Origin of SiO₂-rich components in ordinary chondrites     

Dominik C. Hezel  Herbert Palme  Frank E. Brenker 《Geochimica et cosmochimica acta》2006,70(6):1548-1564

Silica-rich objects are common minor components in ordinary chondrites (OC), occurring as fragments and as chondrules. Their typical paragenesis is orthopyroxene + SiO₂ (with bulk SiO₂ >65 wt%) and occasionally with additional olivine and/or spinel. Individual silica-rich components (SRC) have previously been studied in various types of OCs, although there is only one comprehensive study of these objects by Brigham et al. [Brigham, C.A., Murrell, M.T., Yabuki, H., Ouyang, Z., El Goresy, A., 1986. Silica-bearing chondrules and clasts in ordinary chondrites. Geochim. Cosmochim. Acta 50, 1655-1666]. Several different explanations of how SRCs formed have been published. The main question is how silica-enrichment was achieved, because CI-chondritic atomic Mg/Si-ratio is 1.07 and as a consequence only olivine and pyroxene, but no free silica should be stable. There are two basic possibilities for the SiO₂-enrichment: (1) a RedOx-mechanism or magmatic fractionation on the parent body and (2) fractional condensation or recycling of chondrule mesostasis in the solar nebula. To better constrain the origin of these objects, we measured major and rare earth elements in SRCs of various types of ordinary chondrites, and in addition, we studied silica polymorphism in these objects using an in situ micro-Raman technique. Bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene. The SRCs extend these lines to an SiO₂ end member. In contrast, magmatic trends grossly deviate from these mixing lines. Concentrations of CaO, Al₂O₃, and REE in the pyroxenes of the SRCs are low (0.01 to 1× CI) and the CI-normalized REE-patterns are virtually flat, typical of bulk chondrules, but untypical of magmatic trends. We therefore conclude that SiO₂-rich objects are not of magmatic origin. They are the result of fractional condensation in the solar nebula. The silica in SRCs occurs mainly as tridymite and sometimes as cristobalite or—in very rare cases—as quartz. Some SiO₂-phases yielded a yet unknown micro-Raman spectrum, which we were unable to identify. The often chondrule-like shape of SRCs as well as the presence of high-temperature SiO₂-polymorphs lead to the following model for the origin of SRCs: formation of SiO₂-rich precursors in the solar nebula by fractional condensation, reheating to temperatures between 1140 and >1968 K, thereby forming the SRCs,—probably during the chondrule-forming process—followed by rapid cooling.  相似文献   

Sources of Sediment in Lake Pepin on the Upper Mississippi River in Response to Holocene Climatic Changes     

David W. Kelley  Stefanie A. Brachfeld  Edward A. Nater  Herbert E. Wright Jr. 《Journal of Paleolimnology》2006,35(1):193-206

Sediments from Lake Pepin on the Mississippi River, southeastern Minnesota, are used as provenance tracers to assess variations in hydrology and sediment-transport during the middle Holocene. Three rivers contribute sediment to Lake Pepin, and each catchment is characterized by a distinctly different geologic terrain. The geochemical fingerprint for each drainage basin was determined from the elemental composition of heavy minerals in the silt-sized fraction of modern sediment samples. Down-core elemental abundances were compared with these fingerprints by use of a chemical-mass-balance model that apportions sediment to the source areas. We observed a decreased contribution from the Minnesota River during the interval ~6700–5500 ¹⁴C yr BP, which we attribute to decreased discharge of the Minnesota River, likely controlled by a combination of precipitation, snow melt, and groundwater input to the river. This hydrologic condition coincides with the mid-Holocene prairie period recorded by fossil pollen data. The occurrence of this feature in a proxy record for hydrologic variations supports the hypothesis that the mid-Holocene prairie period reflects drier conditions than before or after in midwestern North America.  相似文献   

Heavy metal retention in secondary precipitates from a mine rock dump and underlying soil, Dalarna, Sweden   总被引：7，自引：2，他引：7  

Zhixun Lin  Roger B. Herbert Jr. 《Environmental Geology》1997,33(1):1-12

This study investigates the retention of heavy metals in secondary precipitates from a sulfidic mine rock dump and underlying podzolic soils by means of mineralogical and chemical extraction methods. The rock dump, which is at least 50 years old, consists of a 5–10-cm-thick leached zone and an underlying 110–115-cm-thick accumulation zone. Optical microscopy and electron microprobe analyses confirm that pyrrhotite weathering has proceeded much further in the leached horizon relative to the accumulation horizon. The weathering of sulfides in the leached zone has resulted in the migration of most heavy metals to the accumulation zone or underlying soils, where they are retained in more stable phases such as secondary ferric minerals, including goethite and jarosite. Some metals are temporarily retained in hydrated ferrous sulfates (e.g., melanterite, rozenite). Received: 28 October 1996 · Accepted: 24 February 1997  相似文献   

Origin of chondritic forsterite grains     

Andreas Pack  Herbert Palme 《Geochimica et cosmochimica acta》2005,69(12):3159-3182

