Magma residence times from diffusion chronology on zoned alkali feldspar phenocrysts from the Agnano-Monte Spina Eruption (4.7 ka), Campi Flegrei caldera (Napoli, southern Italy)

The zoning patterns of crystals are a valuable tool to reconstructs processes occurred in a magmatic system prior to eruption. In particular detailed analyses of the mineral zoning and modeling based on chemical diffusion laws allows to determinate the durations of magmatic processes, providing important constraints for volcanic hazard of active volcanoes, such as the Campi Flegrei (Southern Italy) caldera. The Agnano Monte Spina eruption (A-MS; 4.7 ka; VEI = 4; 0.85 km3 D.R.E), the highest magnitude recent event in this area, produced pyroclastic deposits with variable 87Sr/86Sr and trace elements, suggesting magma mixing between two evolved end-members. Alkali feldspar phenocrysts were hand-picked from samples of A-MS pumice under the binocular microscope, and thin sections were prepared for electron microprobe analyses. Based on back-scattered electron (BSE) images were selected phenocrysts displaying a suitable zoning pattern near the crystal margin, likely representing the last mixing event prior to eruption. The zoning patterns have been analysed by using two different approaches: (1) quantitative BaO compositional profiles by point analyses (at a distance of ~10mm) across the crystals and (2) gray-scale swath profiles, from accumulated BSE, taken along a short transect crossing growth discontinuities parallel to the point profiles. In both cases, each profile was interpolated through a non-linear Boltzmann fit curve with Mathematica® software. However, for the second approach also ImageJ® software was used in order to extract a numerical gray-scale value. Our estimates of diffusion times based on a temperature of 930°C are within tens to hundreds years but showed drastic differences between the two types of zonation profiles: (1) zonation gradients based on quantitative point analyses gave residence times from 8 to 1195 years (2) gray-scale swath profiles gave residence times between 1 and 171 yrs. Thus, residence times from the gray-scale modeling are always younger by about an order of magnitude compared to those derived from BaO concentration profiles. The reason for this deviation is that the spatial resolution of quantitative point measurements is too low and thus interpolated profiles are much smoother than the grayscale profiles. Because of this artifact shorter diffusion times derived from gray-value profiles should be more reliable. The majority of crystals show residence times of < 60 years. However, a few crystals with significantly longer residence (up to 160 years) also occur. Volcanological and geochronological data on the activity in the Agnano-San Vito area predating the Agnano-Monte Spina eruption suggest that the timescales estimated by diffusion chronology are similar to the time intervals observed between eruptions occurred the last 6 kyrs at Campi Flegrei 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.