Jean-Claude Tanguy

Magma dynamics at Mt Etna: Constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas

Abstract 238U-230Th-226Ra-210Pb disequilibria and87Sr/86Sr ratios have been measured in a suite of prehistoric and historical lava flows from Mt. Etna. Whereas Th isotope ratios remain nearly constant during the whole period studied,226Ra and87/Sr86Sr data allow us to distinguish two mainepisodes in the volcanic history. For most of the past two millenia until 1970 A.D., Ra and Ba exhibit a similar behaviour (decreasing (226Ra230Th) andBa/Th ratios with increasing Th contents), which is mainly controlled by plagioclase fractionation during differentiation from hawaiites to mugearites. This differentiation occurs on a short timescale (≤ 200 yr) and the parental hawaiitic magma seems to have maintained a constaant (226Ra230Th) ratio during the whole period. The226Ra-230Th data are interpreted in the context of a steady-state deep reservoir: an upper limit of 1500 yr is derived for the residence time of the magma in this reservoir, which could have a maximum volume of 150–300 km3, much smaller than the volume inferred from seismic studies. The present period of activity, after 1970 A.D., is characterized by the injection in the shallow plumbing system of a new basic magma enriched in Ra, K, Rb and Cs, with a higher Sr isotope ratio (0.70359 compared to 0.70340 just before 1970 A.D.). These peculiarities probably result from selective crustal contamination. Mixing of this magma with that of the pre-1970 period in the upper plumbing system explains the geochemical variability of the present lavas.228Ra and210Pb data and consideration of effusion rates suggest that the injection of the contaminated magma in the shallow plumbing system took place in 1950 A.D. The volume of this shallow system is rather small (∼ 0.5 km3), with a transfer and residence time of the magma of the order of a few tens of years. Finally,226Ra-230Th disequilibria provide a new method for dating prehistoric (

Activity of Mount Etna preceding the February 1999 fissure eruption: inferred mechanism from seismological and geochemical data

Abstract The February 1999 eruption of Mt. Etna took place through a fissure on the SSE flank of the cone of the summit SE Crater. This event was preceded by continuous activity since 1995, sometimes accompanied by violent outbursts from one or more of the three other summit craters (NE Crater, Voragine or Chasm, and Bocca Nuova), and finally by a series of 20 short-lived eruptions from the SE Crater between September 1998 and January 1999. These phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive, basic and gas-rich magma coming from depth. The shallow “chamber” is more likely to be a plexus of dikes, which had developed during the previous years (1995–1997), following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle. Magma injection and mixing is evidenced through geochemistry, whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms. The behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano, although not always directly linked to the relative energy of degassing phenomena.

Comparative temperature measurements on mount etna lavas: problems and techniques

Abstract On the basis of investigations carried out since 1966 at Mount Etna, the temperatures of erupting magmas are shown to be determined within a few °C in the range 1000–1200°C, by using suitable techniques and apparatus. The best measurements are obtained from sheathed thermocouples that are briefly described. Particularly, a new continuously recording multithermocouple system has been designed, tested satisfactorily, and compared with the performances of other pyrometers. However, a certain standardization of measurements is necessary to obtain and discuss the results: measurements should be made at lava vents or a few meters away and at least 30–50 cm into the flows, the highest values only being considered as significant. The magmatic temperatures and petrological characters appear closely related to the volcanic activity. In the normal state of moderate persistent activity of Mount Etna, the alkaline basic magma extrudes at a nearly constant temperature of 1080°C (corresponding to about 50% crystalline phases), meanwhile during stronger paroxysmal eruptions the magma temperature is higher (1125°C and possibly more) with a lower observed content of phenocrysts.

The 1974 eruption of mount etna

The 1974 Etna eruption that occurred on the lower, west part of the volcano is rather exceptional as it appeared independent of the central main vent activity. The products of this eruption also differ from those commonly emitted throughout historic times. They are almost aphyric and display an unusual order of crystallization, clinopyroxene being a near liquidus phase instead of plagioclase that is typical in all other recent lavas. The chemical composition is also slightly more basic than that of pyroclastics contemporaneously erupted by the summit Central Crater.These volcanological and petrological features are symptomatic of a separation at depth of the 1974 magma from the central conduit. The eruption itself, however, appears to have been governed by the general structural environment of the volcano. From this stand-point, there is evident interdependence between the various paroxysms of the past five years and it is suggested that the volcanic activity itself has direct relations with the regional tectonics. A model of the superficial structure of Mount Etna is given that best accounts for the observations.

Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires

High precision methods of sampling and measurement developed at Saint-Maur laboratory added to a critical examination of written documents pertaining to the history of Etna and Vesuvius enable us to reconstruct the path of the geomagnetic field direction in southern Italy for the past 2 000 years. This curve agrees well with that obtained from archaeomagnetism in France and may confidently be used either for geophysical purposes or for dating high-temperature-emplaced volcanic products and archaeological structures in the time span considered.

A MODEL OF THE ONSET OF THE 1991-1993 ERUPTION OF ETNA (ITALY)

Abstract Eruptive activity at Etna resumed in the early morning of 14 December 1991, preceded by a 9 month inflation and a seismic swarm of appreciable intensity. In the early hours of 15 December, eruptive vents opened on the western wall of Valle del Bove at about 2300m elevation and fed the main lava flow. The fracturing episode associated with the eruption outburst affected the principal local structural systems, trending NNE-SSW and NNW-SSE, respectively, which in recent years have produced several lava flows. We propose a model of the eruptive mechanism based on a number of seismological, volcanological and field data, discussed in the framework of the available information from ground deformation measurements. In particular, the model hypothesizes the depressurization of a shallow batch of magma, owing to the rapid magma withdrawal, which caused the downdropping of one side of the eruptive fissure by normal faulting. After a momentary stop, a shallow inflation drove the restart of the eruption by the opening of the lower edge of the fissure.

Kilometer-scale heterogeneities inside volcanoes revealed by using a set of geophysical methods: variable stress field at Mount Etna, Sicily

Abstract A multidisciplinary study involving geological and geophysical techniques has been carried out on the lower southern slope of Mt. Etna, with the aim of discovering kilometer-scale heterogeneities, which are crucial in understanding how the volcano works. In this area, faults and ancient eruptive fractures outcrop, with a NNW–SSE trend, together with volcanic structures, such as elongated hills, also trending NNW–SSE or E–W, which had never been evidenced in the Etna literature. The old landscape has been revealed by considerable erosion. Gravimetry and seismic tomography prospecting, added to geoelectric and drilling surveys, show that the morphology of the sedimentary substratum forms a N–S trending horst, limited to the east and west by depressions, where erosion products and lava flows from the overlying volcanic pile have accumulated. There is also evidence at very shallow depth (≅1 km below sea level) of an elongated body with a NNW–SSE direction, which is interpreted as a small magma chamber that has now almost completely solidified. This shallow magma system is likely to have fed the 122 BC and ≅1150 AD eruptions, which historical accounts suggest are located very close to the city of Catania.