Pre-eruptive magmatic reservoir conditions controlling explosivity of a trachytic Plinian eruption: case for the Rungwe Pumice (Tanzania)

Abstract

The eruptive style and explosivity of volcanic eruptions primarily depend on conduit dynamics (e.g., ascent rate and degassing efficiency) and the rheological behaviour of the pre-eruptive magma, which is controlled by its composition and conditions within the reservoir. In the case of highly alkaline magmas (i.e., agpaitic index > 1), the depolymerisation of silica bonds exerted by alkaline elements promotes a relatively low-viscosity rheological response and therefore theoretically less explosive eruptive behaviour, even for silica-rich magmas. However, several well-studied eruptions show that peralkaline magmas, like trachyte and phonolite, can also experience highly explosive Plinian events, suggesting other parameters might influence the eruption style. In the East African Rift, several such volcanoes with peralkaline magma compositions have erupted both explosively and effusively in the past. We investigated the pre-eruptive magmatic system of the Plinian eruption that produced the Rungwe Pumice (RP) deposit in southern Tanzania. The type section of the RP consists of a ~2.5 m thick fall deposit of peralkaline trachytic/phonolitic pumices. Syn-eruptive volatile exsolution is a crucial parameter controlling the eruptive style and is related to the volatile budget within the reservoir. Therefore, water concentrations in haüyne-hosted melt inclusions (MIs) were characterised using transmitted Fourier-transformed infrared spectroscopy. Preliminary results indicate substantial water concentrations within the reservoir (i.e., 2.60-5.59 wt.%) while the water retained by the glassy matrix of pumices reveals that the magma was almost entirely degassed during ascent. The modelled evolution of magmatic composition within the reservoir suggests closed conduit fractional crystallisation (~80% fractionation) of a parental mafic magma to generate the RP trachyte/phonolite within a compositionally homogeneous magmatic reservoir. Finally, by applying petrological models, we estimated a MI entrapping temperature of ~1147 ± 92 K and a minimum lithostatic pressure ranging from ~28 to 184 MPa (~1-7 km depth).