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

Multiple magma storage levels are commonly recognized beneath magmatic systems and playan important role in the processes leading to the build-up of large silicic magma chambers in the crust, with possiblecritical implications for the occurrence of explosive eruptions. Within such reservoirs, interactions betweendifferent magmas due to new recharge events are common processes, as demonstrated by the presence of maficenclaves, which also reveal the occurrence of magma immiscibility conditions.At Nisyros volcano (Greece), the two most recent eruptive events, the caldera-forming explosive eruption of theUpper Pumice (UP) and the following effusive activity of the Post Caldera Domes (PCD), emplaced athick pyroclastic deposit and six main lava domes, both hosting mafic juvenile products like 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 extentof the mingling processes, indicating that the magma interaction conditions were different, possibly related toa 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 bythese two eruptive episodes, exploring their crystallization histories and the possible variations in physical conditionsto reconstruct the structure of the plumbing system throughout the two phases of activities. Our resultsrevealed the occurrence of evident mineral disequilibria within CRCs and enclaves related to their rapid crystallizationdue to the undercooling within the host. In the PCD systems, mineral disequilibria are also related tothe extensive crystal transfer from the host to the enclaves and vice versa, generating a microscale mingling,which increases with time. The application of geothermobarometers highlight a progressively increase in pressure fromthe 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. We infer that between the two periods, an interconnectedevolved 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.