A new technique for precise uranium-series dating of travertine micro-samples

Secondary carbonate formations, such as travertine and calcareous tufa deposits, are important archives for quaternary continental climate studies and archaeology. The extremely complex growth mechanisms result in some serious problems for precise mass spectrometric uranium-series dating. Often, detrital and organic particles contaminate the carbonate and large pore volumes yield a great potential for open system behavior. We utilized microscopic, mineralogical and geochemical methods prior to sample selection to determine the abundance of primary calcite, i.e. micrite and spar. Furthermore, the state of alteration was characterized by cathodoluminescence and trace-element analysis. We conclude that travertine and calcareous tufa are appropriate for precise U-series age determination if a) micrite and/or spar are the dominant phases; b) cathodoluminescence of both phases is weak or absent; c) Fe and Al levels are low; and d) Sr concentrations are close to the average of the studied site. We mapped and sampled solely areas of major micrite/spar abundance having minor alteration for accurate U-series dating. When this new method was applied, travertines located in eastern Germany (sites Bad Langensalza, Burgtonna and Weimar-Ehringsdorf) gave single ²³⁰Th/²³⁸U-ages consistent with the lithological growth sequence and greatly improved compared to previously published chronologies. In addition, we determined ²³⁰Th/U isochron ages on bulk samples that confirm our single ages. In contrast to primary calcite, pore cements are homogeneously distributed throughout the travertine fabric and reflect early diagenetic processes and/or weathering.     

Plagioclase morphologies in assimilation experiments. Implications for disequilibrium melting in the generation of granodiorite rocks

Summary Assimilation experiments have been performed using a crustal contaminant and several basaltic compositions in order to study the morphologies of plagioclase crystals produced in the course of the assimilation reactions. A Crd-Bt anatectic gneiss was used as a crustal contaminant. Both a synthetic basaltic glass, with the composition of a high-Al ocean tholeiite, and a natural Hb gabbro were used as basaltic compositions. Experiments were carried out at temperatures in the range 900–1100 °C and at pressures of 4, 5 and 10 kbar. Plagioclase crystals with skeletal and honeycomb morphologies are produced in the assimilation experiments. These morphologies result from destabilisation of the crystal-melt interface caused by the disequilibrium production of a Ca-rich melt. The production of the Ca-rich metastable melt is the result of reaction between two compositionally dissimilar systems, pelite and basalt. This interpretation, based on the kinetics of plagioclase and melt production, may be applied to granodiorite rocks characterised by plagioclase with Ca-rich, skeletal cores.

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Zusammenfassung Plagioklas-Morphologie und Assimilations-Experimente: Die Bedeutung von Ungleichgewichts-Schmelzvorg?ngen bei der Entstehung von Granodioriten Assimilations-Experimente mit einer krustalen Komponente und verschiedenen basaltischen Zusammensetzungen wurden durchgeführt, um die Morphologien von Plagioklas-Kristallen, die im Laufe von Assimilations-Reaktionen entstehen, zu studieren. Ein CRD-PT anatektischer Gneis wurde als krustale Beimengung benützt. Ein synthetisches basaltisches Glas mit der Zusammensetzung eines Al-reichen ozeanischen Tholeiites und ein natürlicher Hb-Gabbro wurden als basaltische Zusammensetzungen verwendet. Die Experimente wurden bei Temperaturen zwischen 900 und 1.100° und bei Drucken von 4, 5 und 10 kb durchgeführt. Plagioklas-Kristalle mit skelettartigen und Bienenwaben-f?rmigen Morphologien entstehen bei den Assimilations-Experimenten. Diese Morphologien sind das Ergebnis der Destabilisation der Kristall-Schmelze-Kontaktfl?che, die durch die Entstehung einer im Ungleichgewicht befindlichen Ca-reichen Schmelze gef?rdert wird. Die Entstehung von Ca-reichen metastabilen Schmelzen geht auf die Reaktion zwischen zwei in der Zusammensetzung verschiedenen Systemen, Pelit und Basalt, zurück. Diese Interpretation, die auf der Kinetik von Plagioklas und Schmelz-Entstehung basiert, kann auf granodioritische Gesteine angewendet werden, die durch Plagioklasse mit Ca-reichen, skelettf?rmigen Kernen gekennzeichnet sind.

