The role of volatiles in Cameroon Volcanic Line magmatism

喀麦隆火山线(CVL)是一条1600公里长的火山链，从乍得边界附近的非洲大陆内陆一直延伸到几内亚湾的岛屿，活动了40 Ma。CVL是目前最大的活跃的大陆板内火山省，CVL上最大的火山喀麦隆山从这里上升到4000多米。喀麦隆火山是世界上最活跃的空中火山之一，在20世纪内喷发了7次。由于近50万人居住在喀麦隆火山及其周围，火山位于喀麦隆的S唯一炼油厂及其深水港附近，了解这座火山和CVL火山沿线的其他火山喷发的方式和原因具有重要的科学意义和社会意义。然而，CVL沿线岩浆活动的原因却知之甚少，因为它不能仅用板块构造和热点/热柱理论来解释。尽管火山线横跨大陆-海洋分界线，但在这一分界线上，几乎没有迹象表明火山源特征发生了变化，也没有任何年龄递进的火山活动历史可以像预期的热点背景那样被辨认出来。CVL火山建立在构造高地(而不是裂谷中)，这排除了板内伸展的裂谷作用是唯一原因(例如东非裂谷火山作用)。拟议的“热线”模式是不可信的，因为很难维持一个(300公里深)热源超过&gt；40毫安的活动。刚果克拉通边界或海洋-大陆边界的地幔对流可能是热源的原因，但其他海洋-大陆或前寒武纪克拉通边界缺乏类似的火山线，这与此相反。由于之前提出的将减压或加热作为融化的主要原因的模型都不能令人信服地解释CVL，我们建议调查地幔中增强的挥发性活动是否可能是这种巨大的、长期存在的火山特征的原因。这将是一个可行的解释，因为即使是微量的挥发分也可以显著降低地幔的固相指数，并且CVL与高挥发分含量，特别是二氧化碳有着密切的关系。最好的例子是尼奥斯湖，它在1986年的脱气过程中窒息了1700人，但CVL沿线的其他马伊尔人也被认为二氧化碳含量很高。此外，喀麦隆熔岩被发现具有较高的挥发性含量。我们计划通过对CVL沿线岩浆熔体和流体包裹体的主要、微量元素和挥发分研究，来研究深部原始岩浆的原始挥发分含量。此外，我们打算通过调查挥发物在时间和空间上的分布来研究挥发物的来源，并通过产生放射性和稳定的(碳和氧)同位素来识别挥发物的不同潜在来源。

The Cameroon Volcanic Line (CVL) is a 1600 km-long chain of volcanoes which extends from the interior of the African continent near the Chad border to the islands in the Gulf of Guinea and has been active for 40 Ma. The CVL is the largest presently active continental intraplate volcanic province from which the largest volcano on the CVL, Mount Cameroon rises to more than 4000m. Mount Cameroon is one of the most active subaerial volcanoes in the world, having erupted 7 times within the 20th century. With almost half a million people living on or around Mount Cameroon, and the volcano lying close to both Cameroon´s only oil refinery and its deep water harbour, knowledge of how and why this volcano and the other volcanoes along the CVL erupt is of both scientific and societal relevance. However, the cause for magmatism along the CVL is poorly understood as it cannot be explained through plate tectonics and hotspot/plume theory alone. Although the volcanic line crosses the continent-ocean divide there is little sign of a change in source characteristics at this divide, nor is any age-progressive history of volcanism discernible as would be expected for a hotspot setting. CVL volcanoes are built on structural highs (rather than in rifts) which excludes rifting by intraplate extension as the sole cause (such as for the E- African Rift volcanism). The proposed model of a "hotline" is not plausible since it is difficult to sustain a (<300 km deep) heat source over >40 Ma of activity. Mantle convection at the boundary of the Congo Craton or the ocean-continent boundary could account for the heat source, but the lack of similar volcanic lines at other ocean-continent or Precambrian Craton boundaries argues against this. Since none of the previously proposed models that focused on decompression or heating as the main cause of melting can convincingly explain the CVL, we propose to investigate whether enhanced volatile activity in the mantle could be responsible for this huge, long-lived volcanic feature. This would be a feasible explanation since even trace amounts of volatiles can significantly lower mantle solidi and the CVL has an intimate relationship with high volatile contents, particularly CO2. The best example of this is Lake Nyos that by degassing in 1986 suffocated 1700 people, but also other maars along the CVL are known to have high CO2. In addition, Mount Cameroon lavas are found to have relatively high volatile contents. We plan to investigate the original volatile content of the primitive magmas at depth by a major, trace element and volatile study of melt and fluid inclusions of magmas along the CVL. In addition we intend to study the origin of the volatiles by investigating their distribution along the CVL in time and space and to identify different potential sources for the volatiles by radiogenic and stable (carbon and oxygen) isotopes.