Iron-poor and refractory lithophile element (RLE) rich forsterite grains occur in all major types of unequilibrated chondrites. In our laser ablation inductively coupled mass spectrometry (LA-ICPMS) minor and trace element study we show that refractory forsterites (RF) from carbonaceous (CC), unequilibrated ordinary (UOC) and a Rumuruti chondrite (RC) have similar chemical compositions with high RLE concentrations and low concentrations of Mn, Fe, Co and Ni. Fractionation of RLEs and rare earth elements (REEs) is in agreement with formation by crystallization from a RLE rich silicate melt. Low concentrations and the fractionation of moderately siderophile elements (Fe, Co, Ni) in RFs suggests formation at low oxygen fugacity, possibly in equilibrium with primitive Fe,Ni metal condensates in a gas of solar composition. Anomalously high Ti in the parental melt can be explained by Ti³⁺/Ti⁴⁺ ∼1.5, supporting formation of RF in highly reducing conditions. Low Mn concentrations indicate formation at high temperatures (>∼1160 K). The model of formation of RFs and the accompanying physico-chemical conditions during their formation as well as their relation to non refractory olivine are discussed.  相似文献   

Publishing Ethics     

Giovanni Barla  Herbert Einstein  Kalman Kovari 《Rock Mechanics and Rock Engineering》2014,47(2):333-333

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Calibration of paired watersheds: Utility of moving sums in presence of externalities          下载免费PDF全文

Herbert Ssegane  Devendra M. Amatya  Augustine Muwamba  George M. Chescheir  Tim Appelboom  E.W. Tollner  Jami E. Nettles  Mohamed A. Youssef  François Birgand  R.W. Skaggs 《水文研究》2017,31(20):3458-3471

Historically, paired watershed studies have been used to quantify the hydrological effects of land use and management practices by concurrently monitoring 2 similar watersheds during calibration (pretreatment) and post‐treatment periods. This study characterizes seasonal water table and flow response to rainfall during the calibration period and tests a change detection technique of moving sums of recursive residuals (MOSUM) to select calibration periods for each control–treatment watershed pair when the regression coefficients for daily water table elevation were most stable to minimize regression model uncertainty. The control and treatment watersheds were 1 watershed of 3–4‐year‐old intensely managed loblolly pine (Pinus taeda L.) with natural understory, 1 watershed of 3–4‐year‐old loblolly pine intercropped with switchgrass (Panicum virgatum), 1 watershed of 14–15‐year‐old thinned loblolly pine with natural understory (control), and 1 watershed of switchgrass only. The study period spanned from 2009 to 2012. Silvicultural operational practices during this period acted as external factors, potentially shifting hydrologic calibration relationships between control and treatment watersheds. MOSUM results indicated significant changes in regression parameters due to silvicultural operations and were used to identify stable relationships for water table elevation. None of the calibration relationships developed using this method were significantly different from the classical calibration relationship based on published historical data. We attribute that to the similarity of historical and 2010–2012 leaf area index on control and treatment watersheds as moderated by the emergent vegetation. Although the MOSUM approach does not eliminate the need for true calibration data or replace the classic paired watershed approach, our results show that it may be an effective alternative approach when true data are unavailable, as it minimizes the impacts of external disturbances other than the treatment of interest.  相似文献   

Representation of Uncertainty and Integration of PGIS-based Grazing Intensity Maps Using Evidential Belief Functions     

Jane Bemigisha  John Carranza  rew K. Skidmore  Mike McCall  Chiara Polce  Herbert H.T. Prins 《Transactions in GIS》2009,13(3):273-293

In a project to classify livestock grazing intensity using participatory geographic information systems (PGIS), we encountered the problem of how to synthesize PGIS-based maps of livestock grazing intensity that were prepared separately by local experts. We investigated the utility of evidential belief functions (EBFs) and Dempster's rule of combination to represent classification uncertainty and integrate the PGIS-based grazing intensity maps. These maps were used as individual sets of evidence in the application of EBFs to evaluate the proposition that " This area or pixel belongs to the high, medium, or low grazing intensity class because the local expert(s) says (say) so ". The class-area-weighted averages of EBFs based on each of the PGIS-based maps show that the lowest degree of classification uncertainty is associated with maps in which "vegetation species" was used as the mapping criterion. This criterion, together with local landscape attributes of livestock use may be considered as an appropriate standard measure for grazing intensity. The maps of integrated EBFs of grazing intensity show that classification uncertainty is high when the local experts apply at least two mapping criteria together. This study demonstrates the usefulness of EBFs to represent classification uncertainty and the possibility to use the EBF values in identifying and using criteria for PGIS-based mapping of livestock grazing intensity.  相似文献   

Cadmium release caused by the die-back of the saltmarsh cord grass Spartina anglica in Poole Harbour (UK)     

Ralf Hübner  Roger J.H. Herbert  K. Brian Astin 《Estuarine, Coastal and Shelf Science》2010

