Single crystal U-Pb zircon age and Sr-Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites     

Ferrière L  Koeberl C  Thöni M  Liang C 《Chemical Geology》2010,275(3-4):254-261

The 1.07?Myr old Bosumtwi impact structure (Ghana), excavated in 2.1-2.2?Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U-Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145?±?82?Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb-Sr and Sm-Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb-Sr and Sm-Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n?=?29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846?±?160?Ma, with an initial Sr isotope ratio of 0.703?±?0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible "metamorphic age" of ～?1.8-1.9?Ga tentatively derived from our U-Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb-Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.  相似文献   

Geochemistry and geochronology of HP mélanges from Tinos and Andros,cycladic blueschist belt,Greece     

Florian Bulle  Michael Bröcker  Claudia Gärtner  Alan Keasling 《Lithos》2010,117(1-4):61-81

U–Pb zircon geochronology, Sr–Nd isotope and bulk-rock geochemistry have been applied to meta-igneous and meta-sedimentary rocks from high-pressure metamorphic mélanges exposed on the Cycladic islands of Tinos, Syros and Andros. Ion microprobe (SHRIMP) U–Pb zircon dating of 7 samples representing meta-igneous blocks (Tinos), a blackwall zone (Tinos) and chlorite–talc schists from block-matrix contacts (Syros and Tinos) yielded Cretaceous ages of c. 80 Ma. Many of the criteria commonly used to distinguish between magmatic or metamorphic zircon genesis (internal structure, Th/U ratio, REE characteristics, Ti-in zircon thermometry, enclosed mineral phases) do not provide unambiguous constraints for the mode of formation. However, a magmatic origin for Cretaceous zircon of meta-gabbros and eclogites is considered likely. Supporting evidence for a previously suggested metamorphic origin for c. 80 Ma zircon in eclogite has not been found. Zircon of the same age occurring in chlorite–talc schists is presumably related to non-magmatic processes. Well-defined Cretaceous age groups clustering at c. 79 Ma also occur in the detrital zircon populations of 2 quartz mica schists representing the mélange matrix on Tinos, and suggest a much later time for sediment accumulation than previously assumed. The importance of c. 57 Ma zircon ages remains unclear, but may record either HP metamorphic processes or a post-57 Ma depositional age. The youngest age group in a third quartz mica schist from Tinos, collected outside the main mélange occurrences, clusters at c. 226–238 Ma. In all clastic metasediments from Tinos, most data points plot along the concordia between c. 300 and 900 Ma; single data points indicate concordant ages of c. 2.5 Ga, 2.3 Ga and 1 Ga, respectively. The youngest ²⁰⁶Pb/²³⁸U age group that has been recognized in a felsic paragneiss from Andros indicates an age of 163.1 ± 3.9 Ma, and mostly represents overgrowths around zircon with ages in the range from ～ 272 to ～ 289 Ma. Single data points of other inherited cores provided ²⁰⁶Pb/²³⁸U ages of c. 630 and c. 930 Ma. Meta-gabbros from Tinos show a large compositional variability and were found at 4 locations, each with distinct compositional characteristics, suggesting different crystallization histories, different sources and/or significant post-magmatic disturbance. The geochemistry of mélange blocks and the identical U–Pb zircon ages suggest that the block-matrix associations on Tinos and Syros can be grouped together. On a broader regional scale, there seem to be similarities between some meta-igneous rocks from Tinos and Evvia. Field relationships indicate that the mélanges occurring in southern Andros and northern Tinos can be correlated, but supporting geochemical and/or geochronological evidence for this interpretation could not be established. Previously published Jurassic ages for mafic and felsic mélange blocks from Andros suggest a genetic relationship to the ophiolite occurrences exposed in the larger Balkan region. A similar regional correlation is also considered likely for the Cretaceous meta-gabbros from Tinos and Syros, but cannot be documented with certainty.  相似文献   

Isotopic studies of Pb–Zn–(Ag) and barite Alpine vein deposits in the Iberian Range (NE Spain)     

