Influence of Water on the Viscosity of Magmas from Vesuvius and Phlegrean Fields Volcanic Systems

Daniele Giordano (romano@uniroma3.it)¹, Claudia Romano (brent.poe@uni-bayreuth.de)², Paolo Papale (papale@dst.unipi.it)³, Kai Uve Hess¹ & Don Dingwell (don.dingwell@uni-bayreuth.de)¹

¹ Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, Germania

² Dipartimento di Scienze Geologiche, Facoltà di Scienze M.F. N. Università di Roma Tre, Largo San Leonardo Murialdo, 1 00146 Roma, Italia

³ Dipartimento di Scienze della Terra, Università di Pisa, Italia

The Phlegrean Fields caldera and the Vesuvius Volcano, located in Southern Italy west of the city of Naples, are volcanoes with highly destructive eruptions in their history, and very high danger potential in case of reactivation. Understanding how the magma evolves with time and predicting future eruptive scenarios for these two systems, is thus crucial for the hazard assessment within and around the Naples area. Adequate numerical simulation of such scenarios requires detailed knowledge of the viscosity of the magma. In this study, we have performed an extensive experimental analysis of the viscosities of selected magmas of the Phlegrean Fields system and of the Vesuvius volcano to quantitatively define the volcanic scenarios of these two systems. The glass samples were hydrated via piston cylinder synthesis. The water contents of the run products were subsequently determined by IR spectroscopy. The low temperature anhydrous and hydrous viscosities were measured using a modified micropenetration method in the range of viscosities between 10^8.45 to 10^11.35 Pa s, at 1 atm pressure and in the temperature ranges of 404-814°C. The high-temperature viscosity data for the anhydrous samples were obtained by concentric-cylinder viscometry. The addition of water to the melts results in a large shift of the viscosity-temperature relationship to lower temperatures. The viscosity drops dramatically between 0 and 1 wt% H₂O added to the melt, then tends to level off at higher water content. Low- and high-temperature data have been fitted using VFT-type equations, which have then been used to parameterise the viscosities as input into modelling of the magma response in eruption scenarios. Numerical simulations of magma ascent predict eruptive conditions in agreement with those occurred at the two volcanoes. Our new viscosity parameterizations should therefore be used in future simulations aimed at the definition of the volcanic hazard in the Neapolitan area.

Fe-Al Disordering Experiments on Natural Epidotes: A Single-crystal X-ray Diffraction Study

Gabriele Giuli¹, Paola Bonazzi (pbcry@steno.geo.unifi.it)² & Silvio Menchetti (crystal@cesit1.unifi.it)²

¹ Dip.Scienze della Terra, Università di Camerino, Italy

² Dip.Scienze della Terra, Università di Firenze, Italy

Here we present preliminary results on Fe disordering experiments for natural epidotes. Two crystals were selected coming from the Ellycott city granite pluton (Canada) and Capo Calamita skarn deposit (Elba Isle, Italy) (0.77 and 1.0 Fe a.p.f.u. respectively). Annealing experiments were carried out in air at 600, 700 and 800°C. After each run, single-crystal XRD data were collected on a CAD4 diffractometer (MoK(alpha) radiation) and structure refinements were performed using the program SHELXL93. Disagreement factors (R_obs) are in the range 1.7-1.8%, and the e.s.d.s of the occupancies are less than 0.01 a.p.f.u. No evidence of dehydration or alteration was observed even after two month annealing time. The resulting occupancies are compared with the kinetic experiments by Fehr & Heuss-Assbichler (1997) and the data for synthetic epidotes by Giuli et al. (1999). The data so far obtained point to a much slower kinetics than previously reported in the literature.

Fehr KT & Heuss-Assbichler S, N. Jb. Miner. Abh, 172, 43-67, (1997).

Giuli G, Bonazzi P & Menchetti S, Amer. Miner, 84, 933-936, (1999).

Iron and Magnesium Local Environment in Synthetic Orthopyroxenes: A Combined XANES and Rietveld Study

Gabriele Giuli¹, Eleonora Paris (paris@camars.unicam.it)¹, Wu Ziyu¹, Annibale Mottana (mottana@uniroma3.it)² & Fritz Seifert³

¹ Dip. Scienze della Terra, Università di Camerino, 62032 Camerino (MC), Italy

² Dipartimento di Scienze della Terra, Terza Università di Roma, Roma, Italy

³ Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, Germany

The results of a combined XRD and XANES study of synthetic orthopyroxenes in the entire join enstatite-ferrosilite (En-Fs) are reported. Syntheses were carried out by means of a piston-cylinder apparatus. Starting material, consisting of a finely ground mixture of olivine + quartz, was held at 1000°C and 20 kbar for 24 hours. Run products have been analyzed by electron microprobe and characterized by powder XRD. A Rietveld structural refinement has been carried out on the entire set of samples using the GSAS program. Cell parameters a₀ and b₀ were found to increase linearly with Fe content, whereas c₀ displays a small deviation from linearity. Mean octahedral distances increase linearly with Fe occupancy. Fe and Mg k-edge XANES spectra were collected respectively at the beamlines 4-1 and 3-3 of SSRL synchrotron radiation center (Stanford, USA). The sets of spectra at both edges display continuous variations ascribable to the changing environment around the photoabsorbers as a function of the Fe-Mg substitution. Structural data obtained from XRD refinements have been used to calculate theoretical XANES spectra both at the Fe and Mg K-edges. Theoretical spectra, calculated by means of the CONTINUUM code, perfectly reproduce all the features of the experimental ones and allow to understand how the experimental XANES features reflect changes in the structural and chemical environment around the photoabsorbers. Previous works (Paris and Tyson, 1994; Giuli et al., 1998) already showed the possibility to extract information on the Fe distribution and local chemistry from Fe K-edge XANES spectra. The new sets of theoretical Fe and Mg-XANES spectra obtained using larger clusters and the complete En-Fs synthetic series of structurally characterized samples allows to interpret and quantify the changes observed in the experimental spectra as a function of Fe-Mg substitution and site occupancies.

Paris E & Tyson TA, Phys. Chem. Min., 21, 299-308, (1994).

Giuli G, Paris E, Mottana A & Seifert F, Plinius, 20, 129-130, (1998).