Timescales of mixing from diffusion chronometry on alkali feldspar phenocrysts from the Agnano-Monte Spina Eruption (4.7 ka), Campi Flegrei (southern Italy). Il Pianeta dinamico. Congresso congiunto SIMP-AIV-SoGeI-SGI, Firenze 2-4 Settembre. Rend. Online Soc. Geol. It., Suppl. n. 2 Vol 35, p. 175.

Knowledge of the timescales of magma processes, including rising, stagnation and mingling/mixing occurring in the shallow plumbing system, is crucial for the volcanic hazard assessment of an active volcano. A powerful tool to this purpose is the high spatial resolution analysis of major-, minor- and trace elements on zoned phenocrysts, leading to the definition of the diffusion profiles for some key-elements. The Campi Flegrei (southern Italy) is a volcanic district whose active portion is presently inhabited by ~500,000 people and therefore exposed to a very high risk. The application of geochronometric investigations to the Phlegrean volcanism could be thus of notable impact in the mitigation of the volcanic risk related to such an hazardous site. The present study has concentrated on the products of the Agnano-Monte Spina eruption (A-MS, 4.7 ka), the highest magnitude event in the Campi Flegrei recent activity (VEI = 4, 0.85 km3 D.R.E. of emitted magma). The A-MS eruption has been fed by trachytic magmas whose variable 87Sr/86Sr and trace elements compositions suggest mixing between two different batches. Its volcanological and petrological features make this eruption as a good case-study for the investigation of magma-chamber dynamics, as well as a reliable reference-event for possible future eruptions. The investigated A-MS samples are pumice fragments from which double-polished, 100 µm-thick thin sections have been prepared for the analytical investigations. Back-scattered electrons (BSE) images have been acquired at the scanning electron microscope (SEM) in order to identify alkali feldspar phenocrysts suitable to be analyzed, selected among those in which the zoning pattern is more evident. Alkali feldspar compositions have been determined through combined energy-dispersive and wavelength-dispersive system electron microprobe analyses (EDS-WDS-EMPA) on transects crossing the growth zones of the selected phenocrysts. This allowed to reconstruct the diffusion profiles of Ba and Sr through the growth zones, which combined with BSE observations, were used to construct diffusion models aimed at estimating the timescales of magma processes. The BSE imaging of A-MS alkali feldspars revealed complex zoning and resorption features that might be related to mixing events, thus deserving further investigations. The preliminary time estimates obtained are in the range of 35-183 years. These values can be interpreted as the time required for the magma to collect in the shallow reservoir (2-4 km) prior to the eruption.