Changing magma dynamics and plumbing system architecture at an explosive–effusive transition: the case of Nisyros volcano (Greece)

Abstract. Multiple magma storage levels are commonly recognized beneath magmatic systems and may play an important role in the processes leading to the build-up of large silicic magma chambers in the crust, with possible critical implications for the occurrence of explosive eruptions. Within such reservoirs, interactions between different magmas due to new recharge events are common processes, as demonstrated by the presence of mafic enclaves, which also reveal the occurrence of magma immiscibility conditions. In Nisyros (Greece), the two most recent eruptive events are the caldera-forming explosive eruption of the Upper Pumice (UP) and the following effusive activity of the Post Caldera Domes (PCD), which emplaced a thick pyroclastic deposit and six main lava domes, both hosting mafic components as crystal-rich clasts (CRCs) and enclaves, respectively. These two eruptions show differences in the abundance, petrographic characteristics, mineral chemistry, and geochemical and isotopic signatures of their mafic components, as well as in the extent of their mingling processes, indicating that the magma interaction conditions were different, possibly related to a change in the magma chamber dynamics and/or in the deeper feeding system structure. In this work, we investigated the textural characteristics and mineral chemistry of the products erupted by these two eruptive episodes, exploring their crystallization histories and the possible variations in physical conditions to reconstruct the structure of the plumbing system throughout the two phases of activities. Our results revealed the occurrence of evident mineral disequilibria within CRCs and enclaves related to their rapid crystallization due to the undercooling within the host. In the PCD systems, mineral disequilibria are also related to the extensive crystal transfer from the host to the enclaves and vice versa, generating mingling at the microscale, which increases with time. The application of geothermobarometers records progressively higher pressure from the UP to the PCD under similar temperature conditions. This indicates a deepening of the main eruptible reservoir, sampled by the PCD activity, after the UP–caldera collapse. Between the two periods, an interconnected evolved magma-rich system developed through new inputs of mafic melts that refilled and reheated the system, progressively mingling with the host and generating new conditions for the eruption.