RICERCA ITALIANA     

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

Università degli Studi di Napoli "Federico II"

Abstract

The island of Pantelleria, the type locality for pantellerite, is characterised by the close association of weakly-alkaline basalts, and peralkaline rhyolites and trachytes. The peralkaline eruptions span over a wide range of size and style. As a consequence, Pantelleria offers a good opportunity to study the genesis and differentiation of peralkaline magmas, magma chamber processes, eruption dynamics, transport and depositional mechanisms, and their relationships. All these scientific topics are still strongly debated in the international literature. The debate also arises from the limited knowledge of the rheological and thermodynamic properties of the peralkaline magmas, less diffuse than magmas of other composition. To contribute to the solution of the open problems at Pantelleria and, more in general, at all peralkaline volcanoes, the present project includes a detailed volcanological study of three volcanoes, each representative of one of the different types of volcanoes on the island: a pumice cone, a shield volcano and a partially collapsed edifice. To define the peralkaline magma genesis we will study also the basaltic magmas of Pantelleria. A field survey will be carried out and followed by collection of rock samples on which sedimentological, geochemical (major and trace elements, volatile content), isotope (at a different scale: whole rock, minerals, single mineral, core-rim of selected minerals) analyses will be performed. Furthermore, we will also measure or evaluate thermodynamic and rheological parameters, such as: anydrous and hydrated viscosity at high and low temperature and pressure, heat capacity, glass transition temperature, thermal expansivity coefficient, activation energy and fragility of pantelleritic melts. New methodologies and technologies will be used to carry out some of the planned analyses. The expected results will allow us to define the history of peralkaline magmas from source, through magma chamber and eruption, to deposition. <<<

Principal Investigator

Lucia CIVETTA Università degli Studi di NAPOLI "Federico II"

Research Objectives

The present research project has as main objective the definition of the history of peralkaline magmas at Pantelleria, from source, through magma chamber and eruption, to deposition. Achievement of this goal will contribute to the understanding of magmatological and volcanological processes involving peralkaline magmas, very poorly addressed or strongly debated in the volcanological literature. Particular objectives are the definition of: a) genesis of peralkaline magmas and their differentiation processes; b) possible effects of crustal contamination on mantle-derived magmas; c) shallow magma chamber processes; d) pre-eruption volatiles content; e) P and T of crystallisation; f) rheological properties of peralkaline magmas; g) eruption dynamics, and transport and deposition mechanisms of the products of eruptions building up small volcanic edifices.
Furthermore, the island is one of the eight Italian active volcanoes with a widespread fumarole activity. The last eruption has taken place in 1891, few miles away from the northern coast, in the Sicily Channel. The island is home for about 10,000 people and population increases during summer. Persistent activity of the volcanic system and population exposed to the volcanic hazards in case of renewal of volcanism, make the volcanic risk of the island quite significant. In this framework it is important to assess the type of hazards which could occur investigating the eruption dynamics and the characteristics of the feeding magmas. <<<

