Petrology and geochemistry of spinel peridotite xenoliths from the western Pannonian Basin (Hungary): evidence for an association between enrichment and texture in the upper mantle   总被引：9，自引：0，他引：9  

H. Downes  A. Embey-Isztin  M. F. Thirlwall 《Contributions to Mineralogy and Petrology》1992,109(3):340-354

Twenty spinel peridotite xenoliths from Pliocene alkali basaltic tuffs and lavas of the western Pannonian Basin (Hungary) have been analysed for bulk rock major and trace elements, electron probe mineral compositions, and REE and Sr, Nd isotopes on separated and leached clinopyroxenes. The xenoliths are texturally diverse, including protogranular, porphyroclastic, equigranular and poikilitic textures which can generally be correlated with geochemical features. Protogranular xenoliths are relatively undepleted in Ca, Al, Ti and Na, whereas poikilitic xenoliths are more refractory. LREE-depleted patterns. and MORB-like _Nd and _Sr values are associated with protogranular peridotites. In contrast, xenoliths with complex textures are generally LREE-enriched. Much of the isotopic variation in the suite (_Sr=–20.4 to +10.4, +_Nd=+1.8 to +13.7) can be related to interaction between protogranular mantle and melts resembling the host alkali basalts, but a third (high _Sr) component may be due to Miocene subduction beneath the region.  相似文献   

Spinel-peridotite xenoliths from Kapfenstein (Graz Basin,Eastern Austria): A geochemical and petrological study     

O. Vasellil  H. Downes  M. F. Thirlwall  R. Vannucci  N. Coradossi 《Mineralogy and Petrology》1996,57(1-2):23-50

Summary Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular and protogranular- to- porphyroclastic spinel lherzolites. In addition to these common rocks, a coarse-grained spinel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich lherzolite have been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these xenoliths to the surface is part of the Plio-Pleistocene extension-related alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism which formed the Inner Carpathian arc.Mineralogical and bulk geochemical data for Kapfenstein xenoliths are similar to those for other sub-continental shallow mantle xenolith suites. Correlation diagrams for major and trace elements indicate that their chemical variations are related to different degrees of partial melting due to the extraction of mafic melts. Rare earth element (REE) patterns of separated clinopyroxenes from the xenoliths are generally light rare earth element (LREE)-depleted, although LREE-enriched and more complex patterns have also been found.⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/.¹⁴⁴Nd isotopic ratios of separated clinopyroxenes vary between 0.70193 and 0.70330 (^Sr, = – 36.4 to –17.0) and 0.512824 and 0.513589 (_NdNd = 4.2 to 18.l), respectively. The highest⁸⁷Sr/⁸⁶Sr and lowest¹⁴⁴³Nd/¹⁴⁴Nd.Nd ratios occur in those samples which have the most refractory character and also have the most unusual REE patterns. REE determined by ion microprobe on separated interstitial amphibole grains indicate equilibrium with the host peridotite in most of the xenoliths but disequilibirum conditions have been observed, including some extreme LREE enrichments.Striking textural and some geochemical and isotope geochemical similarities have been recognised between Kapfenstein xenoliths and those from the Eastern Transylvanian Basin, both of which are situated on the edges of the diapiric upwelling located in the central part of the Pannonian Basin. The deformation, depletion and enrichment processes which occurred in these two areas are thought to be similar. The LREE-enriched interstitial amphiboles and the presence of cross-cutting amphibole veins suggest that the shallow depleted lithospheric mantle beneath the Graz Basin has been affected by metasomatising events related to the mafic alkaline host magmas rather than by subduction-related fluids or melts.

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Spinel-peridotit Xenolithe von Kapfenstein (Grazer Becken, Ostösterreich): Eine geochemische und petrologische Studie

