Variability over time in the sources of South Portuguese Zone turbidites: evidence of denudation of different crustal blocks during the assembly of Pangaea     

M. F. Pereira  C. Ribeiro  F. Vilallonga  M. Chichorro  K. Drost  J. B. Silva  L. Albardeiro  M. Hofmann  U. Linnemann 《International Journal of Earth Sciences》2014,103(5):1453-1470

This study combines geochemical and geochronological data in order to decipher the provenance of Carboniferous turbidites from the South Portuguese Zone (SW Iberia). Major and trace elements of 25 samples of graywackes and mudstones from the Mértola (Visean), Mira (Serpukhovian), and Brejeira (Moscovian) Formations were analyzed, and 363 U-Pb ages were obtained on detrital zircons from five samples of graywackes from the Mira and Brejeira Formations using LA-ICPMS. The results indicate that turbiditic sedimentation during the Carboniferous was marked by variability in the sources, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (Mértola Formation and the base of the Mira Formation) inherited from a terrane with intermediate to mafic source rocks. These source rocks were probably formed in relation to Devonian magmatic arcs poorly influenced by sedimentary recycling, as indicated by the almost total absence of pre-Devonian zircons typical of the Gondwana and/or Laurussia basements. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (Mira and Brejeira Formations) included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism. The abundance of Precambrian and Paleozoic zircons reveals strong recycling of the Gondwana and/or Laurussia basements. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (Cadomian-Avalonian and Pan-African zircon-forming events), Paleoproterozoic, and Archean ages. The presence of late Ordovician and Silurian detrital zircons in Brejeira turbidites, which have no correspondence in the Gondwana basement of SW Iberia, indicates Laurussia as their most probable source.  相似文献   

The provenance of northern Kalahari Basin sediments and growth history of the southern Congo Craton reconstructed by U–Pb ages of zircons from recent river sands     

Andreas Gärtner  Ulf Linnemann  Mandy Hofmann 《International Journal of Earth Sciences》2014,103(2):579-595

The southern Congo Craton is widely overlain by Meso- to Cenozoic sediments of the northern Kalahari Basin, which hamper any correlation of basement units. The latter are represented by the Archaean Angola and Kasai Blocks, while the southern cratonic margin is framed by several Meso- to Neoproterozoic orogenic belts. For provenance analysis of the sedimentary cover and reconstruction of the main zircon-forming events, we studied zircons from recent sediments of the largest rivers at the southern margin of the Congo Craton. U–Pb zircon ages suggest a major amount of the sediments to originate from E Lufilian and Kibaran Belts, while input from the S Damara Belt seems to increase to the W. Ages related to the Angola Block were only noticed in the westernmost parts of the working area, which is not in accordance with the SE-trending drainage pattern, proposed to have been onset during the Cretaceous. Thus, it is assumed that the Meso- to Cenozoic sedimentary cover extended further west than today prior to the Mesozoic to Neogene uplift of the Angola Block and that subsequent erosion exhumed the basement stepwise from west to east. A recurrent destabilisation of the southern margin of the Congo Craton at ~2.7, 1.9, 1.0 and 0.6 Ga is supposed to be represented by major peaks in the age distribution pattern of the total amount of concordant zircons. This is in accordance with similar studies in adjacent areas. Additionally, the obtained data fit well to several hypothesised major events during the supercontinent cycle.  相似文献   

Neon produced by solar cosmic rays in ordinary chondrites   总被引：1，自引：0，他引：1       下载免费PDF全文

Antoine S. G. Roth  Reto Trappitsch  Knut Metzler  Beda A. Hofmann  Ingo Leya 《Meteoritics & planetary science》2017,52(6):1155-1172

Solar‐cosmic‐ray‐produced Ne (SCR‐Ne), in the form of low cosmogenic ²¹Ne/²²Ne ratios (²¹Ne/²²Ne_cos <0.8), is more likely to be found in rare meteorite classes, like Martian meteorites, than in ordinary chondrites. This may be the result of a sampling bias: SCR‐Ne is better preserved in meteorites with small preatmospheric radii and these specimens are often only studied if they belong to unusual or rare classes. We measured He and Ne isotopic concentrations and nuclear tracks in 25 small unpaired ordinary chondrites from Oman. Most chondrites have been intensively heated during atmospheric entry as evidenced by the disturbed track records, the low ³He/²¹Ne ratios, the low ⁴He concentrations, and the high peak release temperatures. Concentration depth profiles indicate significant degassing; however, the Ne isotopes are mainly undisturbed. Remarkably, six chondrites have low ²¹Ne/²²Ne_cos in the range 0.711–0.805. Using a new physical model for the calculation of SCR production rates, we show that four of the chondrites contain up to ~20% of SCR‐Ne; they are analyzed in terms of preatmospheric sizes, cosmic ray exposure ages, mass ablation losses, and orbits. We conclude that SCR‐Ne is preserved, regardless of the meteorite class, in specimens with small preatmospheric radii. Sampling bias explains the predominance of SCR‐Ne in rare meteorites, although we cannot exclude that SCR‐Ne is more common in Martian meteorites than it is in small ordinary chondrites.  相似文献   