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Received December 7, 1999; revised version accepted April 14, 2000     

Mineralogical and isotopic evidence for phenocryst-matrix disequilibrium in the Garner Mountain andesite

Petrographic, trace element and isotopic evidence demonstrates that magma mixing preceded the eruption of the Garner Mountain andesite. The flow contains reversely zoned plagioclase phenocrysts and amphibole pseudomorphs composed of plagioclase, clinopyroxene, orthopyroxene and opaque oxides. Partially resorbed quartz grains are also present. In contrast to the isotopically uniform matrix, plagioclase phenocrysts have ⁸⁷Sr/⁸⁶Sr ratios that correlate negatively with matrix Sr and positively with matrix Rb abundances. These observations demonstrate increasing isotopic disequilibrium between the plagioclase and matrix in the more evolved varieties of the flow.Plagioclase phenocrysts and matrix are assumed to record fractionation-assimilation events in different parts of the magma chamber. Early formed plagioclase phenocrysts crystallized under AFC conditions close to the roof of the chamber and were subsequently entrained in a liquid mixture composed of evolved interstitial liquid held in the partly crystallized roof zone and newly injected parental magma.     

Uranium series disequilibrium in a young surficial uranium deposit,northeastern Washington,U.S.A.

A recently discovered ore-grade accumulation of U in organic-rich sediments of late Quaternary age provides an opportunity for studying the early association of U, U-daughters, and organic matter in a natural setting. The U occurs in valley-fill sediments of peat, peaty clay, silt, and sand along the north fork of Flodelle Creek, Stevens County, Washington. Radiometric techniques (delayed neutron, high-resolution gamma-ray spectrometry, thin-source alpha spectrometry) were employed to determine the abundance and distribution of U-series nuclides, the extent of secular equilibrium within the U decay series, and the apparent U-series ages of U incorporation.Sixteen lithologically distinct intervals were sampled from a 292 cm core. Uranium contents range from 140 to 2790 ppm and are positively correlated with organic contents. Measured alpha activity ratios of ²³⁴U/²³⁸U (1.31–1.38) are very similar to those reported in coexisting waters, suggesting a rather constant isotopic composition of introduced U. Much lower Th contents of <10–40 ppm are controlled by the type and abundance of silicate detritus. The youth of the host sediments (<15 000 a) and the paucity of associated radioactivity suggested large excesses of U relative to radioactive daughters and such excesses were observed, particularly in the shallowest intervals. Apparent ages of U emplacement determined by the (alpha) activity ratio of ²³⁰Th daughter to ²³⁴U parent show a general increase with depth and fair agreement with estimated depositional ages. This observation suggests dominantly syndepositional or early post depositional emplacement of U followed by decay-generated buildup of ²³⁰Th daughter with time. However, interval by interval comparisons of the relative abundances of other daughters, particularly ²²⁶Ra and ²¹⁰Pb, indicate variability caused by processes other than closed-system growth and decay, probably because chemically diverse daughters that are decay-generated in situ have differing mobilities and because upwelling ground water continuously adds more U and minor amounts of daughters. If ²³⁰Th is considered the least susceptible to these modifications, the data suggest some addition of ²³⁴U in the deepest intervals and some loss of ²²⁶Ra and/or gain of ²²²Rn throughout the studied core.     

Evidence for the stability of the potential nuclear waste host,sphene, over geological time,from uranium-lead ages and uranium-series measurements

A sphene-based, glass-ceramic material is being investigated as a potential host for Canadian nuclear fuel recycle waste. The ability of sphene to retain certain radionuclides over geological time is examined using observed properties and age measurements of natural sphenes.The long-term stability of sphene is determined by comparing UThPb isotopic age data for sphene, from the literature, with ages determined for zircon and other minerals. Better internal agreement is found between UThPb sphene ages than for zircon ages indicating the resistance of sphene to daughter radionuclide loss over periods as long as 3000 Ma. Excellent agreement was observed between sphene ages and the estimated age of crystallization of the host rocks or vein infillings, except where subsequent metamorphim had reset the sphene age. Complete lead loss was generally found to accompany high-grade metamorphic conditions when temperatures exceeded ∼ 500°C.Stability over shorter periods of geological time was found for vein infilling and mineral separates of sphene using uranium-series disequilibrium analysis. Conditions of secular equilibrium in the decay chain ²³⁸U-²³⁴U-²³⁰Th-²²⁶Ra were observed for most of the 21 sphenes studied indicating that no radionuclide migration had occurred over periods as long as the last 1 Ma. Exceptions to this were four sphenes that had ²³⁴U deficiencies between 6 and 24% and two sphenes that showed ²²⁶Ra/²³⁰Rh disequilibrium. No correlation was found between the extent of radiation damage suffered by the sphenes and the presence of uranium-series disequilibrium. The presence of alteration products in three sphenes did correlate, however, with measured disequilibrium, indicating that some ²³⁴U and ²²⁶Ra had been lost or gained by these samples in the last 1 Ma and 8 ka, respectively.     

