38 research outputs found
Geochemické studium procesů, relevantních pro hodnocení bezpečnosti hlubinného úložistě radioaktivních odpadů: Formy uranu a scénáře jeho retence v prostředí sedimentárních hornin na lokalitě přírodního analogu Ruprechtov
U migration was studied in order to better understand to processes of safety relevant elements, particularly U. The rock sequence on the site can be analogous to a potential rock overburden of deep geology repositories. A multidisciplinary approach was undertaken in order to identify and characterise U mobilisation/immobilisation processes within sedimentary clayey rocks with organic matter enriched interlayers. Both conventional methods and modern sophisticated spectroscopic methods were combined. Sequential extraction, wet chemical method and even spectroscopic methods proved that U prevailed in the U(IV) form in low-oxidised samples. It moved towards more easily releasable fractions with sample ageing (oxidation). The combination of SE, µ-XRF and µ-XAFS results proved U to be unexpectedly associated with As and P, leading to the presumption that U(VI) from groundwater was reduced to U(IV) on As pyrite. The evaluation of the hydrogeochemical conditions and isotope analyses then brought the results into broader context: Sedimentary organic matter within the sedimentary layers was microbially oxidised, releasing dissolved organic matter and providing H+ in order to dissolve sedimentary inorganic carbonates. SO4 2- could be reduced under reducing groundwater conditions, thus causing FeS2 formation....i Abstrakt Studium migrace U v přírodním prostředí představuje významný příspěvek pro poznání procesů významných pro hodnocení bezpečnosti hlubinného úložiště jaderného odpadu. V případě přírodního analogu Ruprechtov lze sedimentární horniny považovat za analogii pokryvných útvarů nad hostitelskými masivy hlubinného úložiště. K identifikaci a charakterizaci procesů mobilizace a imobilizace U v prostředí jílových hornin s obsahem organické hmoty byl využit multidisciplinární přístup. Sekvenční extrakce, chemická analýza na mokré cestě i spektroskopické metody prokázaly, že v málo oxidovaných vzorcích se U vyskytuje převážně ve formě U(IV). Při oxidaci vzorku s časem se U přesunuje směrem ke snáze uvolnitelným formám. Současně metody prokázaly asociaci U s As a P. Tento výsledek vedl k teorii, že se U(VI) z podzemní vody redukoval na U(IV) na povrchu pyritu pokrytém precipitovaným As. Analýza hydrogeologických, hydrogeochemických dat a analýzy stabilních izotopů pomohly zařadit výše uvedené výsledky do obecného konceptu imobilizace U: Oxidace organické hmoty probíhá za mikrobiální katalýzy a vytváří redukční prostředí v blízkosti vrstev bohatých oganickou hmotou. Oxidovatelné látky (např. SO4 2- ) v podzemní vodě jsou redukovány, čímž je umožněn vznik pyritu FeS2. Na jeho povrchu se sráží povlak As, na jehož...Institute of Geochemistry, Mineralogy and Mineral ResourcesÚstav geochemie, mineralogie a nerostných zdrojůFaculty of SciencePřírodovědecká fakult
Tracing water masses and terrestrial inputs with radiogenic neodymium and hafnium isotopes and rare earth elements in the southeastern Atlantic Ocean
The southeastern Atlantic Ocean is a key region to investigate large-scale ocean circulation as the water masses passing this region comprise important parts of the return flow of the Atlantic Meridional Overturning Circulation (AMOC). This doctoral thesis presents the first detailed investigation of water mass mixing processes and the distribution and fluxes of trace elements in the Angola and Cape Basins based on dissolved radiogenic neodymium (εNd) and hafnium (εHf) isotopes as well as rare earth element (REE) concentrations.
Near surface water εNd signatures reaching −17 in the uppermost 200 m of the Angola Basin are mainly a consequence of the admixture of an unradiogenic coastal plume originating from the dissolution of Fe-Mn coatings of particles in the oxygen minimum zone. In contrast, εNd signatures of up to -17.6 in the upper water column of the northern Cape Basin are a consequence of advection of shallow waters via the Agulhas current, which originates from the Mozambique Channel. The Nd isotope compositions of the deep water masses in both basins primarily reflect conservative water mass mixing, while Nd isotope signatures of deep and bottom waters of the central Angola Basin are significantly overprinted by terrestrial inputs. Bottom waters of the Cape Basin show excess Nd concentrations likely originating from resuspended bottom sediments and/or dissolution of dust, but without changing the Nd isotopic composition of the bottom waters significantly due to similar εNd values of particles and seawater.