Zusammenfassung Die ultramafischen Xenolithe aus dem Basaltuff von Kapfenstein (Grazer Becken) bestehen großteils aus protogranularem und porphyroklastischem Spinell-Lherzolith. Abgesehen von diesen weitverbreiteten Gesteinen wurden auch ein grobköringer Spinell-Harzburgit und ein heterogener Spinell-Peridotit, der aus einem Klinopyroxen-armen Harzburgit und einem Klinopyroxen-reichen Lherzolith besteht, gefunden. Interstitielle Amphibole (Pargasit und Kaersutit) sind in den meisten Xenolithen vorhanden. Die magmatischen Prozesse, die diese Xenolithe an die Erdoberfläche befördeten, stehen mit dem mit Extension assoziierten Alkali-Basalt-Vulkanismus, der während des Pliozän/ Pleistozäns dem durch Subduktion ausgelösten Kalk-alkalischen Vulkanismus des inneren Karpathenbogens folgte, in Zusammenhang.Die mineralogischen und geochemischen Vollanalysen der Kapfensteiner Xenolithe sind mit den Daten von anderen Gruppen von subkontinentalen Xenolithen des oberen Mantels vergleichbar. Korrelationsdiagramme für Haupt- und Spurenelemente zeigen, daß die chemische Variation auf verschiedene Grade von Teilschmelzbildung zurück-zuführen ist. Die Verteilungsmuster der Seltenen Erden in Klinopyroxenen der Xenolithe sind generall LREE-verarmt, aber LREE angereichte und kompliziertere Verteilungsmuster kommen vor. Die⁸⁷Sr/⁸⁶Sr und¹⁴³Nd/1⁴⁴Nd Isotopenverhältnisse der Klinopyroxene schwanken zwischen 0.70193 und 0.70330 (_Sr = von – 36.4 bis –17.0), beziechungsweise zwischen 0.512854 und 0.513589 (_Nd=von –4.2 bis 18.l). Die höchsten⁸⁷Sr/⁸⁶Sr und niedrigsten¹⁴³Nd/¹⁴⁴Nd Isotopenverhältnisse treten in den Proben mit dem stärksten refraktären Charakter und den ungewöhnlichsten REE-Verteilungsmustern auf. Die mittels Zonensonde ermittelten Gehalte an REE der interstitiellen Amphibole deuten ein Gleichgewicht mit dem Wirt-Peridotit an, obwohl auch Anzeichen von gestörtem Gleichgewicht, unter anderem extreme Anreicherungen an LREE, beobachtet wurden.Ein Vergleich der Kapfenstein-Xenolithe mit denen aus dem östlichen Transsylvanischen Becken zeigt bemerkenswerte strukturelle und einige geochemische und isotopengeochemische Ähnlichkeiten. Beide Gegenden befinden sich am Rande des Erdmantel diapirs, im Zentrum des Pannonischen Beckens. Die Deformationen und Verarmungsund Anreicherungsprozesse, die in beiden Gebieten stattfanden, sind vergleichbar. Die an LREE angereicherten interstitiellen Amphibole und das Auftreten von diskordanten Amphibolgängen legen nahe, daß der obere verarmte lithosphärische Mantel unter dem Grazer Becken von, mit den mafischen alkalischen Grund-Magmen verwandten, metasomatischen Prozessen erfaßt worden ist, und nicht von mit Subduktion assoziierten Flüssigkeiten oder Schmelzen.

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With 7 Figures  相似文献   

Small-scale variations of 87Sr/86Sr isotope composition of barite in the Madjarovo low-sulphidation epithermal system, SE Bulgaria: implications for sources of Sr, fluid fluxes and pathways of the ore-forming fluids     

Peter Marchev  Hillary Downes  Matthew F. Thirlwall  Robert Moritz 《Mineralium Deposita》2002,37(6-7):669-677

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Silica Springs,Tongariro National Park,New Zealand—analyses of the spring water and characterisation of the alumino-silicate deposit     

N. Wells  C.W. Childs  C.J. Downes 《Geochimica et cosmochimica acta》1977,41(10):1497-1506

The creamy-white deposit in the stream bed below Silica Springs outlet on Mount Ruapehu, Tongariro National Park, New Zealand, has been identified as a hydrous, X-ray-amorphous, aluminosilicate (allophane). The SiO₂/Al₂O₃ mole ratio varies from close to one, to close to two. The elements K, Ca, Mn and Fe are present in low concentrations relative to those in allophanic soil clays, and tend to increase in concentration downstream from where the deposit first occurs. The concentration of S decreases downstream from 0.5% to 0.1%. Surface areas of samples, measured by the ethylene glycol desorption method, are about 200–300 m²/g. The outlet water at Silica Springs contains low dissolved solids and is undersaturated with respect to amorphous silica, but is supersaturated with respect to several alumino-silicate minerals (of which allophane may be considered the precursor) and with respect to CO₂. Gas bubbling at the outlet contains about 10% CO₂ which has a δC¹³_PDB value of ?7.5%..Silica Springs water is derived from the addition of geothermal CO₂ (and possibly H₂S) to near-surface meteoric water from the lava flow above the outlet, and the chemical attack of this water on the andesitic rocks and soil through which it passes. The pH of water at Silica Springs increases from 5.45 at the outlet, to 5.90 where deposition first occurs, to 6.80 below the region of maximum deposition. This rise in pH correlates with loss of excess CO₂ in turbulent regions of the stream, and, through surface charge effects, is probably an important influence on the site of deposition, which begins approx. 100m downstream from the outlet.  相似文献   

