Petrologic evidence of a complex plumbing system feeding the July–August 2001 eruption of Mt. Etna,Sicily, Italy

After the major 1991–1993 eruption, Mt. Etna resumed flank activity in July 2001 through a complex system of eruptive fissures cutting the NE and the S flanks of the volcano and feeding effusive activity, fire fountains, Strombolian and minor phreatomagmatic explosions. Throughout the eruption, magmas with different petrography and composition were erupted. The vents higher than 2,600 m a.s.l. (hereafter Upper vents, UV) erupted porphyritic, plagioclase-rich trachybasalt, typical of present-day summit and flank activity. Differently, the vents located at 2,550 and 2,100 m a.s.l. (hereafter Lower vents, LV) produced slightly more primitive trachybasalt dominated by large clinopyroxene, olivine and uncommon minerals for Etna such as amphibole, apatite and orthopyroxene and containing siliceous and cognate xenoliths. Petrologic investigations carried out on samples collected throughout the eruption provided insights into one of the most intriguing aspects of the 2001 activity, namely the coeval occurrence of distinct magmas. We interpret this evidence as the result of a complex plumbing system. It consists in two separate magma storage systems: a shallow one feeding the activity of the UV and a deeper and more complex storage related to the activity of LV. In this deep storage zone, which is thermally and compositionally zoned, the favourable conditions allow the crystallization of amphibole and the occurrence of cognate xenoliths representing wall cumulates. Throughout 2001 eruption, UV and LV magmas remain clearly distinct and ascended following different paths, ruling out the occurrence of mixing processes between them. Furthermore, integrating the 2001 eruption in the framework of summit activity occurring since 1995, we propose that the 2001 magma feeding the vents lower than 2,600 m a.s.l. is a precursor of a refilling event, which reached its peak during the 2002–2003 Etna flank eruption.     

Crystal population density in not stationary volcanic systems: estimate of olivine growth rate in basalts of Lanzarote (Canary Islands)

Summary We have collected pupulation density data of melanephelinite to olivine tholeiite lavas showing seriated olivine phenocrysts as the first, and often the only, phase occurring on the liquidus. The samples are from the island of Lanzarote, where large volumes of primitive basaltic magmas reach directly the surface from their source regions at a depth between 100 and 50 km. A minimum ascent speed of 1.0 ms^–1, corresponding to times of rise of the magmas in the order of 1 day, was estimated from the occurrence of mantle xenoliths. Modal data have been automatically collected from digitalized images of thin sections. The spatial size distribution densities of olivine show an exponential variation for small crystal dimensions and a nearly constant value for large ones. Theoretical analysis shows that this behaviour can be related to variations of nucleation and growth rate with undercooling. Parameters derived for olivine growth rate equation suggest a growth rate of about 3 x 10^–7 cm s^–1 for undercooling of about 30°C.[/ p]

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Kristall-populations-dichte in nicht-stationären vulkanischen systemen: Abschätzung der wachstumsraten von olivin in basalten auf Lanzarote (Kanarische Inseln).

Zusammenfassung Wir haben Populations-Dichte-Daten von Melanephelinit bis Olivin-Tholeiit-Laven, die Olivin-Kristalle als erste und oft einzige Phase am Liquidus zeigen, erarbeitet. Die Proben stammen von der Insel Lanzarote, wo große Volumina primitiver BasaltMagmen direkt von einer Ursprungsregion in einer Tiefe von 100-50 km an die Oberfläche kommen. Das Vorkommen von Mantel-Xenolithen weist auf eine minimale Aufstiegsgeschwindigkeit von 1.0 ms^–1 hin, und dies entspricht Aufstiegszeiten der Magmen in der Größenordnung von einem Tag. Modale Daten wurden automatisch von digitalisierten Bildern von Dünnschliffen gesammelt. Die räumlichen GrößenverteilungsDichten von Olivin zeigen eine exponentionelle Variation für kleine Kristalle und einen beinahe konstanten Wert für große Kristalle. Dieses Verhaltensteht mit Schwankungen der Nukleations- und Wachstumsraten bei Unterkühlung im Zusammenhang. Parameter, die sich für Olivin-Wachstumsraten ergaben, weisen auf Werte von ungefähr 3 × 10^–7 cm s^–1 für Unterkühlung von 30°C hin.[/ p]

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With 4 Figures     

Geochemical heterogeneities and dynamics of magmas within the plumbing system of a persistently active volcano: evidence from Stromboli

