Abstract
Plinian eruptions are characterized by high intensity and an overall steady character, and result in a stable convective column. The main processes controlling the dynamics of such steady and stable plume systems have been extensively investigated. Conversely, sub-Plinian eruptions are unsteady, as recorded by the large variability of the products and deposits. Our knowledge of the processes creating this unsteadiness on various timescales remains limited, and still requires more observations as well as theoretical and experimental investigation. Here, we focus on the sub-Plinian eruption of the Greenish Pumice (GP, 19,265 ± 105 BP), Mt. Somma-Vesuvius (Italy). On the basis of coupled geochemical and textural analyses of samples from the well-established stratigraphy of the GP deposits, we investigate volatiles (H2O, CO2, F, Cl) to better constrain the unsteady sub-Plinian eruptive style. This allows us to carry out a detailed study of the degassing processes in relation to the eruption dynamics. We find that degassing by open-system processes generally dominates throughout the entire eruption, but alternates with episodes of closed-system degassing. The fluctuating degassing regimes, responsible for the variable magma ascent rate within the conduit, are also responsible for the eruptive column instability. Volatile behavior is well correlated with textural heterogeneities of the eruptive products. Both reflect higher conduit heterogeneity than for Plinian eruptions, where we find a higher horizontal gradient in magma ascent velocity due to a smaller conduit diameter.
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Acknowledgments
We thank A. Michel for analytical support for water measurements. M. Fialin, F. Couffignal, and N. Rividi are also thanked for help support at the electronic microprobe (Camparis, Paris, France) and O. Boudouma for textural analyses by SEM (Paris, France). A. Carandente and P. Belviso helped in sampling fallout deposits. The INSU-CNRS SIMS national facility is acknowledged for isotopic analyses on MI. The manuscript benefits from the constructive review of L. Gurioli and T. Giachetti and English corrections from the associated editor Steve Self and Fran van Wyk de Vries. We also thank the executive editor, Andrew Harris, for his comments. IPGP contribution number: xxx.
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Zdanowicz, G., Boudon, G., Balcone-Boissard, H. et al. Geochemical and textural constraints on degassing processes in sub-Plinian eruptions: case-study of the Greenish Pumice eruption of Mount Somma-Vesuvius. Bull Volcanol 80, 38 (2018). https://doi.org/10.1007/s00445-018-1213-5
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DOI: https://doi.org/10.1007/s00445-018-1213-5