New UPb zircon ages from Tonga (Cameroon): coexisting Eburnean–Transamazonian (2.1 Ga) and Pan-African (0.6 Ga) imprints     

Evine Laure Tanko Njiosseu  Jean-Paul Nzenti  Théophile Njanko  Badibanga Kapajika  Anne Nédélec 《Comptes Rendus Geoscience》2005,337(6):551-562

The central domain of the Pan-African Belt in Cameroon is characterized by abundant porphyritic granitoids, which were emplaced synkinematically and variably orthogneissified in relation with ENE-striking steeply dipping transcurrent shear zones. These plutonic rocks have intermediate to felsic compositions and constitute a high-K calk-alkaline series. Conventional UPb zircon dating yields an age of 618 Ma for this syntectonic Pan-African magmatism in the Tonga area. The country rocks are made of metabasites (garnet amphibolites) and tonalitic to trondhjemitic gneisses, which suffered two distinct orogenic cycles: the first one is the Palaeoproterozoic Eburnean–Transamazonian cycle at 2.1 Ga and the second one is the Pan-African orogenesis. These new ages confirm the existence of an extensive Palaeoproterozoic crust in Cameroon and question the areal extent of the Congo–São Francisco craton towards the north. To cite this article: E.L. Tanko Njiosseu et al., C. R. Geoscience 337 (2005).  相似文献   

Preface     

H. W. Dosso  J. T. Weaver 《Surveys in Geophysics》1983,6(1-2):1-3

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Linear and non-linear appraisal using extremal models of bounded variation     

S. E. Dosso  D. W. Oldenburg 《Geophysical Journal International》1989,99(3):483-495

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An analogue model for studying magnetic variations induced by ocean waves     

T. Miles  H.W. Dosso  T.P. Ng 《Physics of the Earth and Planetary Interiors》1977,14(2):137-142

The scale factors to permit a laboratory analogue model study of the problem of magnetic fields induced by ocean waves in the earth's field are derived. An analogue model employing surface fluid waves in mercury to simulate ocean waves is described. In the analogue model, magnetic field measurements were made 1 cm above a 2 cm deep model mercury ocean for a wave period of 0.21 s. This model simulates measurements 38 m above the surface of a shallow ocean 78 m deep for a wave period of 13 s. The validity of the analogue modelling technique is established by the good agreement obtained in comparing the analogue model measurements of the induced magnetic fields with fields using Podney's expression for an ocean of finite depth.  相似文献   

Comments on the “numerical and analogue model results for electromagnetic induction for an island situated near a coastline”     

V. Ramaswamy  W. Nienaber  H.W. Dosso  F.W. Jones  L.K. Law 《Physics of the Earth and Planetary Interiors》1977,14(4):P28-P30

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Post-depositional realignment of magnetic grains and asymmetrical saw-tooth patterns of magnetization intensity     

Laure Meynadier  Jean-Pierre Valet 《Earth and Planetary Science Letters》1996,140(1-4):123-132

Recent paleomagnetic records suggest that the geomagnetic field intensity has a saw-tooth shape matching the succession of polarity intervals. An alternative hypothesis, that mechanisms linked to the acquisition of magnetization would induce similar saw-toothed records, has been tested by several simulations of post depositional reorientations of magnetic grains. Exponential functions used to date to describe post-depositional magnetization (pDRM) processes do not account for the combination of saw-toothed fluctuations and reasonable delays in the recording of the position of the reversals. At least half of the magnetization must be locked in within a few centimetres below the surface. If not, large delays, which are not observed in the data, are introduced in the stratigraphic positions of the reversals. In addition, the rest of the magnetization must be acquired over depths involving several tens of meters to duplicate a saw-toothed shape. These conclusions are reached with or without incorporating intensity variations across reversals. If the original signal is, in fact, asymmetrical then the pDRM must be very limited to remain coherent with the measurements, since the distortions and the offsets induced by the pDRM smoothing are considerably amplified. We conclude that simulations of saw-tooth patterns of relative paleointensity by pDRM processes have consequences that are difficult to reconcile with our present knowledge of the physical properties of deep-sea sediments. Above all, the hypothesis that the saw-tooth is an artifact of the magnetization acquisition process would have major implications for any sedimentary record of geomagnetic features.  相似文献   

