Abstract. The 2014 eruption at Piton de la Fournaise (PdF), La Réunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4 × 106 m3). The pyroclastic material was composed of golden basaltic pumice along with fluidal, spiny iridescent and spiny opaque basaltic scoria. Density analyses performed on 200 lapilli reveal that while the spiny opaque clasts are the densest (1600 kg m−3) and most crystalline (55 vol. %), the golden pumices are the least dense (400 kg m−3) and crystalline (8 vol. %). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 vol. %) than the spiny (Strombolian-like) clasts (0–5 vol. %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments and lava that mirror the cooler portion of the shallow reservoir. Exponential decay of the magma ascent and output rates through time revealed depressurization of the source during which a stratified storage system was progressively tapped. Increasing syn-eruptive degassing and melt–gas decoupling led to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new input of hot magma into the 2014 reservoir and confirm the emission of a single shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. Fast volatile exsolution and crystal–melt separation (second boiling) were triggered by deep pre-eruptive magma transfer and stress field change. Our study highlights the possibility that shallow magma pockets can be quickly reactivated by deep processes without mass or energy (heat) transfer and produce hazardous eruptions with only short-term elusive precursors.
Malgre l’homogeneite isotopique des laves de La Reunion, une certaine diversite des produits est observee sur les deux principaux volcans de l’ile, le Piton des Neiges et le Piton de la Fournaise, d’un point de vue petrographique et chimique. En effet, si la majorite des laves emises sont des basaltes transitionnels dont l’evolution est principalement controlee par la precipitation et/ou l’accumulation d’olivine, certaines laves plus ou moins anciennes montrent des caracteristiques petrologiques et geochimiques particulieres, qui temoignent de conditions magmatiques variees et de systemes de stockage et transfert relativement complexes. C’est le cas des « Basaltes Porphyriques a Plagioclase », des basaltes des cones excentriques dits « adventifs » et de ceux du cratere Hudson (eruption de 1998), sur lesquels portent les travaux de cette these. L’etude petro-geochimique des laves est couplee a celle des inclusions magmatiques des mineraux, naturellement vitreuses ou homogeneisees par chauffage experimental, afin de caracteriser l’origine des magmas et les processus responsables de leur evolution, en contexte de point chaud oceanique.Les rapports d’elements en traces incompatibles des inclusions magmatiques piegees dans les olivines precoces (Fo > 85) des cones adventifs sont utilises pour identifier la nature de la source du panache mantellique de La Reunion. Les resultats suggerent que les magmas des cones adventifs ont une origine chimiquement comparable a celle de l’ensemble des laves reunionnaises, intermediaire entre un domaine mantellique relativement primitif et un domaine legerement appauvri, presque non-affecte par les processus de recyclage. De faibles degres de fusion partielle de cette source generent les concentrations enrichies en elements en traces des inclusions magmatiques.Les Basaltes Porphyriques a Plagioclases, pouvant contenir jusqu’a 35 % de plagioclases millimetriques, ont ete emis sur les deux volcans. Les compositions des inclusions magmatiques des macrocristaux de plagioclase (An 84.2-71.7 ) et les observations texturales des cristaux mettent en evidence leur caractere herite. Les magmas parentaux des cristaux evoluent essentiellement par cristallisation de clinopyroxene et de plagioclase. Les contrastes de densite entre les phases permettent la segregation des plagioclases par flottaison, et leur accumulation au toit de la chambre. Les Basaltes Porphyriques a Plagioclase sont formes par la remobilisation de ces zones d’accumulation riches en plagioclase lors de l’arrivee d’un nouveau magma. Les periodes tres specifiques d’eruption de ces basaltes correspondraient a une diminution du flux magmatique dans la croissance des volcans, favorisant la cristallisation de plagioclase.Les textures des laves des cones adventifs et du cratere Hudson, ainsi que les inclusions magmatiques des olivines de ces laves temoignent d’une histoire complexe des cristaux, et de l’importance des processus de recyclage dans le systeme d’alimentation magmatique du Piton de la Fournaise. Les olivines sont nettement plus magnesiennes (Fo > 85) que celles des laves historiques. Les compositions chimiques des laves montrent que la majeure partie correspond a des basaltes legerement alcalins, appeles « Mid-Alkaline Basalts », qui sont appauvris en CaO mais enrichis en elements compatibles et incompatibles. Les compositions isotopiques et en elements en traces leurs suggerent une origine commune avec celle des laves historiques, bien que le taux de fusion partielle a l’origine des « Mid-Alkaline Basalts » soit plus faible. Un fractionnement profond de clinopyroxene et de plagioclase expliquerait leur formation. L’ensemble de ces resultats suggere que du magma d’origine profonde peut remonter directement, sans passer par le systeme central du volcan.
Abstract. The 2014 eruption at Piton de La Fournaise (PdF), la Reunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity, and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4 Mm3). The pyroclastic material was composed of spiny-opaque, spiny-iridescent, and fluidal scoria along with golden pumice. Density analyses performed on 200 lapilli reveal that the spiny-opaque clasts are the densest (1600 kg/m3) and richest in crystals (54 vol%), and the golden pumices are the lightest (400 kg/m3) and poorest in crystals (14 vol%). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 %) than the spiny (Strombolian-like) clasts (0–5 %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments which mirror the cooler portion of the shallow reservoir. Progressive tapping of these distinct portions leads to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new hot input of magma and confirm the involvement of a single, shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. We found that the eruption was triggered by water exsolution, favoured by the shallow depth of the reservoir, rather than cooling and chemical evolution of the stored magma.
Plagioclase-bearing ultraphyric basalts, which can have up to 35% accumulative, millimetre-sized plagioclase crystals, were episodically erupted during some stages of building of La Réunion Island volcanoes. Selected rock samples were analysed from the two volcanoes of the island (four samples from Piton des Neiges and two from Piton de la Fournaise). We summarize the results of petrographic and geochemical studies of the whole-rocks and silicate melt inclusions trapped within plagioclase (An84·2–71·7) macrocrysts, which contain aliquots of the parental melts of their hosts. Melt inclusion compositions are used to discuss the origin of La Réunion plagioclase-bearing ultraphyric basalts, with special emphasis on the magma storage system that led to their production. Experimentally re-homogenized melt inclusions were analysed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for their major and trace elements. Together with the textural observations, these results indicate that the host crystals are inherited, although their parental magma compositions are close to that of the host lava. Petrographic lines of evidence suggest that the plagioclase macrocrysts originate from batches of plagioclase-rich mush or fragments of anorthositic gabbro ripped from the magma chamber walls. The parental melts of the plagioclases evolve predominantly by clinopyroxene + plagioclase crystallization. The density contrasts between minerals and liquid allow their efficient segregation. We propose that plagioclases are collected below the magma chamber roof as a flotation cumulate, thereby forming plagioclase-rich mush and anorthositic gabbro, whereas clinopyroxenes sink. The plagioclase-ultraphyric basalts are derived from the remobilization of this mush, or fragments of the anorthositic cumulate, upon the input of a new batch of magma that triggers their eruption. We postulate that the periodic occurrence of these striking basalts during specific periods of the volcano's growth corresponds to decreases in the magma supply, which promoted plagioclase crystallization and its segregation by flotation. Finally, we suggest that the plagioclase-ultraphyric basalts from the Rivière des Remparts and Rivière de l'Est valleys, previously regarded as components of the Piton de la Fournaise volcano, are instead products of the declining activity of an older volcano, possibly Les Alizés volcano.
