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 <60 years, only few profiles gave diffusion times up to 180 years (Fig. 2). BaO point analysis profiles, by contrast, give residence times up to thousands of years. The higher spatial resolution of gray-scale and X-ray profiles results in steeper gradients, providing lower values of diffusion time by almost an order of magnitude compared to profiles based on quantitative point measurements. Thus, shorter diffusion times derived from gray-value swath profiles are more reliable. Based on volcanological and geochronological data a centuries to decades timescale is in agreement with the timing of eruption-triggerung processes preceding the A-MS eruption. In particular we argue that the timescales estimated by diffusion chronology are similar to the inferred time intervals occurred between eruption, and thus may represent the reactivation time of a magma that was residing in a shallow reservoir, after the influx of a new magma batch that triggered the eruption. Such short timescales thus represent the final reactivation/remobilization of a magma from shallow depth in the A-MS plumbing system, after longer residence as testified by the complex core-rim interior zoning of sanidine phenocrysts and our previously obained U-Th isotope dating on phenocryst minerals (Arienzo et al., 2011). Reference: Arienzo I, Heumann A, Wörner G, Civetta L, Orsi G (2011) Time scales of magma crystallization and storage prior to the Campanian Ignimbrite eruption (Campi Flegrei, Italy). Earth Planet Sic Lett 306:217-228