Accessory phases and the generation of LREE-Enriched basalts — A test for disequilibrium melting

The REE contents of tholeiitic rocks can be derived by simple mantle melting models. However this type of model has been less successful in accounting for rocks of the alkali basalt-kimberlite suite with strong LREE enrichment. Rocks from these associations have similar La/Sm ratios over a range of total REE concentrations and their generation by simple melting models requires an enriched source. These requirements conflict with the conclusions reached from Nd-Sm studies.An alternative hypothesis involves the disequilibrium melting of a LREE-rich accessory phase in the mantle. Such a phase will be among the first to melt and thus will dominate the REE patterns of early-formed liquids, even if subsequent partial re-equilibration occurs. This model could account for liquids with strong LREE enrichment and similar La/Sm ratios at different levels of total REE enrichment by partial melting of a simple chondritic mantle. Since the model predicts the existence of an accessory phase in the mantle with a REE pattern parallel to that found in strongly LREE-enriched magma, it provides a sensitive test for disequilibrium melting.     

A model for the explosive synthesis of p nuclei

A model for the synthesis of p nuclei during a supernova explosion of a 25 M _⊙ star is presented. Its main distinguishing feature is the requirement that the yield of s nuclei after the explosion fully correspond to the observed solar-system abundances. A fully self-consistent system of kinetic equations including (n, γ), (p, γ), (α, γ), (p, n), (α, n), and (α, p) reactions and their reverse reactions was solved for the mass fractions of various nuclei, taking into account the dynamics of the propagation of the supernova shock. The absolute abundances of the p nuclei are calculated, and the role of helium layers in the supernova in the synthesis of r nuclei is analyzed. The calculated abundances of p nuclei are in overall good agreement with observations. This makes it possible to synthesize ¹¹³In, ¹³⁸La, and some other problematic isotopes, although the problem of synthesizing molybdenum and ruthenium remains.     

沉积盆地中超压力的形成机理

下列作用可引起超压力:①挤压应力的增大;②孔隙流体或岩石基质体积的改变;③流体的运移或浮力。由于压实作用不均衡,埋藏期间特别是低渗性沉积物迅速沉降期间的负荷能产生相当大的超压力。在构造活动地区,水平应力的改变能迅速产生和耗散大量超压力。与体积变化有关的超压力的形成机理要求必须具备良好的有效的封闭系统。与热液膨胀和粘土脱水有关的流体体积的增加太少,不能产生大的超压,除非有理想的密封体系存在。烃类的形成和裂解为天然气可以产生超压,这主要取决于干酪根的类型、有机质的丰度、温度和岩石的渗透率。然而,由于压力的形成阻碍有机质的进一步变质,所以在封闭系统中这些作用可能受限。由烃类的形成和裂解产生超压的可能性在目前还未被证实。水压头造成的流体运动在浅埋藏的“铅垂”盆地内可能会形成超压。经计算表明,烃的浮力和渗透性所产生的局部超压力很小。在不可压缩的流体中,气体的向上运移也能产生一定的超压力,但需要进一步研究。在许多沉积盆地中与应力相关的机理是产生超压力的最主要原因。     

Petrogenesis and 230Th-238U disequilibrium at Mt. Shasta,California, and in the Cascades

Petrogenesis at Mt. Shasta is dominated by mixing of magmas and/or assimilation of wall rock, as is shown by petrographic, major and trace element chemistry, and ²³⁸U-²³⁰Th disequilibrium data. At least three end- members are involved in these mixing processes. Lavas of very young Cascades lavas, from Mt. Garibaldi in the north to Lassen Peak in the south, are characterized by a large range of thorium isotopic ratios, although series of samples from single volcanoes are characterized by approximately constant (²³⁰Th/²³²Th). There is a monotonic decrease in this ratio from Crater Lake south through Lassen Peak, perhaps reflecting increasing thickness of the underlying crust. Th/U fractionation in Cascades lavas, as evidenced by (²³⁰Th/²³⁸U) 1, is in the opposite sense to that in most island arc lavas. This trend suggests that fluid transport, which is thought to produce uranium enrichment in island arc, is lacking or somehow modified in the petrogenesis of the Cascades lavas.     