I. Subías  I. Fanlo  E. Mateo  K. Billström  C. Recio 《Chemie der Erde / Geochemistry》2010,70(2):149-158

The Iberian Range contains abundant Pb–Zn–(Ag)– and Ba-dominated low temperature veins, mostly formed during the Mesozoic. The hydrothermal activity was related to the extensional regime occurring throughout the Variscan basement of Europe, North Africa and the Appalachians. A stable and radiogenic isotopic study of these veins has identified the reservoir/s of mineralizing components involved in the ore-forming processes. Sulphur isotope ratios indicate that the source of mineralizing components for the base metals veins was a mixture of the Palaeozoic country rocks and Variscan ore deposits. In contrast, either Mesozoic seawater or evaporites supplied most of the sulphur for Ba veins, although a minor contribution of isotopically heavy sulphur derived from the basement is envisaged. The lead isotopic data of galenas define a linear trend in the thorogenic diagram, interpreted in terms of mixing of lead from different reservoirs. The main source is related to local sedimentary country rocks, but a minor contribution from igneous rocks cannot be ruled out. Sr isotope ratios of barites also suggest that most of the components were leached from the basement sequence. When the veins are hosted by carbonate rocks, however, a local source of metals is also available.  相似文献   

Gd-DTPA in the hydrosphere: Kinetics of transmetallation by ions of rare earth elements,Y and Cu     

Peter Möller  Peter Dulski 《Chemie der Erde / Geochemistry》2010,70(2):125-136

The kinetics of transmetallation of Gd³⁺ in diethylenetriaminpentaacetic acid (Gd-DTPA) by rare earth elements (except Yb), Y³⁺ and Cu²⁺ has been studied in test solutions ranging from pH 5 to 6.5 in order to understand the distribution and longevity of their chelates in the hydrosphere. Chelated and non-chelated species were separated by adsorption of the free ions by clay minerals before analysis by ICP-MS. This simple method allows determining the distribution of DTPA chelates at concentration levels as low as a few 10 pmol/l. The transmetallation constants at ionic strength of 0.01 mol/l and room temperature are commensurable with those derived from published formation constants of the corresponding DTPA complexes. It could be proved that DTPA chelates of rare earth elements, Y and Cu are stable at least over 6 months. The pattern of the second-order rate constants shows a strong tetrad effect, which is not displayed by the pattern of transmetallation constants. Based on these results, the rate of transmetallation of Gd-DTPA under natural conditions is derived. The resultant process yields 10% turn-over of initial Gd-DTPA within two months, if Cu²⁺ concentrations are below 1 nmol/l. At higher concentrations, the turn-over increases with elapsed time. Equilibrium among REE and Y is expected only after tens of years. This makes Gd-DTPA a powerful tracer in hydrological studies.  相似文献   

Mafic Late Miocene–Quaternary volcanic rocks in the Kamchatka back arc region: implications for subduction geometry and slab history at the Pacific–Aleutian junction     

Anna O. Volynets  Tatiana G. Churikova  Gerhard Wörner  Boris N. Gordeychik  Paul Layer 《Contributions to Mineralogy and Petrology》2010,159(5):659-687

New ⁴⁰Ar/³⁹Ar and published ¹⁴C ages constrain voluminous mafic volcanism of the Kamchatka back-arc to Miocene (3–6 Ma) and Late Pleistocene to Holocene (<1 Ma) times. Trace elements and isotopic compositions show that older rocks derived from a depleted mantle through subduction fluid-flux melting (>20%). Younger rocks form in a back arc by lower melting degrees involving enriched mantle components. The arc front and Central Kamchatka Depression are also underlain by plateau lavas and shield volcanoes of Late Pleistocene age. The focus of these voluminous eruptions thus migrated in time and may be the result of a high fluid flux in a setting where the Emperor seamount subducts and the slab steepens during rollback during terrain accretions. The northern termination of Holocene volcanism locates the edge of the subducting Pacific plate below Kamchatka, a “slab-edge-effect” is not observed in the back arc region.  相似文献   

