RICERCA ITALIANA     

Research program

Genesis, evolution, eruptive dynamics and depositional processes of peralkaline magmas at Pantelleria.

University Co-ordinator

Università degli Studi di NAPOLI "Federico II" - SCIENZE FISICHE - NAPOLI(NA)

Research Unit Leader

Lucia CIVETTA

Description

The proposed research project is aimed at defining: a) the genesis of the pantelleritic magmas of Pantelleria, b) the magma chamber processes, c) the eruptive dynamics, the transport and depositional mechanisms, and the sin- and post-depositional processes of the peralkaline magmas, d) the relations among magma characteristics, processes operating in magma chambers, and sin- and post- eruptive processes. To reach these goals three different types of pantelleritic volcanoes will be studied in details: a pumice cone, a shield volcano and a partially collapsed volcano.
The objective of the project will be pursued through field and laboratory investigations on the three selected volcanoes, each representing one of the different types of activity previously described. The selected volcanoes are Cuddia di Mida pumice cone (VI silicic cycle; ca. 5 ka; Civetta et al., 1988; Mahood and Hildreth, 1986), Cuddia Sciuvechi shield volcano (IV silicic cycle; ca. 17 ka; Civetta et al., 1988), and Mount Gelfiser partially collapsed edifice (IV silicic cycle; ca. 17 ka; Civetta et al., 1988). The composition of the magmas extruded by these volcanoes varied from first erupted pantellerite to last erupted pantelleritic-trachyte or trachyte.
Field work will be preceded by investigation of aerial photographs aimed at recognising all the morphological and structural characteristics of the selected volcanic edifices. On the basis of these investigations, preliminary maps will be constructed and then revised in the field. Field work will be carried out using the standard methodologies of the geological survey. Firstly the field work will be devoted at reconstructing in detail the stratigraphic sequences of the deposits of each of the selected volcanoes. For each recognised deposit, the sedimentological, mineralogical, textural and structural characteristics will be defined, as well as their horizontal and vertical variations. Lavas, spatter-fed lavas and lava-like bodies will be further investigated in order to define, also using the results of the aerial photographs studies, their length, width and thickness. The components of the non-welded pyroclastic bodies will be defined, and their maximum size will be measured in the largest possible number of exposures. Particular attention will be devoted to the analyses of the variably welded deposits to recognise and measure all the elements, very often not obvious, indicative of the eruption and welding processes. The deformation features (fractures and faults) of the volcanic edifices, particularly of the partially collapsed Mount Gelfiser, will be studied in details in order to detect any indication on the condition and kinematics of the deformation.
On the basis of the results of all the field work, samples will be collected to carry out laboratory analyses.
Samples from non-welded pyroclastic deposits will be collected in order to carry out sedimentological analyses through mechanical sieving. On selected samples, components analysis in variable granulometric fractions will be performed. Morfoscopic ananlyses at SEM will be performed on samples representative of the different recognised facies. The results of these analyses will be used to define the fragmentation dynamics and the transport and depositional mechanisms.
Samples of variably welded deposits will be collected through the stratigraphic sequences. On selected samples will be carry out density measurements and determination of the three axes of the juvenile fragments (pumice and bombs) deformed during welding. The results of these analyses will contribute to define the welding processes and their horizontal and vertical variations.
Laboratory measurements will be also carried out to define the chemical properties of the erupted magmas, and their variation through time. The definition of these characteristics is crucial to understand the differentiation processes and the rheological behaviour (goal of Research Unit 2) of the magmas feeding the different and complex eruption processes. Furthermore, the definition of these properties will also shade light on the post-extrusion behaviour of the erupted materials.
For whole-rock samples, major elements content will be determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES), and trace elements by inductively coupled plasma-mass spectrometry (ICP-MS) at the Centre de Recherches Petrographiques et Geochimiques of Nancy, Cedex (France). Major elements content of mineral phases, glass of the groundmass and glass inclusions in minerals of selected samples, will be analysed by electron microprobe, by combined WDS-EDS technique, using a CAMECA SX50 electron microprobe at the Centro di Studi per il Quaternario e l'Evoluzione Ambientale-CNR (Rome).
H2O and CO2 contents will be determined by infrared spectroscopy (FT-IR) on glass inclusions trapped in crystals and on the matrix glass, using a Nexus Nicolet spectrometer and Continuum microscope available at Osservatorio Vesuviano-INGV, Naples. Knowledge of volatiles concentration will provide important information on both the thermo-barometric conditions of magma crystallization and the degassing history during magma evolution. Furthermore, evaluation of the volatiles content of magmas feeding variable eruption phases, is one of the key elements to understand eruption dynamics.
Geochemical and isotopic analyses (Sr, Nd, Pb measured at the Mass Spectrometry Lab. of the Osservatorio Vesuviano, INGV, Naples) will be carried out on trachytic-pantelleritic rocks and on basaltic rocks, at different scale: whole-rock, separated minerals, and core-rim of selected minerals. The isotopic analyses will be performed on the three volcanoes previously described and on four basaltic units. The selected minerals will be mostly feldspars and clinopiroxenes for the silicic units, and plagioclase and clinopiroxene for the basaltic rocks. In order to understand the genesis of pantellerites, the proposed research will be also devoted to basaltic units of Pantelleria of different ages (i.e.120, 80, 29 and less than 5 ka). The selected silicic units are all characterised by a compositional variation during eruption, from first erupted pantellerite to trachytic pantellerite and trachyte; some contain enclaves rich in crystals, of femic trachytic composition, that offer an unique opportunity to study the less differentiated portions of the magma chamber, and the process responsible of the entrapment of drops of femic trachytic magmas in the pantelleritic melt.
The high-resolution isotopic analyses (at the scale of whole rock, crystal population, single crystals) will allow us to: a) distinguish the effects of the open system processes, such as crustal contamination and recharge; b) track changes in the magma composition through time, from the deduced composition in equilibrium with a given crystal; c) determine the different degree of aggregation of a population of crystals from different environments, rather than crystallise in a single magma chamber.
In conclusion, the interpretation of data relative to the silicic and basic units of different age willl allow to constrain the genesis and differentiation of silicic rocks, the processes occurring in magma chamber and their evolution trough time, to define the relations among magma composition, processes in magma chamber, eruptive dynamics, transport mechanism and post-depositional processes, and to geochemically characterise the mantle source region.