RICERCA ITALIANA     

Research program

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

University Co-ordinator

Università degli Studi "G. d'Annunzio" CHIETI-PESCARA - SCIENZE DELLA TERRA - CHIETI(CH)

Research Unit Leader

Brent Takashi POE

Description

The present research project has as its main objective the definition of the physical volcanology, as well as the eruption, transport and deposition dynamics of peralkaline rocks at Pantelleria. The achievement of this goal will contribute to the understanding of volcanological processes involving peralkaline magmas, which to date have been very poorly addressed in the volcanological literature. Laboratory measurements of melt viscosity, thermodynamic and electric properties will be conducted on selected peralkaline magmas from Pantelleria. Volatile content (H2O, CO2 Cl, F and S) will be measured by research unit 1 and H2O and CO2 contents will be also determined by infrared spectroscopy (FT-IR). The effect of the above mentioned volatile species on the rheological properties of this magma will be evaluated by this research unit.
Investigations of the rheological and thermodynamic properties will be conducted by measuring the viscosity-temperature relationship, glass transition temperature, thermal expansion coefficient and heat capacity. In addition, fragility and activation energy for the viscous flow will be derived from the obtained values for the measured properties. The effect of water on viscosity will be taken into account measuring the viscosity-temperature relationship in anhydrous conditions and after rehydrating the samples via piston cylinder synthesis. The results will be then confronted with the results of the FT-IR measurements regarding the H2O content in the melt inclusions. In addition, 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. Finally, electrical conductivity measurements will be carried out on rocks representative of both magma and wall rock in order to constrain the dimensions and location of magma bodies through 1-D and 2-D forward modelling of apparent resistivity in conjunction with available and future magnetotelluric data below Pantelleria.
The work will be organized as follows: The samples will be collected in collaboration with research unit 1 (Civetta Group) and the sample suite will comprise the widest compositional and textural variations comprising three selected volcanoes (the pumice cone of Cuddia di Mida, il vulcano a scudo di Cuddia Sciuvechi and the partially collapsed edifice of Monte Gelfiser, see section 2.4 of the research project for research unit 1). Sedimentological, mineralogical, textural and structural features of each deposit and their lateral and vertical variations will be investigated in collaboration with research unit 1. In particular, a thorough analisis of these deposits for variable degrees of welding is necessary in order to identify and quantify all indicative features related to both eruptive and post-eruptive processes (see relaxation geospeedometry below).

VISCOSITY
The shear dry viscosity measurements of the pantelleritic and rhyolitic melts, in the high temperature range (1600-1100°C) will be investigated with the method described by Dingwell et al (1996) using a MoSi2 box furnace and a Brookfield viscometer. Samples will then be hydrated via piston cylinder synthesis at P = 10 kbar and T = 1600 °C. The synthesis products and the starting materials will be analysed with the FTIR for H2O content and by electron microprobe. The dry and hydrous viscosity in the low temperature range (400-800°C) will be investigated using the micropenetration method, described in Hess et al. (1995). The measurements are done in), which utilizes a vertical push-rod dilatometer (BÄHR DIL802 V).
Low viscosities at superliquidus temperatures will be determined by falling sphere viscometry using the piston cylinder apparatus in the HP-HT Laboratory INGV Roma (P. Scarlato). Precision sphere displacement can be determined without corrupting the capsule or sample by high resolution x-ray radiography and digital image analysis (external lab, cost-free). Recent experiments by this RU have demonstrated quench-type falling sphere viscometry to be reliable in the range 10 to 10^4 Pa s (Misiti et al, 2005). In the high T regime, viscosity will be determined at variable pressures (up to 20 kbar) for selected samples at selected temperatures in order to gauge its pressure dependence. An accurate evaluation of the T dependence using the combined high and low viscosity data can only be made once the low viscosity P dependence has been determined. FTIR spectroscopy will be used to characterize the water content of samples both before and after viscosity measurements. If IR extinction coefficients are unavailable or their uncertainties are too large, starting materials will be analyzed by Karl-Fischer Titration in order to calibrate near IR band intensities to dissolved water concentrations.
The laboratory viscosity data will be used to accurately describe the variation of viscosity of these liquids as a function of water content, temperature and pressure over a complete range of eruptive and pre-eruptive conditions. These viscosities can then be applied to existing and future models of magmatic processes in order to constrain the rheological behaviour and eruption dynamics predicted by simulation techniques.

