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From source to surface: Explosivity of peralkaline magmas investigated through the Rungwe Pumice Eruption (Tanzania) case study

Lorenzo Cappelli1, Paul A. Wallace12, Gianmarco Buono3, Lucia Pappalardo3, Thomas D. van Gerve4, Olivier Namur4, Vanessa N. Sielenou1, Gerald G.J. Ernst5, Evelyne Mbede6, Shimba Kwelwa7, Edista Abdallah6, Karen Fontijn1

  • Affiliations: 1Department of Geosciences, Environment and Society, Université libre de Bruxelles, Brussels, Belgium; 2Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Munich, Germany; 3Istituto Nazionale di Geofisica e Vulcanologia, Italy; 4Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium; 5Stembert, Belgium; 6School of Mines and Geosciences, University of Dar es Salaam, Dar es Salaam, Tanzania; 7AngloGold Ashanti, Geita, Tanzania 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 197

  • Programme No: 3.2.20

  • Theme 3 > Session 2

Abstract

Magma explosivity is primarily influenced by a combination of pre-eruptive conditions inherited at the storage levels and conduit dynamics (magma ascent style and degassing efficiency), both of which impact the physiochemical magma state. In peralkaline magmas (agpaitic index>1), the depolymerisation effect of excess alkalis lowers melt viscosity for a given temperature and strain rate; therefore, such magmas should favour less explosive behaviour than their calc-alkaline counterparts at equivalent silica contents. In the East African Rift, several peralkaline volcanoes display extensive geological evidence of highly explosive eruptions interspersed by lava effusion, raising questions about the factors regulating eruptive behaviour. To investigate the controls on peralkaline explosivity, this study focuses on the 4 ka Rungwe Pumice (Tanzania) Plinian eruption. We integrated a range of geochemical (FTIR, Raman, EPMA) and imaging techniques (2D/3D textural analysis) to reconstruct the magma evolution, from pre-eruptive storage to syn-eruptive ascent and fragmentation. Our results describe an evolved magmatic body stored at 2--5 km depth, close to volatile saturation. Destabilisation of the shallow magma by gas sparging from deeper reservoir(s) likely triggered rapid magma ascent within the conduit (1-2 MPa s-1). Delayed homogeneous bubble nucleation drastically changed magma's rheology at shallow depths. The limited timescales for bubble growth inhibited the development of permeable pathways, preventing efficient volatile degassing, leading to coupling between the gas and magma phases. Ultimately, high strain rates lead to brittle fragmentation, driving explosive activity. These findings highlight the complex interplay between pre-eruptive conditions and conduit dynamics in controlling the explosivity of peralkaline magmas.