Rheology of primitive alkaline basalts: constraints on the pre-eruptive system of Campi Flegrei (Naples, Italy)

The knowledge of the rheology of magmas at high pressure and temperature is of fundamental importance to model their mobility, ascent rate and volcanic activity at surface. In this study, we determined the viscosity and the atomic structure of the nominally anhydrous APR16 alkali basalt representative of the least-evolved magmas of Phlegrean Volcanic District (Phlegrean Fields, Italy; Bonechi et al., 2017). The sample is a lithic fragment belonging to the Solchiaro hydromagmatic tuff of Procida island, characterized by a porphyritic texture (P.I. 12% volume) with phenocrysts of olivine (Fo85-80), occasionally hosting Cr-spinel grains (Cr# ~0.50), and clinopyroxene (Wo46-48 En46-31 Fs6-23), and a groundmass made of olivine, clinopyroxene, plagioclase (An79-52), Ti-magnetite (Usp4.1-4.4). The APR16 has [Mg/(Mg+Fe)] 0.66, 49 wt% SiO2, 4.4 wt% of total alkali (Na2O+K2O) and ~12 wt% of CaO. The anhydrous glass was prepared at Bayerisches Geoinstitut by using a gasmixing furnace. The rock powder was melted at 1400 °C for 15 minutes at atmospheric pressure and oxygen fugacity buffered at Nickel-Nickel Oxide level using a CO/CO2 gas mixture. The viscosity of glassy APR16 was determined by the in situ falling sphere technique at P of 0.7-2.5 GPa and T of 1335-1700°C using the Paris- Edinburgh press available at the 16BMB beamline at the Advanced Photon Source (Argonne National Lab, Illinois, USA). The falling velocity of the Pt probing spheres was measured by X-ray imaging using a high-speed camera recording 500 frames per second. In addition, analyses of the melt structure were performed by using multi-angle energy dispersive X-ray diffraction technique. Preliminary data show the viscosity being in the range of 0.1-1 Pa·s at the target P and T. These viscosity values can be used to model the mobility and ascent velocity of primitive alkali basalts considered to be parental magmas at Campi Flegrei with important implications for the volcanic hazard of the area over time.