Timescale

24 months

National and international background

Genesis of pantelleritic magmas and relationships among pantelleritic magma rheology, eruption dynamics, and transport and deposition mechanisms are topics still debated in the literature. As a matter of fact, the rheological properties of pantelleritic magmas are poorly known and the controversy on their genesis by crystal fractionation from a basaltic parental magma or partial melting of gabbroic cumulate or underplated basalts is still alive.
The island of Pantelleria, the type locality for pantellerite, is characterised by a bimodal association of mildly alkalic basalts and peralkaline trachytes and rhyolites, and by eruptions of peralkaline magmas covering a wide range of intensity and style. Therefore, it offers a good opportunity to investigate pantelleritic magma genesis and differentiation, magma rheology, eruption dynamics, transport and deposition mechanisms, and their relations.
The island is the emergent portion of a volcanic edifice that rises about 1,000 m above sea floor, in the Sicily Channel. Pantelleria is composed dominantly of volcanic rocks which include lavas and pyroclastic deposits, varying in compositions from pantellerite, through pantelleritic trachyte and comenditic trachyte, to mildly alkalic basalt, in order of decreasing abundance (Civetta et al., 1998). Felsic rocks range in age from 324 to 4 ka (Mahood and Hildreth, 1986; Civetta et al., 1984; 1988), whereas exposed mafic rocks are dated at about 118, 83, 29 and less than 10 ka (Civetta et al., 1984).
The structural and volcanological features of Pantelleria, as well as the mineralogical and geochemical characteristics of the volcanic rocks, have been the topics of a large number of scientific articles (Carmichael, 1962; Rittmann, 1967; Korringa and Noble, 1972; Villari, 1974; Wolff and Wright, 1981; Mahood and Hildreth, 1986; Civetta et al., 1988; 1998; Orsi et al., 1991a, b; Stevenson and Wilson, 1997).
The structural setting of the island is defined by both tectonic and volcano-tectonic lineaments. A NE-SW tensile fault system divides the island into two sectors and most likely represents a crustal discontinuity transverse to the Rift. The north-western sector includes most of the exposed basaltic rocks, whereas the south-eastern sector includes silicic peralkaline rocks. The north-western sector has been affected only by NW-SE crustal fractures through which mafic magmas have reached the surface. In the south-eastern sector, eruption of differentiated magmas and occurrence of calderas, suggest that crustal magma chambers were established, probably at the intersection of the main tectonic lineaments. The volcano-tectonic features of the island include caldera collapses and resurgence of the youngest caldera. Resurgence of the youngest caldera has taken place with uplifting and tilting of the Montagna Grande block, through a simple-shearing mechanism (Orsi et al., 1991a).
The volcanic history of the island is characterised by large explosive eruptions alternating with periods dominated by less energetic eruptions. The volcanic history since the Green Tuff eruption has been subdivided into six silicic cycles, sometimes intercalated with basaltic eruptions. The Green Tuff, representative of the first silicic cycle, is the product of a complex eruption and includes ignimbrites, and fall and surge beds. The chemical composition of the tuff varies from the base upwards from pantellerite to comenditic trachyte. All the other silicic cycles, dated at around 35-29, 22, 20-15, 14-12 and 10-4 ka, respectively (Civetta et al., 1988), are characterised by eruptive products ranging in composition from pantellerite to pantelleritic trachyte, or to comenditic trachyte. For many cycles it has been demonstrated that the most differentiated magmas were erupted earlier. This has been interpreted as the consequence of eruptions tapping a zoned magma chamber at progressively deeper levels during each eruptive cycle.
The genesis of the peralkaline magmas has been investigated through geochemical and isotopic studies on basalts, trachytes and pantellerites (Lowenstern and Mahood, 1991; Civetta et al., 1988, 1998). Lowenstern and Mahood (1991) suggested that pantelleritic magmas were derived by partial melting of an alkaline gabbro, previously formed at the base of the magma chamber by accumulation of mineral phases segregated from alkali basaltic and hawaiitic magmas. On the other hand, Civetta et al. (1998) proposed that the Pantelleria peralkaline suite might result from fractional crystallization of an alkali basaltic parental magma. Partial melting of an alkaline gabbro has been also recently supported by Avanzinelli et al. (2004), on the basis of clinopyroxenes structural data. Also a debate exists on the pre-eruptive H2O content (Lowenstern and Mahood, 1991; Kovalenko et al., 1993), a fundamental parameter of the rheological properties of pantelleritic magmas.
As regards the mantle source region, Civetta et al. (1998) and Mahood and Baker (1986), suggested the occurrence of an heterogeneous mantle source beneath Pantelleria. This source, isotopically similar to those of Linosa, Etna, Iblei, and Ustica, likely represents the asthenospheric mantle below the African plate. Conversely, melting of the subcontinental lithospheric mantle has been suggested for the Pantelleria basalts by Esperanca and Crisci (1993).
It follows that many problems still remain open, such as genesis of the pantelleritic magmas, type and history of the mantle source, and processes operating in the Pantelleria magma chamber(s).
