Isotopic Studies of the Origin of the Cameroon Line

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This award should complete an isotopic investigation into the origins of the Cameroon line, a 1600 km. long chain of volcanoes and plutons which is unique in that it traverses both oceanic and continental lithosphere. Recent published work has shown that: The basalts have identical chemistry in oceanic and continental sectors; Phonolites are common in the oceanic but rare in the continental sector; Rhyolites are common in the continental sector but absent from the oceanic sector, Amphibole is abundant in the oceanic sector but scarce in the continental sector; The basaltic lavas display a progression to more radiogenic Pb with time in both oceanic and continental sectors: There is a remarkable lead isotopic anomaly at the continent/ocean boundary in that the volcanics immediately to either side have a uniform highly radiogenic ("St. Helena type") signature regardless of lead concentration. So far this project has expanded on the published database and presented the first detailed Nd and Pb isotopic studies of some of these centers. The striking near symmetrical change in Pb isotopic composition with distance ffrom the continent/ocean boundary which has been demonstrated, appears to be due to the remelting of upper mantle previously affected by the St. Helena plume. A comprehensive suite of mantle xenoliths from the region has been collected, and these will be the focus of chemical and isotopic studies this next year.