Equilibrium vs. disequilibrium melting relations in theHigher Himalayan Crystalline (HHC) pelitic migmatites,Sikkim Himalaya

Higher Himalayan Crystalline (HHC) complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites. In this study, reaction textures, mineral/bulk rare earth elements (REE), trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis. With the help of equilibrium REE and trace element partitioning model, it is shown that strong enrichment of Effective Bulk Composition (EBC) is responsible for the zoning in garnet in these rocks. The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.     

A dipolar vortex model for the obscuring tori in active galactic nuclei

Motivated by currently available direct observations of occulting tori in Seyfert galaxies and ongoing discussions about tori as important structural components in Active Galactic Nuclei, we discuss the possibility that the “spinning up” of a torus by radiation or winds could transform it into a dipolar toroidal vortex. The vortex motion balances the torus’ self-gravitation, and can explain the existence of cool, thick tori inferred from observations. In turn, the toroidal vortex can be a source of matter to feed the accretion disk. The resulting instability could result in flares accompanied by the ejection of matter. Our numerical estimates of the model parameters for luminosities close to the Eddington limit are consistent with the observational data.     

Study of the physical conditions in active galactic nuclei. A method for estimating the physical parameters of radio sources

A new method for estimating the physical parameters of active galactic nuclei involving the analysis of observations of the compact radio sources in them is proposed. The method is based on an inhomogeneous model for a synchrotron radio source. Theoretical spectra of the radio sources are obtained via numerical solution of the transfer equation. Due to the paucity of observational data, only interval estimates of the magnetic field strength and the energy densities of the magnetic field and relativistic particles can be obtained. A mechanism for the formation of flat radio spectra is proposed.     

The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source

Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO₂ and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years.     

Uranium isotopic disequilibrium for groundwater classification: first results on complexe terminal and continental intercalaire aquifers in Southern Tunisia

A fractionation of uranium (U) series into parent–daughter pairs – ²³⁴U and ²³⁸U – always occurs in natural waters, and the disequilibria between these is commonly used as a tracer of groundwater flow. We report here an interpretation of the U-content and U isotope disequilibria in groundwater sampled from the deep Complexe Terminal and Continental Intercalaire aquifers of southern Tunisia. Variations in both the contents of these isotopes (0.006–2.4 ppb) and ²³⁴U/²³⁸U activity ratios (ARs) (1.7–15.4) were observed. The data could be plotted in two distinct fields of reciprocal U concentration versus ²³⁴U/²³⁸U AR according to groundwater flow and regional bedrock differences. An initial assessment aimed at verifying whether the results of this investigation support those of previous hydrogeological and isotope studies, thereby suggesting that the disequilibrium between U isotopes in groundwater may represent a useful tool for hydrogeological investigations of deep and fossil groundwater. In addition, the disequilibrium can be used for quantifying the recharge or mixing rates between different formations with the aim of delineating the preferential outflow pattern or determining residence times of waters.     

Basin Structure and Numerical Simulation for the Mechanisms of Seismic Disasters

In the present study seismic wave propagation in heterogeneous media is numerically simulated by using the pseudospectral method with the staggered grid RFFT differentiation in order to clarify the cause for the complicated distribution characteristics of strong ground motion in regions with basin structure. The results show that the maximum amplitudes of simulated ground acceleration waveforms are closely related to the basin structure. Interference of seismic waves in the basin strongly affects the distribution of maximum seismic waveforms, which may result in peak disasters during earthquakes. Peak disasters might be away from basin boundaries or earthquake faults. Seismic energy transmitted into the basin from the bedrock can hardly penetrate the bottom of the basin and then travel back into the bedrock region. The seismic energy is absorbed by basin media, and transferred into the kinematical energy of seismic waves with great amplitude in the basin. Seismic waves between basins may result in seriou     

Mechanisms of englacial conduit formation and their implications for subglacial recharge

