The role of volatiles in Cameroon Volcanic Line magmatism
Final Report Abstract
The Cameroon Volcanic Line (CVL) has been active for 40 million years and is the largest presently active volcanic province in a continental intraplate setting. Its largest volcano, Mt. Cameroon has erupted 7 times within the 20th century. Knowledge of how and why this volcano and the other volcanoes along the CVL erupt is thus of both scientific and societal relevance. However, the cause for magmatism along the CVL is poorly understood as its unique features cannot be simply explained by common theories based on plate tectonics or hotspots. Since none of the previously proposed models for the causes of melting, focusing on heating or melting by decompression, can convincingly explain the volcanism of the CVL, this project proposed to investigate if the deeper mantle chemistry could be responsible for this long-lived volcanic feature due to e.g., high volatile contents that cause increased melting. The volcanoes of the CVL have an intimate relationship with high volatile contents, particularly to CO2 that is known from the catastrophic degassing events at the ‘Killer Lakes’ Monoun and Nyos in 1984 and 1986, respectively. Thus, this project aimed to investigate the volatile contents of the deep primitive magmas by studying melt and fluid inclusions in minerals from volcanic rocks, sampled along the CVL. A further aim was to get a better understanding of the magmatic systems that drives volcanism at the CVL, and particularly its biggest active volcano, Mt. Cameroon. The project resulted in the most detailed sample collection of the CVL till date. Samples from Mt. Cameroon, Debunscha Maar, Tombel Graben and the Foumbot Volcanic Field were studied for major and trace element concentrations in whole rocks and minerals; pressure calculations were performed to estimate the depth of magma reservoirs and volatiles; major and trace element and volatile concentrations in primitive melt inclusions were measured. For Mt. Cameroon, the results show that the chemical variations between samples from different eruption episodes are caused by different degrees of crystal fractionation of magma batches that are all derived from the same magma-source for the whole of Mt. Cameroon, and this has not changed significantly over time. Small compositional variations between different volcanic cones of one eruption episode argue for slightly inhomogeneous magmas or latestage crystallization during the eruption. The little overall variation and the lack of systematic variation between historic and pre-historic eruption events, together with a lack of interaction with the continental crust, indicates that eruption mechanisms have not significantly changed over time and that future eruptions are not expected to be more explosive than they have been in the past, which is a good outcome of the study in terms of volcanic hazard assessment. A surprising finding about the magmatic plumbing system below Mt. Cameroon is based on observations from the mineral grains. Minerals in the Mt. Cameroon magmas are formed during two stages: in a first stage of crystallization, olivine and clinopyroxene phenocrysts are formed from earlier magma batches that form a large reservoir of crystal mush at lower crustal depths. These crystals and rock fragments are remobilized in a second stage by new magma batches. At the same depths, rims crystallize around these earlier grown larger phenocrysts and very small crystals crystallize from the magmas, before the lavas rise to the surface to finally erupt within days to weeks. These petrographic observations imply that the magmatic system beneath Mt. Cameroon is much larger than previously thought, with main magmatic reservoirs in the lower crust. Geochemical data from melt inclusions and olivine crystals imply that the mantle source is mostly peridotitic and from the garnet to spinel stability fields, but with low amounts of pyroxenite, although their contents are varying non-systematically along the CVL. The peridotitic mantle source displays signs of metasomatism, which is indicated by trace elements that show the presence of amphiboles. Further, the high CO2 contents in the melt inclusions and indications for carbonated magma from geochemical signatures in melt inclusion and olivines, point towards high CO2 concentrations in the mantle source. Mantle potential temperatures estimated for the primitive melt inclusions suggest temperatures below 1400°C, which indicates that the CVL is not formed by a hotspot, but instead high volatile activities in the mantle may play an important role in the formation of melts beneath the CVL. The first published results on the Debunscha Maar were publicly shared via outreach posts on the collaborators Facebook page of GEOMAR www.facebook.com/muhs.geomar.
Publications
- (2015) Compositional diversity in Mt. Cameroon eruptions. In: Goldschmidt Conference 2015, 25.-30.08.2015, Prague, Czech Republic. 3232
van der Zwan F.M., Suh C.E., Hansteen T.H., Gerami-Manesch M., Garbe-Schönberg D., Devey C.W., Embui F.V., Tibang E.E.B., Taboko A.
- (2016) Magmatic processes beneath Mt. Cameroon – insights from minerals and melt inclusions In: Goldschmidt Conference 2016, 26.06.- 01.07.2016, Yokohama, Japan. 3251
van der Zwan F.M., Hansteen T.H., Suh C.E., Nordstad S., Gerami-Manesch M., Devey C.W., Garbe-Schönberg D.
- (2017) Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line. Lithos 288-289, 326- 337
Ngwa C.N., Hansteen T.H., Devey C.W., van der Zwan F.M. and Suh C.E.
(See online at https://doi.org/10.1016/j.lithos.2017.06.028) - (2017) Primitive melts and mantle sources of the Cameroon Volcanic Line based on melt inclusion geochemistry In: Goldschmidt Conference 2017, 13.08.-18.08.2017, Paris, France. 4588
van der Zwan F.M., Hansteen T.H., Suh C.E., Gurenko A.A., Devey C.W., Garbe-Schönberg D.