Molecular gas distribution and dynamics in the luminous merger NGC 6240     

L.J. Tacconi  R. Genzel  M. Tecza  J.F. Gallimore  D. Downes  N.Z. Scoville 《Astrophysics and Space Science》1999,266(1-2):157-162

We present the results of our subarcsecond resolution interferometricobservations of the 1.3 mm CO J = 21 line in the luminous merger NGC6240. Roughly half of the CO flux is contained in a rotating and highlyturbulent thick disk centered between the two radio and near-infrarednuclei. In this disk the molecular gas has velocity widths which reachFWZP line widths of up to 1000 km s^-1. The mass of this gasconcentration makes up between 30%–70% of the dynamical mass in thisregion. NGC 6240 may be in an earlier merging stage than typical ULIRGssuch as Arp 220. We compare these results from NGC 6240 with thoseof other luminous, gas-rich interacting galaxies and mergers.  相似文献   

The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008 TC3     

Cyrena Anne Goodrich  Michael E. Zolensky  Anna Maria Fioretti  Muawia H. Shaddad  Hilary Downes  Takahiro Hiroi  Issaku Kohl  Edward D. Young  Noriko T. Kita  Victoria E. Hamilton  My E. I. Riebe  Henner Busemann  Robert J. Macke  M. Fries  D. Kent Ross  Peter Jenniskens 《Meteoritics & planetary science》2019,54(11):2769-2813

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC₃). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC₃ was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC₃. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ¹⁸O = 13.53‰, δ¹⁷O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH^? in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC₃ consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm^?3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm^?3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC₃ in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.  相似文献   

Projected changes in vertical temperature profiles for Australasia     

Ji  Fei  Evans  Jason P.  Di Virgilio  Giovanni  Nishant  Nidhi  Di Luca  Alejandro  Herold  Nicholas  Downes  Stephanie M.  Tam  Eugene  Beyer  Kathleen 《Climate Dynamics》2020,55(9-10):2453-2468

The vertical temperature profile in the atmosphere reflects a balance between radiative and convective processes and interactions with the oceanic and land surfaces. Changes in vertical temperature profiles can affect atmospheric stability, which in turn can impact various aspects of weather systems. In this study, we analyzed recent-past trends of temperature over the Australian region using a homogenized monthly upper-air temperature dataset and four reanalysis datasets (NCEP, ERA-Interim, JRA-55 and MERRA). We also used outputs of 12 historical and future regional climate model (RCM) simulations from the NSW/ACT (New South Wales/Australian Capital Territory) Regional Climate Modelling (NARCliM) project and 6 RCM simulations from the CORDEX (Coordinated Regional Downscaling Experiment) Australasian project to investigate projected changes in vertical temperature profiles. The results show that the currently observed positive trend in the troposphere and negative trend in the lower stratosphere will continue in the future with significant warming over the whole troposphere and largest over the middle to upper troposphere. The increasing temperatures are found to be latitude-dependent with clear seasonal variations, and a strong diurnal variation for the near surface layers and upper levels in tropical regions. Changes in the diurnal variability indicate that near surface layers will be less stable in the afternoon leading to conditions favoring convective systems and more stable in the early morning which is favorable for temperature inversions. The largest differences of future changes in temperature between the simulations are associated with the driving GCMs, suggesting that large-scale circulation plays a dominant role in regional atmospheric temperature change.

