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The role of volatiles in Cameroon Volcanic Line magmatism

挥发物在喀麦隆火山线岩浆作用中的作用

基本信息

批准号：
254022072
负责人：
Dr. Froukje van der Zwan
金额：
--
依托单位：
University of Buea
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2014
资助国家：
德国
起止时间：
2013-12-31 至 2018-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/254022072?language=en
关键词：
role volatiles Cameroon Volcanic Line

项目摘要

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.

喀麦隆火山线（CVL）是一条1600公里长的火山链，从乍得边境附近的非洲大陆内部延伸到几内亚湾的岛屿，已经活跃了40 Ma。CVL是目前最大的活跃大陆板内火山区，CVL上最大的火山喀麦隆山海拔超过4000米。喀麦隆火山是世界上最活跃的陆上火山之一，在世纪曾爆发过7次。喀麦隆山附近有近50万人居住，火山靠近喀麦隆唯一的炼油厂和深水港，了解这座火山和沿着CVL的其他火山如何以及为什么爆发具有科学和社会意义。然而，岩浆沿着CVL的原因是知之甚少，因为它不能通过板块构造和热点/羽理论单独解释。虽然火山线穿过大陆-海洋分水岭，但在这个分水岭上，几乎没有迹象表明源特征发生了变化，也没有任何火山活动的年龄递进历史，就像热点环境所预期的那样。CVL火山是建立在构造高点（而不是在裂谷），这排除了裂谷板内扩张作为唯一的原因（如东非裂谷火山活动）。所提出的“热线”模型是不合理的，因为很难维持一个（深度小于300公里）热源超过40 Ma的活动。在刚果板块边界或大洋-大陆边界的地幔对流可以解释热源，但在其他大洋-大陆或前寒武纪板块边界缺乏类似的火山线反对这一观点。由于没有以前提出的模型，专注于减压或加热融化的主要原因，可以令人信服地解释CVL，我们建议调查是否在地幔中增强的挥发性活动可能是负责这个巨大的，长寿的火山功能。这是一个可行的解释，因为即使是微量的挥发分也能显著降低地幔固体，CVL与高挥发分含量，特别是CO2含量有密切的关系。最好的例子是尼奥斯湖，1986年的脱气导致1700人窒息，但CVL沿着的其他玛珥也被认为具有高二氧化碳。此外，喀麦隆火山熔岩被发现具有相对较高的挥发物含量。我们计划通过对沿着CVL的岩浆熔体和流体包裹体的主量、微量元素和挥发分的研究，来研究原始岩浆的原始挥发分含量。此外，我们打算研究挥发物的起源，通过调查他们的分布沿着CVL在时间和空间上，并确定不同的潜在来源的挥发物的放射性和稳定的（碳和氧）同位素。