The Congo River is the second largest river by discharge in the world and its plume carries extraordinarily high Nd and Hf concentrations of up to 4000 pmol/kg and 54 pmol/kg into the northern Angola Basin. Its freshwater is characterized by εNd and εHf values ranging between -15.6 and -16.4 and between 0.35 and -1.4, respectively. Rapid scavenging and removal of light rare earth elements (LREEs) and middle rare earth elements (MREEs) by coagulation processes form typical seawater REE patterns at salinities between 0 and 23. However, particle-seawater interactions in the low salinity zone may also result in Nd release from or exchange with Congo-derived particulate phases, as is indicated by elevated Congo-shelf-zone REE and Hf fluxes. Yet, this process is not reflected by changes in Nd and Hf isotopic compositions given that the latter are identical in the dissolved and particulate pools of the Congo River. Having passed the estuary, the Nd and Hf concentrations and isotopic signatures in the Congo freshwater plume are mixed conservatively for up to 1000 km northwest of the river mouth. Intermediate and deep waters below the plume and in the open northern Angola Basin are strongly affected by inputs from the Congo River resulting in less radiogenic signatures compared to what is expected from water mass mixing only.
Surface waters of the Angolan coast near the Angola Benguela Front (ABF) are characterized by unradiogenic εNd signatures of up to -21 and elevated REE concentrations (35 pmol/kg for Nd), which are likely caused by dissolution of Fe-Mn oxide coatings of coastal sediment particles in the prevailing oxygen minimum zone. Decreasing REE concentrations in intermediate and deep waters suggest removal via scavenging due to high vertical particle fluxes, while Nd signatures of about -15 indicate release of unradiogenic Nd. Surface waters off the Namibian coast are more radiogenic (εNd -12) and have lower REE concentrations (12 pmol/kg for Nd) suggesting minor terrestrial inputs from the Namib and Kalahari or the Orange River. Waters off the Namibian coast are strongly influenced by upwelling and resuspension of particles from shelf sediments into the oxygen minimum zone, where REEs and distinct Nd isotope signatures are released and advected into the surface waters.
The investigations support the application of Nd isotopes as a quasi-conservative tracer of present and past deep water mass mixing in the southeastern Atlantic Ocean. Nd isotopes and REE concentrations trace advection of near surface water masses with distinct isotope composition such as the Agulhas current but also serve to monitor non-conservative terrestrial inputs. In coastal areas, Nd isotopes can be used to examine seawater-particle exchange processes and in combination with Hf isotope compositions and element distributions serve to quantify riverine fluxes of elements to the open ocean, such as across the Congo estuary. Overall, this study demonstrates that combined REE concentrations and Nd and Hf isotopes help to better understand the mixing and advection of the water masses of the AMOC in the SE Atlantic Ocean and the importance of local terrestrial inputs, which has implications for the application of these geochemical tools in other parts of the world ocean and for the reconstruction of past ocean circulation
The petrogenesis of the Loch Scridain xenolithic sill complex, Isle of Mull
Intruded into the Palaeocene lava field and underlying Moine (Neoproterozoic) crystalline basement rocks around Loch Scridain, Isle of Mull, Scotland, is a suite of high-level, inclined, xenolithic sheets, ranging in composition from basalt, through andesite and dacite, to rhyolite. These sheets, associated with the Mull central volcano, were emplaced at approximately 58Ma. Three distinct (magma) groups are recognised. Group I consists of fine-grained, aphyric basalts and basaltic andesites, the most primitive of which has MORB-like chemical affinities. Group II predominantly comprises aphyric two-pyroxene andesites, and glassy plagioclase- and pyroxene-phyric dacites (pitchstones). Group III consists solely of fine-grained rhyolites.