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Regional metamorphism and the alteration response of selected Silurian to Devonian mineral systems in the Nymagee area,Central Lachlan Orogen,New South Wales: a HyLogger™ case study     

P. M. Downes  D. B. Tilley  J. A. Fitzherbert  M. E. Clissold 《Australian Journal of Earth Sciences》2016,63(8):1027-1052

The southern Cobar Superbasin includes the Mount Hope, Cobar and Rast troughs and adjacent Winduck, Mouramba and Kopyje shelves. These stratotectonic units host a range of mineral systems within deformed upper Silurian to Lower Devonian rocks. Despite the metallogenic importance of the terrane the regional metamorphic- and alteration-related mineral assemblages have not been systematically described until now. Here, we present a study using the HyLogger? spectral scanner to systematically map changes in both background and alteration-related mineralogy for 14 mineral systems across the Nymagee area. The study found that, with the exception of the late diagenetic zone assemblages associated with the Winduck Shelf, the mineral assemblages for zones away from mineralisation are consistent with uniform sub- to lowest-greenschist facies metamorphic conditions, with no evidence for a regional, thermal field gradient across the basin. By contrast, proximal to mineralisation, there are significant changes in alteration-related mineral abundances and species reflecting elevated fluid temperatures. For several zones, including Browns Reef, Great Central, Hera, May Day, Nymagee and Shuttleton, there is a systematic change in chlorite composition from Fe- and/or Fe–Mg-chlorites to more Mg-rich varieties towards mineralisation. In addition, talc was noted for May Day, Mineral Hill and Shuttleton. The change in chlorite composition suggests that these ore-forming fluids were undersaturated with respect to iron. Furthermore, although present in discrete zones, carbonate alteration is not a dominant alteration type. However, at Manuka and Blue Mountain, the mineralisation is associated with carbonate-rich (calcite, ankerite, dolomite) units that have been dolomitised (Mg alteration) but not all dolomite-rich zones are mineralised. For Mineral Hill, there are systematic changes to mineral species/chemistry with observed data consistent with Parkers Hill being proximal to the centre of a mineralising system. The G and H Lode area (Southern Ore Zone) is slightly more distal whereas the Pearse deposit may represent a zone more distal again.  相似文献   

A three-dimensional numerical method for modelling weakly ionized plasmas     

Stephen O'Sullivan  Turlough P. Downes 《Monthly notices of the Royal Astronomical Society》2007,376(4):1648-1658

Astrophysical fluids under the influence of magnetic fields are often subjected to single- or two-fluid approximations. In the case of weakly ionized plasmas, however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and non-collisional forces. As a result, in dense molecular clouds and protostellar accretion discs, for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics.
Diffusive processes are known to restrict the stability of conventional numerical schemes which are not implicit in nature. Furthermore, recent work establishes that a large Hall term can impose an additional severe stability limit on standard explicit schemes. Following a previous paper, which presented the one-dimensional case, we describe a fully three-dimensional method which relaxes the normal restrictions on explicit schemes for multifluid processes. This is achieved by applying the little-known Super TimeStepping technique to the symmetric (ambipolar) component of the evolution operator for the magnetic field in the local plasma rest frame, and the new Hall Diffusion Scheme to the skew-symmetric (Hall) component.  相似文献   

Relativistic blast waves and synchrotron emission     

T. P. Downes †  P. Duffy  S. S. Komissarov 《Monthly notices of the Royal Astronomical Society》2002,332(1):144-154

Relativistic shocks can accelerate particles by the first-order Fermi mechanism; the particles then emit synchrotron emission in the post-shock gas. This process is of particular interest in the models used for the afterglow of gamma-ray bursts. In this paper we use recent results in the theory of particle acceleration at highly relativistic shocks to model the synchrotron emission in an evolving, inhomogeneous and highly relativistic flow. We have developed a numerical code that integrates the relativistic Euler equations for fluid dynamics with a general equation of state, together with a simple transport equation for the accelerated particles. We present tests of this code and, in addition, we use it to study the gamma-ray burst afterglow predicted by the fireball model, along with the hydrodynamics of a spherically-symmetric relativistic blast wave.
We find that, while broadly speaking the behaviour of the emission is similar to that already predicted with semi-analytic approaches, the detailed behaviour is somewhat different. The 'breaks' in the synchrotron spectrum behave differently with time, and the spectrum above the final break is harder than had previously been expected. These effects are due to the incorporation of the geometry of the (spherical) blast wave, along with relativistic beaming and adiabatic cooling of the energetic particles leading to a mix, in the observed spectrum, between recently injected 'uncooled' particles and the older 'cooled' population in different parts of the evolving, inhomogeneous flow.  相似文献