The ultimate 1631 CE sub-plinian eruption of Mt. Vesuvius: hints on pre-eruptive magmatic processes from zoned clinopyroxenes

The 1631 CE eruption is the most violent and destructive event of the last millennium at Mt. Vesuvius, followed by a persistent small magnitude activity that has lasted until the 1944 CE. Selected units of the proximal stratigraphic section cropping out at Cognoli di Ottaviano (Monte Somma), <2 km from the current Vesuvius crater, have been sampled. To retrieve information on types and timescales of magmatic processes and to possibly link them to precursory phenomena, the chemical composition of clinopyroxene crystals has been acquired. Mineral chemistry (Si, Ti, Al, Fe, Mg, Mn, Ca, Na and Cr) has been obtained on the core and the rim of 50 unzoned crystals well-characterized in terms of texture. Mineral microanlaysis were realized along core-to-rim transects, varying from 40 μm to 500 μm in length, in 59 zoned crystals using a beam diameter of 2 μm and 1 μm spacing between individual points.

The mineral composition allows to characterize the zoning patterns and provides information on the environments in which crystals grew. About 70% of the zoned crystals show patchy zoned cores with-or-without- complex zoning in the outer bands. The other crystals show normal zoning, normal-to-reverse zoning, reverse zoning and complex zoning (e.g., oscillatory zoning). Zoned clinopyroxenes have a diopsidic/Fe-diopsidic composition (Wo_52-48-En_47-30-Fs_17-5). Mg# [molar Mg²⁺/ (Mg²⁺ +Fe_tot) *100] of the zoned clinopyroxene ranges between 90 and 62, with some differences observed in the different bands of the crystals.

The zoning pattern shows two or more bands with constant Fe-Mg and/or Al and/or Ti composition separated by both either sharp or diffuse boundaries. The occurrence of bands of constant composition in the zoning patterns of minerals suggests that crystal growth occurred for certain several periods of time under stable conditions, separated by events involving fast changes of thermodynamic variables (pressure, temperature, oxygen or water fugacity). Different compositional populations have been distinguished in the clinopyroxene zoning pattern, e.g., through the Mg# parameter, but also other elements concentrations such as Al₂O₃. Each population results from growth under a Magmatic Environment (ME), namely specific thermodynamic conditions. The connections between MEs recorded by clinopyroxene zoning pattern allow to accurately reconstruct the history of the magmatic processes across the clinopyroxene growth. As a whole, the zoned clinopyroxenes from the 1631 CE eruption record a huge variety of MEs, testifying to a complex history of crystallization under different conditions, reflecting long-lasting magmatic processes from mantle depth to shallow reservoir(s).