit much greater variability, much of this appears to result from shallow level, crystal fractionation. The data reveal little systematic across-arc change in radiogenic isotopes or trace element ratios. The Nd and Pb isotope data overlap those for Pacific MORB and limit the amount of sediment that could be incorporated in the mantle source region to 〈1% which is insufficient to account for the observed La/Ta ratios (50–68) in the high-MgO lavas. The lack of a positive correlation between La/Ta and depth to the slab suggests that melt–wall rock interaction was not important in controlling this ratio. Instead it is inferred that La/Ta increased during partial melting and that DLa/DTa = 0.11–0.06, possibly due to residual amphibole. Ba, U, Sr and Pb were added to the source by an aqueous fluid from the subducting slab and its inferred isotopic composition indicates that this fluid was derived from the altered oceanic crust. The addition of U resulted in a large range of (238U/232Th) from 0.79–2.48 similar to that observed in the Mariana and Lesser Antilles island arcs. However, (230Th/232Th) = 0.79–2.34, and the majority of samples lie close to the equiline indicating that the time since U/Th fractionation is generally ≥150 thousand years. The large width of the volcanic zone is assumed to reflect protracted fluid release from the subducting slab over the depth interval 170–380 km possibly coupled with extension across the Central Kamchatka Depression. The data from the Aleutians contrast strongly with those from Kamchatka. Radiogenic isotope data indicate that the Aleutian lavas contain a significant recycled sedimentary component, consistent with elevated 10Be/9Be ratios. The Aleutian lavas have (230Th/232Th) = 0.79–2.34 and exhibit a significant range of U/Th disequilibria [(238U/230Th) = 0.75–1.01]. However, 10Be/9Be is positively correlated with (238U/230Th) suggesting that the 10Be signal was carried by the aqueous fluid from the slab. The U/Th disequilibria for the Aleutian lavas lie close to a 30 thousand year reference line suggesting that this fluid was released from the slab ∼30 thousand years ago similar to recent estimates from the Lesser Antilles, Marianas, and Tonga-Kermadec island arcs from which it is inferred that fluid addition was the trigger for partial melting. Given that the rate of convergence in Kamchatka is similar to that in the Aleutians, Marianas and Tonga-Kermadec the inferred greater time since␣fluid release in Kamchatka requires further investigation.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1617-4623
    Keywords: Gene expression (glycine decarboxylase serine hydroxymethyltransferase) ; Pisum sativum ; RFLP mapping
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Restriction fragment length polymorphisms have been used to determine the chromosomal location of the genes encoding the glycine decarboxylase complex (GDC) and serine hydroxymethyltransferase (SHMT) of pea leaf mitochondria. The genes encoding the H subunit of GDC and the genes encoding SHMT both show linkage to the classical group I marker i. In addition, the genes for the P protein of GDC show linkage to the classic group I marker a. The genes for the L and T proteins of GDC are linked to one another and are probably situated on the satellite of chromosome 7. The mRNAs encoding the five polypeptides that make up GDC and SHMT are strongly induced when dark-grown etiolated pea seedlings are placed in the light. Similarly, when mature plants are placed in the dark for 48 h, the levels of both GDC protein and SHMT mRNAs decline dramatically and then are induced strongly when these plants are returned to the light. During both treatments a similar pattern of mRNA induction is observed, with the mRNA encoding the P protein of GDC being the most rapidly induced and the mRNA for the H protein the slowest. Whereas during the greening of etiolated seedlings the polypeptides of GDC and SHMT show patterns of accumulation similar to those of the corresponding mRNAs, very little change in the level of the polypeptides is seen when mature plants are placed in the dark and then re-exposed to the light.
