Timescales and dynamics of magmatic processes in a Plinian eruption's feeding system: The 79 CE eruption of Somma-Vesuvius, Italy

The eruption of Somma-Vesuvius in 79 CE is considered one of the most iconic eruptions of the last two millennia. It was the first documented Plinian eruption and caused the destruction of the Roman cities of Pompeii and Herculaneum. Previous petrological studies provided useful insights into the dynamics of the magma reservoir and pre-eruptive processes, such as the episodic mafic recharge, that is considered the growing mechanism of the 79 CE reservoir. Scattered knowledge exists about reservoir properties and the magmatic processes leading to this Plinian event.

In this study, we performed high-spatial resolution analyses of zoned clinopyroxene crystals from selected eruptive units representing the phonolitic (white pumice) and tephri-phonolitic (grey pumice) magmas of the 79 CE eruption. The Mg# [molar MgO/(MgO+Fe_tot)*100] of clinopyroxene ranges between 91 and 39, with a notable compositional variation ascribable to five distinct compositional populations. The relationship among the compositional populations tracks the sequential growth of crystals in different magmatic environments, reflecting crystallization at different conditions.

The combined use of chemical data on zoned clinopyroxene and whole rocks, of isotopic data and numerical models allowed us to simulate the evolution of a primitive Vesuvius magma and constrain the physico-chemical conditions of the magmas feeding the 79 CE eruption. Magma temperatures, calculated with different thermometric and MELTS models, vary in the range 870–1120°C across the identified populations. Moreover, this approach allows identifying a vertically extended magmatic system beneath Somma-Vesuvius prior to the eruption, with various melt reservoirs at pressures of 2–4.5 kbar. These reservoirs were interconnected, facilitating prolonged crystal transfer. A phono-tephritic reservoir (at ~1010°C and >7.5 km depth) possibly constituted the lower part of the reservoir that fed the Pompeii eruption, where most of the magmatic interaction(s) took place. Diffusion modelling applied to the clinopyroxene zoning pattern allowed us to infer crystals residence times, mostly in this part of the magmatic system. The clinopyroxene crystals resided in the phono-tephritic and phonolitic magma reservoirs for 30–20 years, and most of them for less than 10 years (mostly less than 5 years) before the eruption. This time is related to the last episodes of magma recharge and crystal transfer before the eruption, and aligns with historically verified and archeologically inferred seismic precursors.