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A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

Geoscience Frontiers 2015年第4期6卷 557-570页

作者： asobo n.e.asaah Tetsuya Yokoyama Festus T.Aka Tomohiro Usui Mengnjo J.Wirmvem Boris Chako Tchamabe Takeshi Ohba Gregory Tanyileke J.V.Hell Department of earth & Planetary Sciences Tokyo Institute of Technology2-12-1 Ookayama Meguro-ku Tokyo 152-8551 Japan Department of Mines Ministry of Mines Industry and Technological Development Yaoundé Cameroon Institute of Mining & Geological Research P.O. Box 4110 Yaoundé Cameroon Department of Chemistry School of Science Tokai UniversityHiratsuka 259-1211 Japan

The origin and petrogenesis of the Cameroon Volcanic Line （CVL）, composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, het- erogeneity, and nature of available data. Major and trace elements, and Sr-nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between Si02 58-64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability feld. Melting models suggest small degree （〈3%） partial melting of mantle bearing （6-10%） garnet for Mr. etinde, the ngaoundere Plateau and the Biu Plateau, and 〈5% of garnet for the oceanic sector of the CVL, Mr. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-nd-Pb isotope sys- tematics suggest that mixing in various proportions of Depleted MORB Mantle （DMM） with enriched mantle 1 and 2 （eM1 and eM2） could account for the complex isotopic characteristics of the CVL lavas. Low Mg number （Mg# - 100 x MgO/（MgO ＋FeO）） and ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in nbJTa and Zr/Hf ratios, and Sr-nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle het- erogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

关键词： Petrogenesis Cameroon volcanic line Depleted MORB mantle enriched mantle Mantle heterogeneity

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Origin of major ions in monthly rainfall events at the Bamenda Highlands, north West Cameroon

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Journal of environmental Sciences 2014年第4期26卷 801-809页

作者： Mengnjo J. Wirmvem Takeshi Ohba Wilson Y. Fantong Samuel n. Ayonghe Jonathan n. Hogarh Justice Y. Suila asobo nkengmatia e. asaah Seigo Ooki Gregory Tanyileke Joseph V. Hell Department of Chemistry School of Science Tokai University Hiratsuka 259-1211 Japan Institute of Geological and Mining Research Yaounde Cameroon Department of environmental Science Faculty of Science University of Buea Buea Cameroon Department of environmental Science Kwame Nkrumah University of Science & Technology Kumasi Ghana G.B.H.S. Atiela Nkwen Bamenda Cameroon Department of earth and Planetary Sciences Tokyo Institute of Technology Tokyo 152-8551 Japan

Rainwater characteristics can reveal emissions from various anthropogenic and natural sources into the atmosphere. The physico-chemical characteristics of 44 monthly rainfall events （collected between January and December 2012） from 4 weather stations （Bamenda, ndop plain, ndawara and Kumbo） in the Bamenda Highlands （BH） were investigated. The purpose was to determine the sources of chemical species, their seasonal inputs and suitability of the rainwater for drinking. The mean pH of 5 indicated the slightly acidic nature of the rainwater. Average total dissolved solids （TDS） were low （6.7 mg/L）, characteristic of unpolluted atmospheric moisture/air. Major ion concentrations （mg/L） were low and in the order K＋〉 Ca2＋〉 Mg2~ 〉 na＋ for cations and nO3 〉〉 HCO3 〉 SO] 〉 CI- 〉 PO3- 〉 F- for anions. The average rainwater in the area was mixed Ca-Mg-SO4-CI water type. The CI-/na＋ ratio （1.04） was comparable to that of seawater （1.16）, an indication that n a＋ and CI originated mainly from marine （Atlantic Ocean） aerosols. High enrichments of Ca2＋, Mg2＋ and SO2- to na＋ ratios relative to seawater ratios （constituting 44% of the total ions） demonstrated their terrigenous origin, mainly from Saharan and Sahelian arid dusts. The K＋/na＋ ratio （2.24）, which was similar to tropical vegetation ash （2.38）, and nO3 was essentially from biomass burning. Light （〈 100 mm） pre-monsoon and post-monsoon convective rains were enriched in major ions than the heavy （〉 100 mm） monsoon rains, indicating a high contribution of major ions during the low convective showers. Despite the acidic nature, the TDS and major ion concentrations classified the rainwater as potable based on the WHO guidelines.

关键词： rainwater chemistry tropical atmosphere drinking-water Bamenda Highlands Cameroon

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