Coma Structures in Comet 73P/Schwassmann-Wachmann 3, Components B and C,Between January and May 2006     

Jean-Baptiste Vincent  Hermann Böhnhardt  Ivano Bertini  Luisa-Maria Lara  Michael Küppers  Rafael Rodrigo 《Earth, Moon, and Planets》2010,106(1):27-35

The Jupiter family comet 73P/Schwassmann-Wachmann 3 has been widely observed since 1995 after a nucleus break-up event produced at least five components labeled 73P-A to E. During the 2006 appearance, two of them (B and C) showed very strong coma activity. Our R-filter imaging of 73P-B & C from 21 January to 25 May 2006 revealed the presence of fan-like structures in the comae of both components and evidence for further fragmentation events in component B. As of early April 2006, component C showed two jets emanating from the nucleus, with one continuously visible. Through a simulation of the orbital geometry we infer that the rotation axis of 73P-C has an inclination of 20° to the orbital plane and a longitude of 45° at perihelion. The coma activity of component B was highly variable, displaying signatures of at least 3 fragmentation events. The coma was characterized by the continuous presence of a jet roughly in sunward direction, starting from the beginning of May. The first fragmentation event of component B may have happened between April 16 and April 26, leading to the presence of at least 6 fragments detected in images of May 2. The second one happened on or shortly before May 8, the third one between May 18 and 24. For the rotation axis of 73P-B we infer an inclination of 5°–15° to the orbital plane and a longitude of 20°–30° at perihelion.  相似文献   

The quest for the solar g modes     

T. Appourchaux  K. Belkacem  A.-M. Broomhall  W. J. Chaplin  D. O. Gough  G. Houdek  J. Provost  F. Baudin  P. Boumier  Y. Elsworth  R. A. García  B. N. Andersen  W. Finsterle  C. Fröhlich  A. Gabriel  G. Grec  A. Jiménez  A. Kosovichev  T. Sekii  T. Toutain  S. Turck-Chièze 《Astronomy and Astrophysics Review》2010,18(1-2):197-277

Solar gravity modes (or g modes)—oscillations of the solar interior on which buoyancy acts as the restoring force—have the potential to provide unprecedented inference on the structure and dynamics of the solar core, inference that is not possible with the well-observed acoustic modes (or p modes). The relative high amplitude of the g-mode eigenfunctions in the core and the evanesence of the modes in the convection zone make the modes particularly sensitive to the physical and dynamical conditions in the core. Owing to the existence of the convection zone, the g modes have very low amplitudes at photospheric levels, which makes the modes extremely hard to detect. In this article, we review the current state of play regarding attempts to detect g modes. We review the theory of g modes, including theoretical estimation of the g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the techniques that have been used to try to detect g modes. We review results in the literature, and finish by looking to the future, and the potential advances that can be made—from both data and data-analysis perspectives—to give unambiguous detections of individual g modes. The review ends by concluding that, at the time of writing, there is indeed a consensus amongst the authors that there is currently no undisputed detection of solar g modes.  相似文献   

Explicit algorithmic regularization in the few-body problem for velocity-dependent perturbations     

Christian Hellström  Seppo Mikkola 《Celestial Mechanics and Dynamical Astronomy》2010,106(2):143-156

A new algorithm is presented for the numerical integration of second-order ordinary differential equations with perturbations that depend on the first derivative of the dependent variables with respect to the independent variable; it is especially designed for few-body problems with velocity-dependent perturbations. The algorithm can be used within extrapolation methods for enhanced accuracy, and it is fully explicit, which makes it a competitive alternative to standard discretization methods.  相似文献   

Kamarizaite,Fe<Stack><Subscript>3</Subscript><Superscript>3+</Superscript></Stack>(AsO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>3</Subscript> · 3H<Subscript>2</Subscript>O,a new mineral species,arsenate analogue of tinticite     