As in many other areas, the perennial cord grass Spartina anglica C.E. Hubbard has been extremely successful at colonising the English and Irish south coasts, but it is dying back for reasons that are not completely understood. The present study considers the relationship between die-back and metal concentrations in estuarine sediment. No obvious impact of the metal contamination on the S. anglica growth/die-back could be detected in the study zone, Poole Harbour, although the die-back did seem to have substantially influenced the metal concentrations in the sediments of the estuary. Based on core profiles, the remaining patches of S. anglica still retain elevated concentrations and the overall cadmium concentrations in the sediments have risen since 1925. However, considerable quantities of the cadmium stored in the sediment by S. anglica appear to have been washed out rapidly in the die-back zones. Should serious erosion of the saltmarsh occur, triggered for example by sea-level rise, then sudden and high levels of cadmium release may cause harmful effects to marine biota.  相似文献   

Manuscripts Using Numerical Discrete Element Methods     

Giovanni Barla  Herbert Einstein  Kalman Kovari 《Rock Mechanics and Rock Engineering》2013,46(4):655-655

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Dynamic features in the western Baltic Sea investigated using NOAA-AVHRR data     

Dr. Herbert Siegel  Monika Gerth  Rainer Rudloff  Gisela Tschersich 《Ocean Dynamics》1994,46(3):191-209

Summary The western part of the Baltic Sea is a shallow transition area which is characterized by the water exchange between the Baltic and the North Sea. The dynamic processes in this area are forced by sea-level differences between the North Sea and Baltic Sea as well as by local wind and freshwater input. The response patterns are modified by Earth's rotation, bathymetric structures and the alignment of the coast. Series of satellite images permit synoptical views of these pattern and their temporal and spatial development.Sea surface temperature maps derived from infrared data supplied by the National Oceanic and Atmospheric Administration (NOAA) satellite-borne Advanced Very High Resolution Radiometer (AVHRR) are used to show various features like upwelling filaments and eddies.The River Oder discharges into the Pomeranian Bight is the main source of freshwater input into the western Baltic. The variability of this inflow and of the affected coastal seas is being presented in relation to the prevailing meteorological conditions.

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Dynamische Besonderheiten in der westlichen Ostsee untersucht anhand von NOAA-AVHRR-Daten

Zusammenfassung Die westliche Ostsee ist ein flaches Übergangsgebiet, das durch den Wasseraustausch zwischen Nord- und Ostsee charakterisiert ist. Die dynamischen Prozesse werden in diesem Gebiet durch Wasserstandsdifferenzen zwischen Nord- und Ostsee, durch den lokalen Wind und die Süßwasserzufuhr getrieben. Die Reaktionsmuster werden modifiziert durch die Erdrotation, die Bathymetrie und den Küstenverlauf.Serien von Satellitendaten gestatten die synoptische Betrachtung dieser Muster und ihrer zeitlichen und räumlichen Entwicklung.Karten der Wasseroberflächentemperatur (SST), abgeleitet aus Infrarotdaten des Advanced Very High Resolution Radiometer (AVHRR), werden genutzt, um verschiedene Strukturen wie Auftrieb, Filamente und Wirbel zu präsentieren. Das Radiometer wird durch die National Oceanic and Atmospheric Administration (NOAA) auf Wettersatelliten betrieben.Der Flußeintrag der Oder in die Pommersche Bucht ist die bedeutendste Süßwasserzufuhr in die westliche Ostsee. Die Veränderlichkeit des Einstroms und die beeinflußten Gebiete sind in Abhängigkeit von den meteorologischen Bedingungen dargestellt.

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Recherche du caractéristiques dynamiques en mer Baltique ouest á l'aide de données NOAA-AVHRR

Résumé La partie ouest de la mer Baltique est une zone de transition de profondeur réduite, caractérisée par un échange des eaux entre la mer du Nord et la mer Baltique.Dans cette zone, les processus dynamiques proviennent des différences de niveau de la mer du Nord et de la mer Baltique, des vents locaux et de l'apport en eau douce. Les modèles créés sont modifiés à partir de la rotation terrestre, de la bathymétrie et de la morphologie côtière.Les données satellite permettent d'établir une étude synoptique de ces processus dans l'ensemble de la zone ainsi que leur développement spatio-temporel au moyen de séries.Des cartes de température superficielle (SST) obtenues à partir des données radiométriques infrarouges AVHRR (Advanced Very High Resolution Radiometer) sont utilisées pour présenter des exemples SST caractéristiques de résultats de processus dynamiques de l'ouest de la mer Baltique. Les données radiométriques proviennent du National Oceanic and Atmospheric Administration (NOAA) à partir de satellites météorologiques.La dépendance des développements spatio-temporels de modèles en fonction des conditions météorologiques est discutée à partir de scènes et de séries particulières.Des exemples d'upwelling devant les côtes allemandes, suédoises et polonaises ainsi que des fronts, des filaments et des tourbillons sont présentés.La plus grande partie de l'apport en eau douce à l'ouest de la mer Baltique provient du fleuve Oder, en baie de Poméranie. L'influence des facteurs météorologiques sur la variation de ce déversement et sur les zones influencées est présentée.

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