We report here the most complete dataset for major and trace elements, as well as Sr isotopic compositions, of magmas erupted by Stromboli since the onset of present-day activity 1,800 years ago. Our data relate to both porphyritic scoria and lava originating in the uppermost parts of the feeding system, plus crystal-poor pumice produced by paroxysmal explosive eruption of deep-seated, fast ascending, magma. The geochemical variations recorded by Stromboli’s products allow us to identify changes in magma dynamics affecting the entire plumbing system. Deep-seated magmas vary in composition between two end-members having different key ratios in strongly incompatible trace elements and Sr isotopes. These features may be ascribed to mantle source processes (fluid/melt enrichment, variable degrees of melting) and occasional contamination by deep, mafic, cumulates. Temporal trends reveal three phases during which magmas with distinct geochemical signatures were erupted. The first phase occurred between the third and fourteenth centuries AD and was characterised by the eruption of evolved magmas sharing geochemical and Sr isotopic compositions similar to those of earlier periods of activity (<12 ka—Neostromboli and San Bartolo). The second phase, which began in the sixteenth century and lasted until the first half of the twentieth century, produced more primitive, less radiogenic, magmas with the lowest Ba/La and Rb/Th ratios of our dataset. The last phase is ongoing and is marked by a magma having the lowest Sr isotopic composition and highest Rb/Th ratio of the dataset. While this new magma can be clearly identified in the pumice erupted during the last two paroxysmal eruptions of 2003 and 2007, shallow degassed magma extruded during this time span records significant geochemical and isotopic heterogeneities. We thus suggest that the shallow reservoir has been only partially homogenised by this new magma influx. We conclude that compositional variations within the shallow magma system of a persistently active volcano provide only a biassed signal of ongoing geochemical changes induced by deep magma refilling. We argue that source changes can only be identified by interpreting the geochemistry of pumice, because it reliably represents magma transferred directly from deep portions of the plumbing system to the surface.     

Paroxysmal activity at Stromboli: lessons from the past

The persistent normal activity of Stromboli is occasionally interrupted by sudden and highly energetic explosive events called Strombolian paroxysms. These phenomena together with landslide-generated tsunamis represent the most hazardous manifestations of present-day volcanic activity at Stromboli. The most recent paroxysms, on 5 April 2003 and 15 March 2007, have drawn attention to these energetic events because they significantly threatened inhabitants and tourists. Historical accounts and field evidence indicate, however, that even larger paroxysms, in terms of volume, dispersal of products and intensity of explosive phenomena, occurred in the recent past. During these paroxysms incipiently welded spatter deposits mantled the north and south rims of the Sciara del Fuoco down to low elevations, extending much farther than the similar deposits from recent observed events (5 April 2003 and 15 March 2007). In order to identify, characterize and discriminate among products of these outstanding spatter-forming eruptions, more than 50 stratigraphic sections were measured and sampled. Stratigraphic, sedimentological and radiometric (¹⁴C) data indicate that only two paroxysms produced spatter that reached very low elevations and inhabited areas: the first occurred in the 16th century and the last in AD 1930. Analysis of texture and deposit components reveals that the early phases of the two eruptions were driven by distinctly different eruptive dynamics. Both identified paroxysms are at least one order of magnitude greater than any similar event observed by monitoring systems at Stromboli. These two large paroxysms were the most powerful volcanic events at Stromboli in the last eighteen centuries.     

Effects of magma storage and ascent on the kinetics of crystal growth

The size distributions of crystals of olivine, plagioclase and oxides of the 1991/93 eruption at Mt. Etna (Italy) are analyzed. The simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas. Three distinct episodes are detectable: a storage of the magma in a deep reservoir, characterized by nearly constant and low nucleation and growth rates (near to equilibrium); an ascent phase, with an ever increasing nucleation rate related to volatile exsolution; and finally a quenching phase. In addition to geochemical and geophysical evidence, the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of Etnean lavas are due to mixing of different populations. This strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates.     

Compositional variation in the 1983 and other recent Etnean lavas: Insights on the shallow feeding system

A systematic petrographic and chemical study of chronologically ordered lava samples collected during the 1983 Etna eruption, has shown minor fluctuating variations in crystallinity and chemistry, apparently correlated with variations in the rate of effusion. Comparison with the compositional variations observed in the lavas emitted during the 1971–1983 period, has revealed a repeated evolutional tendency suggesting a small size shallow storage system periodically refilled, as in 1983, by more primitive magma.     

Paleo-environmental and volcano-tectonic evolution of the southeastern flank of Mt. Etna during the last 225 ka inferred from the volcanic succession of the ‘Timpe’, Acireale, Sicily

The tectonic escarpments locally known as ‘Timpe’ cut a large sector of the eastern flank of Etna, and allow an ancient volcanic succession dating back to 225 ka to be exposed. Geological and volcanological investigations carried out on this succession have allowed us to recognize relevant angular unconformities and volcanic features which are the remnants of eruptive fissures, as well as important changes in the nature, composition and magmatic affinity of the exposed volcanics. In particular, the recognition in the lower part of the succession of important and unequivocal evidence of ancient eruptive fissures led us to propose a local origin for these volcanics and to revise previous interpretations which attributed their westward-dipping to the progressive tectonic tilting of strata. These elements led us to reinterpret the main features of the volcanic activity occurring since 250 ka BP and their relationship with tectonic structures active in the eastern flank of Etna. We propose a complex paleo-environmental and volcano-tectonic evolution of the southeastern flank of Mt. Etna, in which the Timpe fault system played the role of the crustal structure that allowed the rise and eruption of magmas in the above considered time span.