Changes in Silicic Melt Structure Between the Two Bandelier Caldera-Forming Eruptions, New Mexico, USA: Evidence from Zirconium and Light Rare Earth Elements     

STIX  JOHN; GORTON  MICHAEL P. 《Journal of Petrology》1990,31(6):1261-1283

The Cerro Toledo Rhyolite is a group of high-silica rhyolitedomes and tephras which were erupted during the period betweenthe Lower Bandelier Tuff (LBT) at 1?45 Ma and the Upper BandelierTuff (UBT) at 1?12 Ma. The tephra sequence reflects the changingcomposition of the most fractionated liquids at the top of themagma chamber or chambers owing to crystallization during this0?33-Ma interval, over which it is possible to trace magma evolutionin detail. Incompatible elements such as Cs and Lu generallyincrease in concentration upsection through the tephra sequence.By contrast, Zr initially decreases upsection from 153 ppm to134 ppm in the middle, then increases upward to 320 ppm in themost evolved rhyolites. The Zr/Cs and Hf/Cs ratios initiallydecline, then become constant. The dramatic rise in Zr at nearlyconstant Zr/Cs suggests suppression of zircon crystallizationand a change from a high degree of zircon oversaturation toone of marginal oversaturation in the most evolved magmas. Theobserved Zr trends in the Cerro Toledo Rhyolite are oppositeto those (1) predicted by experimental studics of Zr solubilityin silicic magmas and (2) observed in the Bishop Tuff. LREEshow broadly similar relations to Zr and Hf. We have examined two parameters—iron content and volatilecontent of the magma—which may have controlled the crystallizationbehavior of zircon and the LREE-rich phase(s). The dramaticincrease in Zr and LREE in the most evolved Cerro Toledo Rhyolitesand basal UBT plinian tephras is accompanied by similar increasesin iron and halogen contents. If changa in the halogen contentsof matrix glasses are indicative of changes in overall volatilelevels at the top of the magma chamber(s), the increase in volatilesand iron may have modified the structural state of the magmaand increased the solubilities of zircon and the LREE-rich phase(s),thereby raising the saturation levels of Zr and LREE and stabilizingthese elements in the melt. Disruption of the melt structuremay have resulted from (1) Fe as a network modifier or quasi-molecularcomplex, (2) breaking of bridging oxygens by anions such asOH^– and F^–, and/or (3) complexing of Al and alkalisby OH, F, and/or Cl.  相似文献   

Anatomy and origin of a Cretaceous phosphorite-greensand giant, Egypt   总被引：4，自引：0，他引：4  

CRAIG R. GLENN  MICHAEL A. ARTHUR† 《Sedimentology》1990,37(1):123-154

Late Cretaceous epicontinental phosphorites, porcelanites/cherts, dark-coloured shales, glauconitic sandstones and bioclastic and fine-grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon-rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea-level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution-collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron-oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters. This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters.  相似文献   

Age and extent of the Scandinavian ice sheet in northwest Russia     

EILIV LARSEN  ASTRID LYSÅ  IGOR DEMIDOV  SVEND FUNDER  MICHAEL HOUMARK-NIELSEN  KURT H. KJÆR  REW S. MURRAY 《Boreas: An International Journal of Quaternary Research》1999,28(1):115-132

The last glacial maximum (LGM) of the Scandinavian ice sheet in the Arkhangelsk region has been identified morphologically as ridges and hummocks in an otherwise flat topography. Stratigraphically the limit is marked by the presence of till above Mikhulinian (last interglacial) sediments inside the ridges and by the absence of till outside the ridges. During the LGM, ice flowed into the region from the north and northwest forming a lobe in the Dvina-Vaga depression. The continuation northward, northeast of Arkhangelsk, is still somewhat uncertain, but evidence suggests that the outer margin of the Scandinavian ice sheet was situated in the Mezen drainage basin. Luminescence and radiocarbon dates suggest that the maximum position was attained after some 17 ka ago, and that deglaciation started close to 15 ka ago. This age for the maximum position is younger than the maximum position in the western peripheral areas of the Scandinavian ice sheet. This may be accounted for by initial ice build-up in the west followed by a successive migration of the ice divide(s) to the east as ice growth continued. Deglaciation was either by lateral retreat or isolation of dead ice masses causing areal downwasting.  相似文献   

