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1.
his study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U1, U2 and U3. Interpretation of these features indicates that U1 consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U2, a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith- and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under “dry” and “wet” conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U2 and U3, would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar’s polygenetic origin.  相似文献   
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Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts. An erratum to this article can be found at  相似文献   
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5.
Airphotos have been used for topographic survey since the Twenties. The “European Space Agency” (ESA), the “Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt” (DFVLR) and “Carl Zeiss” have modified this successful technique for spacecraft and recently have tested the system over four continents. Each of these satellite photos covers a region of 189×189 km to the scale of 1∶800000 and may be enlarged to 1∶250000 or 1∶100000. Between the altitudes of 200 and 300 km, mapmaking or-revision seems to be economic in zones or in countries, where existing maps are of minor quality or outdated. In this paper, examples from deserts and semideserts in the Sudan are presented.  相似文献   
6.
. The continental flood basalts of the East Greenland volcanic rifted margin were extruded during continental breakup above the ancestral Iceland mantle plume at 55 Ma. Three distinct magma types, the low-Ti, high-Ti and very high-Ti series (LTS, HTS and VHTS respectively), are found intercalated in the ~6-km-thick Plateau Lava sequence. Incompatible trace elements indicate that the LTS are derived from a more depleted mantle source compared to HTS and VHTS. The LTS is characterised by increasing Cu (105 to 248 ppm) and Pd (7 to 24 ppb), constant Cu/Pd ratio (~10,000), and decreasing Ir (1.1 to <0.05 ppb) and Ru (1.8 to <0.3 ppb) concentrations during magmatic differentiation (16 to 7 wt% MgO). The constant Cu/Pd ratio reflects silicate- and chromite-dominated fractionation without concurrent segregation of sulphide. S-undersaturated differentiation is also indicated in the HTS, which also displays increasing Pd (6-16 ppb) and decreasing Ir concentrations (1 to <0.05 ppb) during differentiation, and the Cu/Pd ratios for the entire series average 21,000. However, some HTS samples have elevated Cu/Pd ratios (up to 33,000). Cu/Pd ratios in the HTS do not correlate with MgO, and this is interpreted to reflect varying Cu/Pd ratios of HTS parental magmas rather than S-saturated differentiation. During S-undersaturated differentiation of the LTS and HTS, Pt/Pd ratios decrease from 1.3 to 0.11 and 1.1 to 0.2 respectively, which indicates that Pd is much more incompatible than Pt during S-undersaturated differentiation. The VHTS consists exclusively of highly evolved samples with low MgO (6.6-6.1 wt%) and Pd/Ir ratios 98-228. Here, Cu/Pd ratios increase from 17,500 to 35,000 with decreasing Cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide. The LTS represent melting of a depleted source and show high PGE concentrations and constant Cu/Pd ratios during S-undersaturated differentiation. Melting of a normal depleted upper mantle source generates S-saturated melts (MORB), and the depleted LTS source is therefore considered an extraordinary S-poor component within the ancestral Iceland plume. Of the three series, the VHTS contain the largest contribution from enriched mantle portions. The VHTS have similar PGE but much higher Nb concentrations for instance compared to the most evolved LTS and HTS samples, indicating that the enriched source contributes Nb but not PGE.  相似文献   
7.

Bétaré-Oya is one of the gold mining districts in the eastern region of Cameroon. Structural controls on gold mineralization were examined along the Bétaré-Oya Shear Zone, providing further clues on favorable areas for mineral exploration. GIS-based methods combining point pattern (i.e., quadrat count, Fry analysis) and distance distribution analysis were employed here to delineate the spatial patterns of known gold deposits and to evaluate their spatial association with geological structures. Results show that the gold deposits in this area are spatially clustered. At the regional scale, the Fry plot indicates an alignment of deposits, suggesting that gold mineralization is controlled by structures oriented NNE–SSW and NE–SW. At the deposit scale, an alignment is also evident, indicating that the mineralization is also controlled by ENE–WSW-trending structures. The cumulative relative frequency distribution of distances from lineament features to gold occurrence points (DM) and to non-occurrence points (DN) ratio (DM/DN) was used to rank these two major structural trends and their relative importance as mineralization control. The yielded grades show that NE–SW-trending lineaments, akin to P-type structures, play a major role in controlling the gold mineralization in the area compared to other structures. Beyond the goal to foster mineral prospection in the Bétaré-Oya gold district, information yielded in the present study provides relevant criteria for further exploration in the eastern region of Cameroon.

