Controls on the architecture and dynamics of the Campi Flegrei magmatic system in the last 15 kyr

Abstract

Caldera-forming volcanoes typically remain restless over long time periods following major collapse events; this continued activity provides a unique hazard as subsequent vents are disseminated across a wide area within the pre-existing collapse structure. One such volcano is Campi Flegrei (Naples, southern Italy), considered one of the most hazardous volcanoes on Earth having produced >70 eruptions in the past 15 kyr, and currently showing significant signs of unrest within a densely populated part of Europe. Post-15 ka eruptions span a range of eruptive styles (effusive lava domes to Plinian events) and compositions (shoshonite to trachyte-phonolite), which broadly correlate with the spatial and structural location of vents within the large caldera. These compositional and eruptive differences suggest an intrinsic link between their volcano-tectonic setting and processes operating within the sub-volcanic magmatic system. This thesis uses a combination of geochemical analysis and modelling to investigate controls on the architecture and dynamics of the Campi Flegrei magmatic system in the last 15 kyr. We first investigated the major element variation between eruptions in the last 15 kyr, using Rhyolite-MELTS thermodynamic modelling. We tested models of fractional crystallisation of a single parental magma, which allowed us to constrain the most likely storage conditions but could not fully reproduce the observed major element variation. However, models of assimilation-fractional crystallisation improved the measured fit between modelled and observed compositions across the caldera. The assimilant composition which produced the best-fit model varied across the caldera. Compositions of eruptions from the caldera rim were best reproduced by models where small amounts of Palaeozoic metamorphic basement was added, whereas erupted compositions from the centre and west of the caldera were best reproduced by adding syenite to the models. Characterising the oxygen, strontium, neodymium and lead isotopic composition of mineral and glass separates from post-15 ka Campi Flegrei eruptions, alongside basement rocks which may have contaminated the magmas, permitted further investigation of assimilation processes, and whether interaction between magma and country rock varies across the caldera. We found significant variation in isotopic composition between eruptions, which we attribute to assimilation of crustal material. Comparison with the isotopic composition of the basement rocks suggests small amounts of Palaeozoic metamorphic basement contaminated eruptions from northern/eastern caldera rim faults, whereas more extensive interaction with syenitic restite affected the isotopic composition of eruptions from the centre and west of the caldera, in agreement with our thermodynamic modelling of major element variation. Processes occurring before volcanic eruptions are also preserved in crystals formed within the sub-volcanic plumbing system. We combined imaging of crystal textures, trace element mapping, in-situ Sr isotope analysis and thermobarometry in clinopyroxenes from Minopoli 2 and Astroni 6 eruptions of Campi Flegrei, to investigate magma evolution at the crystal scale. This revealed a magmatic history of fractional crystallisation, assimilation, magma recharge and mixing before eruption, within a complex sub-volcanic plumbing system. Comparison between eruptions suggests these processes operate with different frequencies, likely linked to the contrasting volcano-tectonic settings of each vent. The results of this thesis provide new insights on processes operating in the sub- volcanic magmatic system beneath Campi Flegrei prior to an eruption and demonstrate an important link between these processes and the spatial and structural location of vents within the caldera.