AVIFAUNAL RESEARCH AND GEOGRAPHICAL PERSPECTIVES     

MICHAEL K. STEINBERG 《Geographical review》2010,100(2):iii-iv

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The sensitivity of supercritical atmospheric boundary-layer flow along a coastal mountain barrier     

MICHAEL TJERNSTRÖm 《地球，A辑:动力气象学与海洋学》1999,51(5):880-901

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Sedimentary facies and geomorphic evolution of a blocked‐valley lake: Lake Futululu,northern Kwazulu‐Natal,South Africa     

SUZANNE E. GRENFELL  WILLIAM N. ELLERY  MICHAEL C. GRENFELL  LISA F. RAMSAY  TYREL J. FLÜGEL 《Sedimentology》2010,57(5):1159-1174

Blocked‐valley lakes are formed when tributaries are impounded by the relatively rapid aggradation of a large river and its floodplain. These features are common in the landscape, and have been identified in the floodplains of the Solimões‐Amazon (Brazil) and Fly‐Strickland Rivers (Papua New Guinea), for example, but their inaccessibility has resulted in studies being limited to remotely sensed image analysis. This paper documents the sedimentology and geomorphic evolution of a blocked‐valley lake, Lake Futululu on the Mfolozi River floodplain margin, in South Africa, while also offering a context for the formation of lakes and wetlands at tributary junctions. The study combines aerial photography, elevation data from orthophotographs and field survey, and longitudinal sedimentology determined from a series of cores, which were sub‐sampled for organic content and particle size analysis. Radiocarbon dating was used to gauge the rate and timing of peat accumulation. Results indicate that following the last glacial maximum, rising sea‐levels caused aggradation of the Mfolozi River floodplain. By 3980 years bp , aggradation on the floodplain had impounded the Futululu drainage line, creating conditions suitable for peat formation, which has since occurred at a constant average rate of 0·13 cm year^?1. Continued aggradation on the Mfolozi River floodplain has raised the base level of the Futululu drainage line, resulting in a series of back‐stepping sedimentary facies with fluvially derived sand and silt episodically prograding over lacustrine peat deposits. Blocked‐valley lakes form where the trunk river has a much larger sediment load and catchment than the tributary stream. Similarly, when the relative difference in sediment loads is less, palustrine wetlands, rather than lakes, may be the result. In contrast, where tributaries drain a steep, well‐connected catchment, they may impound much larger trunk rivers, creating lakes or wetlands upstream.  相似文献   

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging     

By RALPH KAHN  REAS PETZOLD  MANFRED WENDISCH  EIKE BIERWIRTH  TILMAN DINTER  MICHAEL ESSELBORN  MARCUS FIEBIG  BIRGIT HEESE  PETER KNIPPERTZ  DETLEF MÜLLER  ALEXANDER SCHLADITZ  WOLFGANG VON HOYNINGEN-HUENE 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):239-251

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite's larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR's ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.  相似文献   

Regional Saharan dust modelling during the SAMUM 2006 campaign     

By BERND HEINOLD  INA TEGEN  MICHAEL ESSELBORN  KONRAD KANDLER  PETER KNIPPERTZ  DETLEF MÜLLER  ALEXANDER SCHLADITZ  MATTHIAS TESCHE  BERNADETT WEINZIERL  ALBERT ANSMANN  DIETRICH ALTHAUSEN  BENOIT LAURENT  REAS MASSLING  THOMAS MÜLLER  REAS PETZOLD  KERSTIN SCHEPANSKI  ALFRED WIEDENSOHLER 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):307-324

The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006 SAMUM field campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations. However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust source placement by the model. While simulated winds and dust distributions are well matched for dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.  相似文献   

Implications of garnet resorption for the Lu–Hf garnet geochronometer: an example from the contact aureole of the Makhavinekh Lake Pluton,Labrador     