N. V. Chukanov  I. V. Pekov  S. Möckel  A. A. Mukhanova  D. I. Belakovsky  L. A. Levitskaya  G. K. Bekenova 《Geology of Ore Deposits》2010,52(7):599-605

Kamarizaite, a new mineral species, has been identified in the dump of the Kamariza Mine, Lavrion mining district, Attica Region, Greece, in association with goethite, scorodite, and jarosite. It was named after type locality. Kamarizaite occurs as fine-grained monomineralic aggregates (up to 3 cm across) composed of platy crystals up to 1 μm in size and submicron kidney-shaped segregations. The new mineral is yellow to beige, with light yellow streak. The Mohs hardness is about 3. No cleavage is observed. The density measured by hydrostatic weighing is 3.16(1) g/cm³, and the calculated density is 3.12 g/cm³. The wavenumbers of absorption bands in the IR spectrum of kamarizaite are (cm^?1; s is strong band, w is weak band): 3552, 3315s, 3115, 1650w, 1620w, 1089, 911s, 888s, 870, 835s, 808s, 614w, 540, 500, 478, 429. According to TG and IR data, complete dehydration and dehydroxylation in vacuum (with a weight loss of 15.3(1)%) occurs in the temperature range 110–420°C. Mössbauer data indicate that all iron in kamarizaite is octahedrally coordinated Fe³⁺. Kamarizaite is optically biaxial, positive: n _min = 1.825, n _max = 1.835, n _mean = 1.83(1) (for a fine-grained aggregate). The chemical composition of kamarizaite (electron microprobe, average of four point analyses) is as follows, wt %: 0.35 CaO, 41.78 Fe₂O₃, 39.89 As₂O₅, 1.49 SO₃, 15.3 H₂O (from TG data); the total is 98.81. The empirical formula calculated on the basis of (AsO₄,SO₄)₂ is Ca_0.03Fe _2.86 ³⁺ (AsO₄)_1.90(SO₄)_0.10(OH)_2.74 · 3.27H₂O. The idealized formula is Fe ₃ ³⁺ (AsO₄)₂(OH)₃ · 3H₂O. Kamarizaite is an arsenate analogue of orthorhombic tinticite, space group Pccm, Pcc2, Pcmm, Pcm2₁, or Pc2m; a = 21.32(1), b = 13.666(6), c =15.80(1) Å, V= 4603.29(5) ų, Z= 16. The strongest reflections of the X-ray powder diffraction pattern [\(\bar d\), Å (I, %) (hkl)] are: 6.61 (37) (112, 120), 5.85 (52) (311), 3.947 (100) (004, 032, 511), 3.396 (37) (133, 431), 3.332 (60) (314), 3.085 (58) (621, 414, 324). The type material of kamarizaite is deposited in the Mineralogical Collection of Technische Universität Bergakademie Freiberg, Germany, inventory number 82199.  相似文献   

A new method in palaeoecology: fish community structure indicates environmental changes     

Ulrich Schmölcke  Kenneth Ritchie 《International Journal of Earth Sciences》2010,99(8):1763-1772

A new method to reconstruct aquatic palaeoenvironments is presented. It is based on a non-metrical ‘fish environment reconstruction index’ (FERI), calculated for the total fish community recorded at an archaeological site. As an example, a FERI is generated for the Baltic Sea using the ecological requirements of northern European fish species. The present study evaluates the proposed method by using fish bone assemblages from a region (the middle Holocene Baltic Sea coast) with well-studied hydrographic history. The bones originate from consecutive human riparian and coastal settlements of hunter-gatherers. The results obtained for the parameters salinity and sediment structure correlate well with geological knowledge. The new method shows a successive change from freshwater to brackish and finally to nearly marine conditions before, during, and towards the end of the marine transgression that created the present Baltic Sea. Additionally, a shift in the sediment structure from muddy to sandy/rocky conditions is recognisable.  相似文献