Isotopic constraints on volcano-tectonically controlled assimilation at Campi Flegrei, Italy

Campi Flegrei (southern Italy) has produced >70 eruptions in the last 15 kyr and is currently showing significant signs of unrest within a densely populated part of Europe. These eruptions span a wide range of eruptive styles and compositions and have erupted from different vent locations within a larger caldera system. Here, we characterised 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, to investigate whether interaction between magma and country rock varies across the caldera. Our data point to an important role of assimilation in the system, with differences between individual eruptions attesting to variation in the lithology and amount of assimilated material. These differences correlate with the vent location within the caldera; eruptions along northern/eastern caldera rim faults have isotopic compositions which deviate further from typical mantle source values than those erupted from the centre of the caldera or along western caldera rim faults. Comparison with the isotopic composition of the basement rocks suggests small amounts (<10%) of Palaeozoic metamorphic basement contaminated the eruptions from northern/eastern caldera rim faults, whereas more extensive (<30%) interaction with syenitic cumulate residue (i.e. pre-existing magma mush) affected the isotopic composition of eruptions from the centre and west of the caldera. Our results from vents located across the caldera provide new insights and further constraints on processes operating in the sub-volcanic magmatic system beneath Campi Flegrei prior to an eruption.