ARE MAGMAS BENEATH THE CAMEROON VOLCANIC LINE ENRICHED IN CARBON DIOXIDE? -RELATIONSHIP WITH THE LAKE NYOS-TYPE GAS DISASTERS-

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The Cameroon Volcanic Line (CVL) is a 1600 km long, linear chain of volcanoes that trends in NE-SW direction across Cameroon, West Africa. CVL hosts numerous crater lakes that include notorious Lakes Nyos and Monoun for their gas disasters in 1986 and 1984. Scientific surveys after the disasters indicate that both lakes contain a huge amount of magmatic COィイD22ィエD2 in their bottom waters and are considered to have a high potential to explode again due to continuous supply of COィイD22ィエD2 through the lake bottom. Many soda springs occur along CVL.The purposes of this study are two fold : To obtain the current physico-chemical situations of these gassy lakes, and to understand the geochemical features of CVL magmas. To this end, the following field and laboratory work was undertaken ; (1) determination of COィイD22ィエD2 profiles and isotopic compositions of rare gases dissloved in the lakes, (2) systematic sampling of soda springs along southern part of CVL to characterize their chemistry and rare gas isotopic compositions, and (3) systematic sampling of volcanic rocks and mantle xenoliths for their K-1r ages, chemical compositions and rare gas elemental/isotopic compositions.The results are : (1) The COィイD22ィエD2 concentration in these lakes is steadily increasing, (2) The COィイD22ィエD2 originates from mantle, (3) Volcanism of CVL took place during Pleistocene-Holocene along the whole CVL, and (4) the upper mantle beneath CVL is geochemically heterogeneous with respect to (U+Th)/ィイD13ィエD1He rations with the highest ratio occurring at present ocean-continent boundary. The last interpretation is based on a clear observation that ィイD13ィエD1He/ィイD14ィエD1He ratios of phenocrysts and xenoliths from volcanic rocks of this region are definitely lower than those from the other regions.