THERMODYNAMIC PROPERTIES
Using the Netzsch STA 449C equipment, a Differential Scanning Calorimetry method will be applied in order to obtain data for determination of glass transition temperature (Tg), and heat capacity (Cp). The heat capacity (Cp) is specific for each magma composition and varies according to the energy needed to change the structure of a melt in response to a temperature variation (Courtial and Richet, 1993). In addition, a Netzsch Dilatometer DIL 402 C (Webb et al. 1992) will be used for measuring both the Tg and the thermal expansion coefficient (α), which is a measure of the relative volume or length variation of the melt and strongly depends on composition and temperature changes. The above mentioned experimental measurements will be carry out at the Institute for Mineralogy, Petrology and Geochemistry (IMPG) of the Ludwig Maximilians University in Munich, Germany (D.B. Dingwell). All these data can contribute to determine the influence of temperature and rheological properties of trachyte to pantellerite melts.
The presence of well-defined flow morphologies on Pantelleria allows the analysis of the thermal regime of spatter, welding and subsequent rheomorphic flow. Using the techniques of Wilding et al. (1995, 1996) the thermal history of the flows will be investigated using relaxation geospeedometry. Calorimetric determinations of the hysteresis of the enthalpy relaxation will be obtained using differential calorimetry. The resulting traces, combined with calibration traces for rate cooled samples, yield cooling rates for the volcanic glasses of interest.
These techniques have been used with great success on phonolitic (Teide) and pantelleritic (Mayor Island) systems in the past (Gottsmann and Dingwell, 2001a, 2002). Using these techniques we can distinguish between primary spatter and welded air fall deposits. Using careful geometrical control in an outcrop / flow front scale we can obtain thermal models for the cooling of such flows (Gottsmann and Dingwell, 2001b).

ELECTRICAL PROPERTIES
Complex impedance spectroscopy will be performed at moderate pressure (ca. 1 GPa) in a multianvil apparatus located at the HP-HT Laboratory for Experimental Geophysics and Volcanology (Scarlato, INGV Roma) for rocks both representative of the magma and surrounding wallrock. As electrical conductivity is relatively insensitive to pressure, the data will be useful for correlation to thermal distribution, such as defining the size and location of a hot magma body within cooler surrounding rock. Some experiments will be preceded by rehydration of volcanic rocks to assess the influence of dissolved water on magma electrical conductivity. Other experiments will involve re-equilibration of the starting materials at varying pressure and temperature conditions in order to determine the influence of melt content on bulk conductivity. Ambient pressure, high-temperature electrical conductivity experiments are also proposed for which much larger samples and thus higher precision measurements can be made on coarse-grained, heterogeneous rock samples that may not be suitable for the smaller sample dimensions required in the multianvil apparatus. Such larger volume measurements can be performed under better controlled temperature and chemical environments without the aid of the high pressure apparatus. We request funding for the acquisition of a high-temperature (max. 1400°C) furnace to be located in the Dipartimento di Scienze della Terra, Università degli Studi di "G. d'Annunzio" (Poe, Novembre). The instrumentation to be used for the ambient pressure measurements will be the same as that for the high-pressure measurements such that the acquisition of complex impedance spectra will be possible. The larger sample dimensions, however, will allow the application of a four-electrode technique, which will eliminate any contributions to the spectra due to the sample-electrode interface. In this manner, we will be able to cross-check the spectra obtained by the necessary two-electrode technique from our high-pressure measurements for any possible misinterpretation of those data. After a sufficient amount of data has been collected for a variety of rock types at various thermodynamic conditions, forward modelling of the conductivity data will be used to generate 1-D and 2-D apparent resistivity sections for different magma/wall rock configurations.

To satisfy the objectives of this unit's research project, the following five tasks are expected to be carried out:
1) collection of samples (basaltic and trachyitic-pantelleritic) in collaboration with research unit 1
2) viscosity measurements at both high and low temperatures and ambient and high pressures, for samples both anhydrous and hydrous, using the above-described methods
3) both calorimetric and dilatometric measurements to determine Cp, Tg and thermal expansion coefficient
4) relaxation geospeedometry measurements to determine thermal histories and cooling rates of flow deposits
5) complex impedance measurements to determine electric conductivities of both magma and wallrock for 1-D and 2-D forward calculations of magma chamber size and depth
A comprehensive analyses of all the obtained data will permit interpretations that likely will contribute to understand the eruption behaviour of pantelleritic magmas, presently poorly known.