Ideas about the character and evolution of englacial drainage systems have been deeply influenced by the theoretical model developed by Shreve [1972. Movement of water in glaciers. Journal of Glaciology 11(62), 205–214]. The Shreve model is based on three main assumptions: (1) englacial drainage is in steady state; (2) englacial water will flow along the steepest hydraulic gradient within the glacier; and (3) pressure head equals the pressure of the surrounding ice minus a small component due to melting of the walls. The Shreve model has been widely adopted as a fundamental component of englacial drainage theory. There is no evidence, however, that the model provides a realistic picture of actual glacial drainage systems.To evaluate Shreve's theory, we used speleological techniques to directly survey englacial conduits. We mapped a total of 8.25 km of passage in 27 distinct englacial conduits in temperate, polythermal, cold-based and debris-covered glaciers between 2005 and 2008. New information reported here is supplemented by published data on 40 other englacial conduits located worldwide and surveyed to ice depths of 176 m using speleological techniques. In all cases, englacial drainage systems consisted of a single unbranching conduit. Englacial conduit morphologies were found to be intimately linked to the orientation of a glacier's principal stresses or the presence of pre-existing lines of high hydraulic conductivity. If a sufficient supply of water is available, hydrofracturing forms vertical conduits in zones of longitudinal extension and subhorizontal conduits where longitudinal stresses are compressive. On unfractured glacier surfaces, relatively shallow subhorizontal conduits with migrating nickpoints form by cut-and-closure provided channel incision is significantly faster than surface lowering. Conduits can also form along permeable debris-filled crevasse traces that connect supraglacial lake basins of different potential. Our results suggest that Shreve-type englacial drainage systems do not exist and implies that englacial conduits can only penetrate through thick ice to recharge the bed where supraglacial water bodies either intersect, or are advected through, zones of acceleration.     

淤长型泥质潮滩双凸形剖面形成机制

以江苏中部淤长型泥质潮滩为例,建立了基于过程的潮滩动力地貌演变数学模型,研究了在沿岸潮流作用、供沙充分情况下双凸形潮滩横剖面形成机制。在向岸方向,流速自潮下带至低潮位线附近急速减小,潮间带区域流速则缓慢减小,泥沙在流速急变区域迅速堆积形成上凸点。潮下带中部在小潮期的淤积量大于大潮期冲刷量,总体淤积率较高;加之潮间带中部较弱的落潮流不足以将其上风处底沙掀动并向海输运,导致落潮后期潮下带上部含沙量小、沉积率相对较低,最终在潮下带中部形成下凸点。随着滩面淤长抬升,上、下凸点位置逐步向高、低潮位线附近移动。与前人关于双凸形剖面形成机制的定性分析成果相比,尚有不一致之处,需通过现场观测等进一步探讨。     

超压封存箱的压力封闭机制研究进展综述

超压封存箱的封闭机制研究是封存箱研究中的难点之一,倍受许多专家学者关注。深入研究和揭示封隔层的封存机制和异常高压的保存时间对超压成因和油气成藏条件诸多方面的研究都非常重要。已有的关于异常高压的封闭机制研究可分为静态和动态两种观点。静态观点认为:极低渗透率(渗透率10-11μm2)的致密岩层可作为封闭层阻止流体流动,如蒸发岩或经历强烈碳酸盐矿化作用的碎屑岩可形成近于非渗透性岩层从而形成有效封闭;在砂泥岩频繁互层的地层中,气-水两相界面可产生足够大的毛细管压力,而且毛细管压力具有可叠加性,进而形成有效封闭。动态观点认为:在封存箱和封闭层处于连续水相的地质环境中,剩余压力将通过低速扩散方式逐步同其环境压力达到平衡,但由于异常高压通过低渗透岩层(渗透率10-8~10-10μm2)的扩散速度极其缓慢,从而也可使封存系统长期保持超压状态。超压封闭机制的静态和动态两种观点都各自具有事实依据,但具有普遍意义的封存机制可能是气-水两相界面毛细管压力封闭和低速扩散机制。     

On search for tracks of heavy and superheavy cosmic-ray nuclei in crystals from pallasites

The results of a search for far transuranic elements in the primary cosmic radiation are presented. It is shown that olivines from pallasites are very suitable for such investigations. The sensitivity of olivines to charged particles and the fading effect of latent tracks under space conditions have been studied. In the Lipovsky and Marjalahti pallasites, the distributions of tracks of nuclei with Z > 36 have been measured, and the abundance of nuclei with Z ? 70 in these crystals has been tentatively set at (2–5) × 10^?6 with respect to the Fe group nuclei.