The role of volatiles in Cameroon Volcanic Line magmatism

• Applicant: Dr. Froukje van der Zwan
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 254022072

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.

• DFG Programme: Research Grants
• International Connection: Cameroon
• Participating Persons: Dr. Carl-Dieter Garbe-Schönberg; Privatdozent Dr. Thor Henrik Hansteen; Professor Dr. Emmanuel Suh