Protracted storage of CR chondrules in a region of the disk transparent to galactic cosmic rays          下载免费PDF全文

Antoine S. G. Roth  Knut Metzler  Lukas P. Baumgartner  Beda A. Hofmann  Ingo Leya 《Meteoritics & planetary science》2017,52(10):2166-2177

Renazzo‐type carbonaceous (CR) chondrites are accretionary breccias that formed last. As such they are ideal samples to study precompaction exposures to cosmic rays. Here, we present noble gas data for 24 chondrules and 3 dark inclusion samples (DIs) from Shi?r 033 (CR2). The meteorite was selected based on the absence of implanted solar wind noble gases and an anomalous oxygen isotopic composition of the DIs; the oxygen isotopes match those in CV3 and CO3 chondrites. Our samples contain variable mixtures of galactic cosmic ray (GCR)‐produced cosmogenic noble gases and trapped noble gases of presolar origin. Remarkably, all chondrules have cosmogenic ³He and ²¹Ne concentrations up to 4.3 and 7.1 times higher than the DIs, respectively. We derived an average ³He‐²¹Ne cosmic ray exposure (CRE) age for Shi?r 033 of 2.03 ± 0.20 Ma (2 SD) and excesses in cosmogenic ³He and ²¹Ne in chondrules (relative to the DIs) in the range (in 10^?8 cm³STP/g) 3.99–7.76 and 0.94–1.71, respectively. Assuming present‐day GCR flux density, the excesses translate into average precompaction ³He‐²¹Ne CRE ages of 3.1–27.3 Ma depending on the exposure geometry. The data can be interpreted assuming a protracted storage of a single chondrule generation prior to the final assembly of the Shi?r 033 parent body in a region of the disk transparent to GCRs.  相似文献   

High resolution polarimetric measurements in a sunspot     

T. Horn  A. Hofmann  H. Balthasar 《Solar physics》1996,164(1-2):321-332

A Fabry-Perot interferometer is being used for two-dimensional spectropolarimetric measurements. We demonstrate the suitability of the setup for the measurement of the magnetic field and present some preliminary results from first observations.  相似文献   

F. Sanches,M. Collados,and M. Vazquez (eds.),solar observations: techniques and interpretation (first canary islands winter school of astrophysics), with 4 authors,Cambridge University Press, 1992, xii + 246 pp., US $64.95, ISBN 0 521 40251 4 hardback     

A. Hofmann 《Astrophysics and Space Science》1993,201(1):151-151

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Bipolar outflow on the asymptotic giant branch – the case of IRC+10011     

Dejan Vinkovi&#;  Thomas Blöcker  Karl-Heinz Hofmann  Moshe Elitzur  Gerd Weigelt 《Monthly notices of the Royal Astronomical Society》2004,352(3):852-862

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The earth system model of intermediate complexity CLIMBER-3α. Part I: description and performance for present-day conditions     

Marisa Montoya  Alexa Griesel  Anders Levermann  Juliette Mignot  Matthias Hofmann  Andrey Ganopolski  Stefan Rahmstorf 《Climate Dynamics》2006,26(2-3):327-328

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Sharp images of young stellar objects     

N. Ageorges  A. Eckart  R. Hofmann 《Astrophysics and Space Science》1995,224(1-2):411-414

We present high resolution infrared images of the pre-main sequence star V536 Aql and polarization maps of the reflection nebula IRN Cha observed with the SHARP camera on the ESO NTT. Extended structures are observed near V536 Aql. The polarization pattern of IRN Cha is typical of a centrally illuminated source but with a central band of anomalously oriented polarization believed to occur in an optically thick disk.  相似文献   

Uranium accumulation during weathering of Cañon Diablo meteoritic iron     

Beda A. Hofmann 《Meteoritics & planetary science》1992,27(1):101-103

Abstract— Cañon Diablo meteoritic iron oxide consists mainly of goethite and maghemite and contains 209 to 630 ppb uranium compared to <0.02 ppb in the unweathered octahedrite. Significant radioactive disequilibria between ²³⁸U, ²³⁴U and ²³⁰Th indicate that uranium was sorbed from soil porewater during terrestrial weathering after meteorite impact 50 ka ago. Depending on the model assumed for U uptake, corrected ²³⁰Th-²³⁴U ages of 24 to 48 ka were obtained. While the data presented may not allow an unambiguous interpretation, the potential of this approach in obtaining minimum terrestrial ages for weathered meteorites is demonstrated.  相似文献