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Atouba L.C.O.,Institute of Geological and Mining Research IRGM | Chazot G.,CNRS Oceanic Domains Laboratory | Moundi A.,University of Yaounde I | Agranier A.,CNRS Oceanic Domains Laboratory | And 4 more authors.
Arabian Journal of Geosciences

The age and origin of the volcanism along the Cameroon Volcanic Line (CVL) are still a matter of debate. We present major and trace element compositions as well as Sr–Nd–Pb–Hf isotopic results for mafic rocks from the Bamoun area, in the central part of the Cameroon Volcanic Line, as well as for two samples from recent eruptions of Mt. Cameroon. Lava flows are mostly basalts and hawaiites with transitional affinity. Some samples are among the oldest rocks of the CVL, with ages older than 51 My while some rocks are very young, around 0.05 My. All the samples are enriched in incompatible elements, indicating that melts were formed in a garnet-bearing mantle source. Different mantle sources participated to the formation of the Bamoun lavas. One source is isotopically similar to the main volcanic rocks of the CVL and probably represents an important part of the subcontinental lithospheric mantle. The second source is enriched in incompatible elements and shows a marked positive Eu anomaly, probably related to the participation of pyroxenites in the partial melting processes. The third mantle source is similar to the source of the Mt. Cameroon. This mantle source was known previously only in the Mt. Cameroon lavas, and we report its occurrence for the first time in old lavas and in other location along the CVL. © 2016, Saudi Society for Geosciences. Source

Fantong W.Y.,Institute of Geological and Mining Research IRGM | Fantong W.Y.,University of Toyama | Kamtchueng B.T.,University of Toyama | Yamaguchi K.,Mitsubishi Group | And 9 more authors.
Journal of African Earth Sciences

For the first time, comprehensive study of hydrogeochemistry of water seeps, role of chemical weathering on dam failure, estimation of minimum width of dam to resist failure and simulation of changes in dissolved ions and secondary mineral was conducted on the Lake Nyos dam. The salient results and conclusions were; the dam spring water represented a mixture of 60-70% rainwater and 30-40% Lake water (from 0 to -40m). The chemistry of the observed waters was Ca-HCO3 for rainwater, Ca-Mg-HCO3 in boreholes, and Mg-Ca-HCO3 - for spring water. The relative rate at which ions dissolved in water was HCO3 ->Mg2+>Ca2+>Na+>SiO2>K+>NO3 ->SO4 2 ->Cl-. Weathering of rocks resulted in the formation of clay minerals such as kaolinite and smectite. Relative mobility of elements compared to Alumina (Al2O3) indicated that in monzonites there was a loss of CaO, Na2O, K2O, P2O5 and gain of SiO2, Fe2O3, TiO2, MnO and MgO, while in basalts there was a loss of SiO2, Fe2O3, Ca2O, NaO, MgO and gain of TiO2, K2O and P2O5. Values of chemical alteration index that ranged from 49 to 82 suggest a weak to intermediate categories of chemical weathering that occurred at a rate of 5.7mm/year. Paired to that rate, which suggests that the dam is not vulnerable to failure at the previously thought time scale, some other processes (physical weathering, secondary mineral formation and lake overflow) can cause instant failure. Hydrostatic pressure of 1.6GN generated by Lake water can be supported only when the width of the dam is greater than 19m. PHREEQC-based simulation for 10years indicates decoupling of Ca and Mg, and Na and Mg. Multidisciplinary monitoring of the dam is advocated. © 2014 Elsevier Ltd. Source

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