Isotopic Studies of the Origin of the Cameroon Line

该奖项将完成对喀麦隆线起源的同位素调查，喀麦隆线是一条1600公里长的火山和岩浆链，其独特之处在于它穿越了海洋和大陆岩石圈。最近发表的研究表明：玄武岩在海洋和大陆板块具有相同的化学性质；空粒岩在大洋板块很常见，但在大陆板块很少见；流纹岩在大陆板块普遍存在，而在大洋板块中缺乏；角闪孔在大洋板块中丰富，而在大陆板块中较少；玄武岩熔岩在海洋和大陆板块都显示出随着时间的推移向放射性成因铅的发展：在大陆/海洋边界有一个显著的铅同位素异常，因为与铅浓度无关的任何一侧的火山都具有统一的高放射性成因（“圣赫勒拿岛型”）特征。到目前为止，该项目已经扩展了已发表的数据库，并首次详细介绍了其中一些中心的Nd和Pb同位素研究。铅同位素组成随着离大陆/海洋边界的距离的接近对称的显著变化已被证明，这似乎是由于先前受圣赫勒拿岛羽流影响的上地幔重熔所致。从该地区收集了一整套地幔捕虏体，这些将是明年化学和同位素研究的重点。

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.