Determining the depths and timescales of pre-eruptive processes before the historical 1538 Monte Nuovo eruption (Campi Flegrei caldera, Italy)

Abstract

Campi Flegrei, Italy is one of the most populated caldera volcanoes on Earth and a detailed understanding of its sub-volcanic magma system and pre-eruptive processes are essential for effective hazard mitigation. Of particular interest are the depths of magma storage and timescales of magma recharge, as this information can inform the interpretation of volcano monitoring data that is critical to civil protection. This study places new constraints on these important aspects of the Campi Flegrei magmatic system through detailed petrological analysis of clinopyroxene crystals produced in the most recent, AD 1538 Monte Nuovo eruption, which was preceded by important precursory activity at the Earth s surface, recorded in historic chronicles.

Through backscattered electron imaging (BSE) of >200 erupted crystals, I identified four texturally distinct clinopyroxene populations in Monte Nuovo eruption deposits, characterised by their zoning patterns. Using BSE images as a guide, we characterised the full compositional diversity of these clinopyroxenes by electron microprobe analysis (EPMA). This data was input into the Python tool Thermobar (Wieser, 2022) to find equilibrium matches with published Monte Nuovo whole-rock, melt inclusion and glass data, to determine clinopyroxene crystallisation pressures and temperatures using a geothermobarometer calibrated for alkali systems (Masotta et al., 2013).

Compositional zoning preserved in the Monte Nuovo clinopyroxenes is consistent with an open magmatic system where mafic and silicic magma often mix. Our barometric modelling suggests that all the erupted clinopyroxene crystals formed at temperatures and pressures of ~986 ̊C and ~1.4-2.4 kbar. These pressures equate to a magma storage depth of ~6-11 km, in agreement with a deep melt zone identified beneath Campi Flegrei by seismic tomography (Zollo et al., 2008). The crystals do not display evidence of crystallisation at shallow depths, suggesting that the erupted magmas did not undergo prolonged storage at the depth of recent bradyseismic activity(Armienti et al., 1983). Hence, our data agree with previous studies (Stock et al., 2018b) which have suggested that recent Campi Flegrei eruptions are fed by magmas ascending directly from the mid lower crust and undergo minimal interaction with magmas that have stalled at shallow depths.

After constraining the structure of the Monte Nuovo magma system, we extracted compositional profiles across the outermost zones of clinopyroxene crystals; these rim zones are interpreted as recording the transfer of crystals from one magmatic environment to another, shortly before eruption. We used Fe-Mg diffusion modelling to constrain the timing of these pre-eruptive processes. Our results show a correlation with a period of uplift which began in the Pozzuoli region 36 years prior to the Monte Nuovo eruption. This new insight and gives a robust temporal constraint which can inform the interpretation of current monitoring data at Campi Flegrei and provide important information for future eruptive scenarios.