Many of the sheets are composite. The phase relationships and major-element geochemistry suggest that many of the Group I specimens evolved via fractional crystallization of olivine + palgioclase + augite, followed by plagioclase + augite + low-Ca pyroxene, from the more primitive samples. Evidence for this fractionation scheme is supported by the occurrence of gabbroic cumulate xenoliths in many of the Group I and Group II sheets. Group II evolved via the mixing between basic magmas and crustally derived silicic melts, followed by the fractionation of plagioclase + pigeonite. However, Sr-Nd isotope analyses and trace-element characteristics, in particular the LREE enrichment and Nb depletion of all but the least evolved samples, suggest that both Groups I and II contain significant amounts of crustally derived material. Strontium initial ratios correlate positively with degree of fractionation throughout Groups I and II, with the least evolved members of Group I (Mg# ˜ 0.3) having (87Sr/86Sr)58 of around 0.7037. More evolved members of Group I and the dacites of Group II have (87Sr/86Sr)58 in excess of 0.7167.
Phenocryst population evidence and phase relationships suggest that some Group I samples are the result of mixing between evolved and more basic magmas, implying that the magma-storage reservoir for the Loch Scridain Sill Complex (LSSC) was continually being replenished with fresh batches of basic magma
Workshop on the Martian Surface and Atmosphere Through Time
The purpose of the workshop was to bring together the Mars Surface and Atmosphere Through Time (MSATT) Community and interested researchers to begin to explore the interdisciplinary nature of, and to determine the relationships between, various aspects of Mars science that involve the geological and chemical evolution of its surface, the structure and dynamics of its atmosphere, interactions between the surface and atmosphere, and the present and past states of its volatile endowment and climate system
The Twenty-Fifth Lunar and Planetary Science Conference. Part 2: H-O
Various papers on lunar and planetary science are presented, covering such topics as: planetary geology, lunar geology, meteorites, shock loads, cometary collisions, planetary mapping, planetary atmospheres, chondrites, chondrules, planetary surfaces, impact craters, lava flow, achondrites, geochemistry, stratigraphy, micrometeorites, tectonics, mineralogy, petrology, geomorphology, and volcanology
Papers presented to the International Colloquium on Venus
This volume contains short papers that have been accepted for the International Colloquium on Venus, August 10-12, Pasadena, California. The Program Committee consisted of Stephen Saunders (Jet Propulsion Laboratory) and Sean C. Solomon (Massachusetts Institute of Technology). Chairmen: Raymond Arvison (Washington University); Vassily Moroz (Institute for Space Research); Donald B. Campbell (Cornell University); Thomas Donahue (University of Michigan); James W. Head III (Brown University); Pamela Jones (Lunar and Planetary Institute); Mona Jasnow, Andrew Morrison, Timothy Pardker, Jeffrey Plaut, Ellen Stofan, Tommy Thompson, Cathy Weitz (Jet Propulsion Laboratory); Gordon Pettengil (Massachusetts Institute of Technology); and Janet Luhmann (University of California, Los Angeles)
Workshop on Pristine Highlands Rocks and the early History of the Moon
Oxide composition of the Moon, evidence for an initially totally molten Moon, geophysical contraints on lunar composition, random sampling of a layered intrusion, lunar highland rocks, early evolution of the Moon, mineralogy and petrology of the pristine rocks, relationship of the pristine nonmore rocks to the highlands soils and breccias, ferroan anorthositic norite, early lunar igneous history, compositional variation in ferroan anosthosites, a lunar magma ocean, deposits of lunar pristine rocks, lunar and planetary compositions and early fractionation in the solar nebula, Moon composition models, petrogenesis in a Moon with a chondritic refractory lithophile pattern, a terrestrial analog of lunar ilmenite bearing camulates, and the lunar magma ocean are summarized
Twenty-Fourth Lunar and Planetary Science Conference. Part 3: N-Z
Papers from the conference are presented, and the topics covered include the following: planetary geology, meteorites, planetary composition, meteoritic composition, planetary craters, lunar craters, meteorite craters, petrology, petrography, volcanology, planetary crusts, geochronology, geomorphism, mineralogy, lithology, planetary atmospheres, impact melts, K-T Boundary Layer, volcanoes, planetary evolution, tectonics, planetary mapping, asteroids, comets, lunar soil, lunar rocks, lunar geology, metamorphism, chemical composition, meteorite craters, planetary mantles, and space exploration