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8.
The Planck mission is the most sensitive all-sky cosmic microwave background (CMB) experiment currently planned. The High-Frequency Instrument (HFI) will be especially suited for observing clusters of galaxies by their thermal Sunyaev–Zel'dovich (SZ) effect. In order to assess Planck 's SZ capabilities in the presence of spurious signals, a simulation is presented that combines maps of the thermal and kinetic SZ effects with a realization of the CMB, in addition to Galactic foregrounds (synchrotron emission, free–free emission, thermal emission from dust, CO-line radiation) as well as the submillimetric emission from celestial bodies of our Solar system. Additionally, observational issues such as the finite angular resolution and spatially non-uniform instrumental noise of Planck 's sky maps are taken into account, yielding a set of all-sky flux maps, the autocorrelation and cross-correlation properties of which are examined in detail. In the second part of the paper, filtering schemes based on scale-adaptive and matched filtering are extended to spherical data sets, that enable the amplification of the weak SZ signal in the presence of all contaminations stated above. The theory of scale-adaptive and matched filtering in the framework of spherical maps is developed, the resulting filter kernel shapes are discussed and their functionality is verified.  相似文献   
9.
Rutile, as an important component in alluvial or eluvial heavy mineral deposits, is known in southern Cameroon. These deposits are underlain by the Neoproterozoic low- to high-grade Yaoundé Group. Geochemical, thermometric, fluid inclusion and Pb isotopic studies of the rutile from alluvial and eluvial concentrates and from medium-grade micaschist from the nearby Yaoundé region permit the following conclusions: (1) alluvial and eluvial rutile of the Yaoundé region are derived from the degradation of metapelites, metamafic rocks and pegmatites of the nearby Yaoundé Group; (2) rutile in the Yaoundé Group formed during the Pan-African metamorphism, or was inherited as detrital rutile from a 900 Ma source. The study also shows that the rutile can be used to trace the history of the Pan-African belt north of the Congo craton.  相似文献   
10.
In 1986, carbon dioxide gas exploded from Lake Nyos and killed about 1,800 people. After that disaster, various administrative and research activities have been conducted to mitigate subsequent disasters. However, none of those endeavors have characterized the groundwater chemistry to identify hydrogeochemical processes that control the water chemistry, and the quality of the water for domestic and agricultural uses that support the lives of un-official resettlers around Lake Nyos. Conventional hydrochemical techniques coupled with statistical and graphical analysis were therefore employed to establish the baseline hydrochemical conditions, assess processes controlling solutes distribution in shallow groundwater in the Lake Nyos catchment and explore its usability. Groundwater samples were analyzed for their physical and chemical properties. The wide ranges of electrical conductivity and total dissolved solid values reveal the heterogeneous distribution of groundwater within the watershed. The relative abundance of major dissolved species was Ca > Mg > Na > K for cations and HCO3 >>> Cl > SO4 > NO3 for anions. Piper diagram classified almost all water samples into mixed CaMg–HCO3 water type. Major ion geochemistry reveals that, in addition to silicates weathering (water–rock interaction), ion exchange processes regulate the groundwater chemistry. Principal component analysis supports the occurrence of water rock interaction. Hierarchical cluster analysis showed that the chemistry of groundwater in the study area is controlled by three main factors, and suggests no hydraulic connectivity between deep lake water and groundwater in the catchment. The quality assessment of the groundwater showed that groundwater parameters are within the acceptable limit of the World Health Organization and Nigeria guidelines for drinking and domestic uses, and water found to be good for irrigation.  相似文献   
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