Isolation and characterization of Aeromonas species from an eutrophic tropical estuary     

Kouassi Aka Marcel  Adingra Ama Antoinette  Dosso Mireille 《Marine pollution bulletin》2002,44(12)

Isolation and characterization of Aeromonas species were undertaken in the Ebrié lagoon (Abidjan, Côte d’Ivoire) over a one year period. Overall 63% of 501 water samples were identified as positive for Aeromonas spp. A. sobria, A. hydrophila and A. caviae represent respectively 49.20%, 20.63% and 30.15% of the positive samples. Thirty nine of the positive samples were recovered from surface water samples; while 24 bottom water samples were positive for Aeromonas. Aeromonas species are most frequently (82.53) present in the Ebrié lagoon urban area during the rainy and flood seasons when the salinity is low and below 10‰. A lower isolation frequency is noticed during the dry season when water salinity is over 10‰. Finally, a higher isolation frequency occurred in samples with highest counts of Escherichia coli.  相似文献   

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere   总被引：2，自引：0，他引：2  

Flurin Vils  Laure Pelletier  Othmar Müntener 《Geochimica et cosmochimica acta》2008,72(22):5475-5504

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen-Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44-2.8 μg g⁻¹) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 μg g⁻¹), depleted in Li (most values below 1 μg g⁻¹) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4-65.0 μg g⁻¹), H₂O and Cl contents and to lower Li contents (0.07-3.37 μg g⁻¹) of peridotites, implying that—contrary to alteration of oceanic crust—B is fractionated from Li and that the B and Li inventory should depend essentially on rock-water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17-307 kg in a column of 1 m × 1 m × thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25-904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.  相似文献   

Geochemical and isotopic signatures of magmatic products in the MAR rift valley at 12°49′–17°23′ N and 29°59′–33°41′ N: Evidence of Two contrasting sources of the parental melts     

S. A. Silantyev  L. V. Danyushevsky  A. A. Plechova  L. Dosso  B. A. Bazylev  V. E. Bel’tenev 《Petrology》2008,16(1):36-62

Data presented in the paper suggest significant differences between the thermodynamic conditions under which magmatic complexes were formed in MAR at 29°–34° N and 12°–18° N. The melts occurring at 29°–34° N were derived by the melting of a mantle source with a homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their rapid ascent. The MAR segments between 12° and 18° N combine contrasting geodynamic environments of magmatism, which predetermined the development of a large plume region with the widespread mixing of the melting products of geochemically distinct mantle sources. At the same time, this region is characterized by conditions favorable for the origin of localized zones of anomalous plume magmatism. These sporadic magmatic sources were spatially restricted to MAR fragments with the Hess crust, whose compositional and mechanical properties were, perhaps, favorable for the focusing and localization of plume magmatism. The plume source between 12° and 18°N beneath MAR may be geochemically heterogeneous.  相似文献   

Geochemistry of the Hollister Ridge: relation with the Louisville hotspot and the Pacific–Antarctic Ridge     

I. Vlastelic  L. Dosso  H. Guillou  H. Bougault  L. Geli  J. Etoubleau  J. L. Joron 《Earth and Planetary Science Letters》1998,160(3-4):777-793

The Hollister Ridge is located on the western flank of the Pacific–Antarctic Ridge (PAR), between the Udintsev fracture zone (FZ) and the Eltanin fault system. It is a linear aseismic structure, 450 km long, oblique with respect to the PAR. Data show that the most recent activity is located in the central part of the chain, which can be considered as being still volcanically active. Both major/trace element and isotopic data suggest that some interaction occurred between the Pacific–Antarctic Ridge and the Hollister Ridge. The source of the Hollister Ridge samples has its own geochemical characteristics. The geochemical variations observed along the ridge can be explained by mixing between two major end-member components: (1) a PAR depleted source, and (2) a Hollister enriched source. A small contribution (20% maximum) of Louisville plume material is likely to exist in the middle of Hollister Ridge. These data unequivocally reject the possibility that the Hollister Ridge could be the present location of the Louisville hotspot. Ages and geochemistry data support the idea of an influence of intraplate deformation as a probable cause of the origin of the Hollister Ridge.  相似文献