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  • 5
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    Copenhagen: Danish Institute for International Studies (DIIS)
    Publication Date: 2018-07-03
    Description: This working paper explores the changing roles of Diasporas in post-conflict Burundi in terms of contributing to development, reconciliation and peace building. Burundi is in a state of post-conflict recovery after decades of civil war and widespread ethnic violence. Due to repressive regimes and to extensive violence, a large proportion of the country's Hutu population left the country to take refuge in neighbouring countries or in Europe and North America where they involved themselves in political activities. The evolving new situation with better security has led to a diversification of Diaspora engagements. First, it is now possible for members of the Diaspora to invest in the country, either with the prospect of returning in the future or simply to make a profit. Second, the Diaspora is increasingly involved in development projects. A third area of Diaspora engagement after conflict is the return or circulation of "brains". In this working paper, we explore the various initiatives that have been taken by various actors in order to try to engage the Diaspora in peace building and development. Such initiatives are taken at all levels from local grassroots and individual entrepreneurs to states and international organizations. They include private and public sectors and may be initiated both inside Burundi and among the Diaspora. The paper demonstrates that it is a great challenge for the conflict-generated Diaspora to redefine its role from being in political opposition to the home country to being part of the development process. We argue that the Burundian Diaspora is very heterogeneous and that the various 'Diasporas' react differently to the new situation. Furthermore, we argue that the initiatives to engage the Diaspora are precarious and not always in tune with the needs and preoccupations of a conflict-generated Diaspora. In particular, the majority of initiatives lack awareness about the political engagement and identity of Burundians in exile, thus often alienating them from the process.
    Keywords: ddc:300 ; Bürgerkrieg ; Kriegsfolgen ; Friedenssicherung ; Burundi
    Repository Name: EconStor: OA server of the German National Library of Economics - Leibniz Information Centre for Economics
    Language: English
    Type: doc-type:workingPaper
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  • 6
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    Copenhagen: Danish Institute for International Studies (DIIS)
    Publication Date: 2018-07-03
    Description: Since the massive violence in the 1990s, Rwanda and Burundi have moved in two very different directions in terms of peace and state building. Rwanda is following a path of social engineering, creating a radically new national identity with a new citizenry in order to break with what is perceived as a national culture that led to the genocide. Because the conflict ended with a clear winner in 1994, the state has the moral legitimacy to carry through with its policies, and any opposition may be dismissed as supporting divisionism and genocidal ideologies. While Rwanda is concerned with social engineering, Burundi is concerned with what we might term 'consociational engineering'. In other words, the complex, time consuming and often frustrating path out of conflict has been characterised by constant negotiations with countless factions and complex power-sharing deals, trying to take the concerns of all parties into account. The report argues that Rwanda appears to be ahead of Burundi in terms of economic growth, political stability and statebuilding. Likewise, it appears that national unity has replaced ethnic tensions. On closer scrutiny, however, these achievements may only be temporary. In the long term, Rwanda risks renewed violence because of a lack of social justice and popular legitimacy of the state. Burundi, on the other hand, appears to be achieving meagre economic growth and is experiencing political instability. However, the report argues that the long and complex path that Burundi has chosen, may prove to be more durable in the long run.
    Keywords: ddc:300
    Repository Name: EconStor: OA server of the German National Library of Economics - Leibniz Information Centre for Economics
    Language: English
    Type: doc-type:report
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  • 7
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillan Magazines Ltd.
    Nature 389 (1997), S. 568-573 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Subduction processes are central to plate tectonics and to crust–mantle recycling and differentiation. Here we present a study of lavas from the Tonga–Kermadec island arc which places important constraints on the processes and rates involved. The mantle wedge overlying the subducting ...
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  • 8
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The Tibetan plateau (Fig.l) formed in response to the collision of India and Asia around 50-45 Myr ago. Since then, India has moved northwards with respect to Asia at a constant rate (4-5 cm yr"1), apparently accommodated by internal deformation (see for example ref. 5) of India and Asia, and ...
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  • 9
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Evidence for the deep recycling of surficial materials through the Earth’s mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and ...
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  • 10
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The long-standing paradigm that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumes—hot, buoyant upwelling regions beneath the Earth’s lithosphere—has recently been the focus of controversy. Whether mantle plumes exist or not is pivotal ...
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