Poorly known also are the eruption dynamics and their relations with magma composition, magma rheology, magma chamber processes, as well as transport and deposition mechanisms. Eruptions fed by peralkaline magmas are the result of a complex combination of physico-chemical proprierties and processes, resulting from changes in pressure, temperature, density, bubble distribution, volatile and crystal content during magma storage and ascent. High-magnitude events form widespread and variably welded pyroclastic sheets, while medium- to small-magnitude eruptions generally build up small and complex volcanic edifices (Schmincke, 1974; Wright, 1980; Wolff and Wright, 1981; Orsi and Sheridan, 1984; Mahood and Hildreth, 1986; Civetta et al., 1988; Orsi et al., 1989; 1991b; Houghton et al., 1992; Stevenson and Wilson, 1997).
At Pantelleria, the peralkaline rhyolitic volcanoes can be grouped in three basic types: pumice cones, shield volcanoes and partially collapsed edifices. Pumice cones are small features composed of pumice and bombs fallout layers with intercalation of welded layers. Orsi et al. (1989, 1991b) have described the Serra della Fastuca Tephra, the deposit of one of the largest pumice-cone forming eruptions, and have calculated the physical parameters of the eruption. Stratigraphy, and sedimentological and textural characteristics of the deposits of the shield volcanoes suggest that the eruptions began with explosive phases producing fallout deposits, continued with generation of spatter-fed agglutinates, and ended with extrusion of massive lavas (Orsi et al., 1989; 1991b; Stevenson and Wilson, 1997). Typical shield volcanoes are Cuddia Mueggen, Cuddia Patite and Cuddia Sciuvechi. Recently, Stevenson and Wilson (1997) have investigated in detail both physical volcanology and eruption dynamics of the small rhyolitic edifice of Cala Tramontana by field, experimental and theoretical approaches. The partially collapsed edifices, whose best examples are Mt. Gelkamar, Mt. Gelfiser, Fossa del Russo, Mt. Gibile and Cuddia Randazzo-Khaggiar, have not been studied in details.
To contribute to solve the open problems at Pantelleria and, more in general relatively to the peralkaline magmas, detailed volcanological studies and measurements of chemical and physical parameters are needed. Measurements can be performed also by using new methodologies and technologies. Some of those, particularly relevant for the proposed project, will be briefly summarized.
Isotope geochemistry techniques in the last years have greatly improved, mostly due to the use of new generation multicollector mass spectrometers and of mechanical microsampling instruments, which permit precise analyses of small portions of minerals. Important questions on source, genesis and differentiation (i.e. crystal fractionation, magma mingling/mixing, crustal contamination) of the Pantelleria magmas, can be addressed by investigating the isotopic record within rocks and minerals with the use of the new technologies.
Silicate-melt inclusions (MI) provide important information on the magma evolution and its volatile content (Johnson et al., 1994; Lowenstern, 1995). In particular, composition and concentration of volatile components in MI within variable crystals, allows the estimation of the pre-eruptive volatile evolution in the magma, its depth of equilibration and ascent history. Such information for pantelleritic magmas, at present, is poorly known and debated (Lowenstern and Mahood, 1991; Webster et al., 1993; Kovalenko et al., 1993; Lowenstern, 1994).
The peralkaline magmas are characterized by the lowest viscosities among the magmas of the peralkaline suite. The viscosity of these magmas is also rediced by the presence of high quantity of volatile content.
To accurately parameterise the viscosity of a hydrous natural silicate melt, both high and low T data are required, such that rheological properties can be interpolated to eruptive temperatures. At low T, and thus high viscosities, in hydrated and anydrous condtions, the micropenetration technique is used. H2O exsolution in this regime is negligible and thus the measurements can be made at atmospheric pressure. However, at higher T, and thus low viscosities H2O exsolution is unavoidable and samples must be confined to high pressures in order to maintain a fixed volatile content. Thus,in hydrated conditions and high temperature, the falling sphere viscometry is required. Because such experiments are carried out at high pressure, to mantain the H2O content in the sample, the P dependence of the viscosity must also be determined. In such a way it is possible to correlate the high T viscosity data with a low T viscosity data. The third metodology used to study the viscosity of anidrous samples is the viscometer at concentric cylinders, which works in the temperature interval 1000-1600°C. The combination of data resulting from these three different methodologies can be used to create a general equation for viscosity as a function of temperature, composition of the different products analysed and volatile content, which will then be applied to specific eruptive processes related to Pantelleria.
Relaxation geospeedometry (Wilding et al, 1995; Wilding et al, 1996) will be applied to extract thermal history of the flows, and differential calorimetry will be used to measure the hysteresis of the enthalphy relaxation in order to constrain cooling rates.The electrical properties of rocks also vary strongly with with P and T, and thus laboratory data can be useful for interpreting existing and future field-based apparent resistivity data. Electrical conductivity is strongly temperature dependent, and thus can be used to detect in field prospections small volumes of magma such as a dike or partially molten rocks. Thus, laboratory data can be used in the interpretation of geophysical data not only to estimate temperature contrasts, but also chemical and physical properties of the magma itself such as bulk chemistry, % melt and H2O content. <<<