Pre-eruptive conditions at satellite vent eruptions at Teide-Pico Viejo complex (Tenerife, Canary Islands)
Résumé
The Teide-Pico Viejo (T-PV) stratovolcanoes constitute one of the most potentially active volcanic complexes in Europe. T-PV was traditionally considered as a non-explosive system, but recent studies have pointed out the explosive character of their phonolitic magmas, including plinian and subplinian eruptions and the generation of pyroclastic density currents, as is evidenced in the volcanological record of the last 30 kyr. This explosive activity is mostly associated with satellite dome-like vents, which are characterised by presenting progressive transitions between explosive and effusive activity. In order to improve our understanding of these types of eruptions and their controlling mechanisms, we conducted a petrological and mineral characterisation of the products from the different eruptive phases of Pico Cabras dome. This permitted us to constrain the pre-eruptive conditions of the magma and to infer the potential factors that control the changes in the volcanic activity of dome-forming eruptions at T-PV and, in particular, the transition between explosive and effusive phases.
Feldspar (anorthoclase and sanidine), clinopyroxene (diopside and augite), biotite, amphibole (kaersutite), magnetite, ilmenite, sodalite, and glass samples were analysed using EPMA and Micro-XRF. Our results suggest the presence of a compositionally, thermally, and volatile stratified magma chamber at 100 ± 50 MPa prior to the Pico Cabras eruption (between 9210 and 5911 years BP), which was characterised by a progressive change in the style of activity, from an efficiently fragmented magma dispersed by a sustained plume, to a poorly-fragmented, low-height, non-convective fountain (forming the fountain-fed lava dome) to a glass-bearing lava flow activity. The transition in the eruptive style was controlled by changes in temperature and amount of volatiles dissolved in the phonolitic melt (i.e., differences in magma rheology). The initial explosive phase was related to the emission of highly evolved phonolitic magma ponding at 725–800 °C while containing 3.5–5 wt% H2O and located in the upper part of the magma reservoir. The magma that fed the phonolitic lava flow formed the main body of the reservoir and was stored below the preceding melt at 880 ± 30 °C and with 2.5–3 wt% H2O dissolved. Feldspar zonation suggests that the eruption was triggered by the intrusion of hotter mafic magma at the base of the phonolitic chamber, which induced the overturn of the interior of the reservoir. However, the interaction between the internal magmas was short enough to prevent the hybridization phenomena among them. The compositions of some feldspars from the most explosive phase are equivalent to those found in the El Abrigo eruption, the last caldera-forming episode (ca. 190 ka), demonstrating that T-PV volcanic system can already produce evolved and highly explosive magmas, a factor that should be considered in future hazard assessment for Tenerife.
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