Ba-zonation modelling on sanidine phenocrysts from the Agnano-Monte Spina eruption (4.7ka), Campi Flegrei caldera (Napoli, southern Italy)

We applied Ba diffusion chronometry to sanidine phenocrysts from the trachytic Agnano-Monte Spina eruption (A-MS ~4.7 ka) in order to constrains the time between reactivation and eruption of magma batches in the Campi Flegrei caldera (CFc, Napoli, southern Italy), one of the most hazardous volcanic areas on Earth. The A-MS products display variable 87Sr/86Sr and trace elements features that suggest magma mixing between two magma end-members. Resulting zonation patterns in phenocryst minerals are an ideal case to study timescales of magma mobilization at the CFc. Ba zonation profiles of 50 sanidine phenocrysts (Fig. 1) have been determined through combined energy-dispersive and wavelength-dispersive electron microprobe analyses (EDS-WDS-EMPA). We focused on compositional breaks near the rim of the crystals that represent the last mixing event prior to eruption. Profiles were measured by approaches: (1) quantitative BaO point-measurements at 10 μm spatial resolution, (2) gray-scale swath profiles from accumulated BSE images and (3) Ba X-ray scans. Since Ba dominates the backscattered electron intensities in sanidines, grayscale gradients extracted from the images using ImageJ® are a diffusive tracer proxy. Each profile from the different approaches was interpolated through a non-linear Boltzmann fit curve with Mathematica® software. We always choose the steepest gradients close to the crystal rims. However, any effects from cutting angles or crystal orientation always give longer apparent diffusion times. Our diffusion time estimates are thus minimum values. Gray-scale swath profiles and X-ray scans modelled for 930°C give short diffusion times of