E. D. KELLY  W. D. CARLSON  J. N. CONNELLY 《Journal of Metamorphic Geology》2011,29(8):901-916

In the contact aureole of the Makhavinekh Lake Pluton (MLP), Labrador, garnet resorption caused redistribution of Lu and loss of Hf, creating spuriously young Lu–Hf garnet ages. Garnet grew during granulite facies regional metamorphism at 1860–1850 Ma. At 1322 Ma, garnet rims were replaced by coronas of cordierite and orthopyroxene during contact metamorphism. Garnet–ilmenite Lu–Hf geochronology using bulk‐garnet separates yields apparent ages that young from 1876 ± 21 Ma at 4025 m from the contact to 1396 ± 8 Ma at 450 m from the contact. Toward the contact, garnet crystals are progressively more resorbed. Concentrations of Lu measured by LA‐ICP‐MS along radial traverses on central sections through relict garnet decrease gently away from the cores but rise steeply within 50–200 μm of the edges of the relicts. Enrichments of Lu in rims of relict garnet demonstrates strong partitioning of Lu into garnet during resorption and modest intracrystalline diffusion. Hafnium distributions could not be measured, but considering the strong incompatibility of Hf with garnet, it is likely that nearly all Hf in resorbed portions of the garnet was lost from the crystals. Lu–Hf ages in the aureole are thus controlled predominantly by this retention of Lu and loss of Hf during garnet resorption. This deduction was tested with a simple numerical model in which the partial retention of Lu and loss of Hf is tracked as a population of garnet is resorbed. Assuming a spherical geometry for garnet porphyroblasts, Rayleigh fractionation is used to approximate initial Lu zoning profiles ranging from flat to steeply decreasing toward garnet rims. The model simulates: (i) Lu–Hf decay for a specified period before resorption; (ii) instantaneous resorption with retention of Lu and loss of Hf from the resorbed portion of the crystal and (iii) Lu–Hf decay during a specified period after resorption. Several parameters influence the modelled age, but garnet resorption and Lu retention are the primary factors. When all other parameters are held constant, larger amounts of resorption and higher degrees of Lu retention produce younger apparent ages (false ages). Similarly, flatter initial Lu profiles yield younger apparent ages as a consequence of the larger proportion of Lu and Hf that resides in the outer portions of the porphyroblast. The difference between the apparent and actual ages is greater if the duration of the pre‐resorption decay period is large relative to the post‐resorption decay period. Larger crystals in a Gaussian crystal‐size distribution (CSD) generally dominate the Lu–Hf budget and produce an older apparent age relative to the age of the mean crystal size. Compared to a symmetrical Gaussian CSD, positively skewed CSDs result in reduced resorption of large crystals and produce an older apparent age. Application of the model to the MLP aureole, positing growth at 1850 Ma and resorption at 1320 Ma, yields model ages that young from 1850 to 1374 Ma toward the contact, in good agreement with the apparent ages determined from geochronology.  相似文献   

Microstructure and thermal history of metal particles in CH chondrites     

J. I. GOLDSTEIN  R. H. JONES  P. G. KOTULA  J. R. MICHAEL 《Meteoritics & planetary science》2007,42(6):913-933

Abstract We have studied metal microstructures in four CH chondrites, Patuxent Range (PAT) 91546, Allan Hills (ALH) 85085, Acfer 214, and Northwest Africa (NWA) 739, to examine details of the thermal histories of individual particles. Four types of metal particles are common in all of these chondrites. Zoned and unzoned particles probably formed as condensates from a gas of chondritic composition in a monotonic cooling regime, as has been shown previously. We have demonstrated that these particles were cooled rapidly to temperatures below 500 K after they formed, and that condensation effectively closed around 700 K. Zoned and unzoned particles with exsolution precipitates, predominantly high‐Ni taenite, have considerably more complex thermal histories. Precipitates grew in reheating episodes, but the details of the heating events vary among individual grains. Reheating temperatures are typically in the range 800–1000 K. Reheating could have been the result of impact events on the CH parent body. Some particles with precipitates may have been incorporated into chondrules, with further brief heating episodes taking place during chondrule formation. In addition to the four dominant types of metal particles, rare Ni‐rich metal particles and Si‐rich metal particles indicate that the metal assemblage in CH chondrites was a mixture of material that formed at different redox conditions. Metal in CH chondrites consists of a mechanical mixture of particles that underwent a variety of thermal histories prior to being assembled into the existing brecciated meteorites.  相似文献