Volcano-tectonic controls on magmatic evolution at Campi Flegrei, Italy: insights from thermodynamic modelling

Fay M Amstutz, Michael Stock, Victoria C Smith, Roberto Isaia, Stefano Vitale, Elliot J Carter, Jacopo Natale

Campi Flegrei caldera (Naples, southern Italy) is one of the most hazardous volcanoes on Earth, having produced more than 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 and compositions, which broadly correlate with the spatial and structural location of vents within the large caldera: eruptions from vents along northern and eastern caldera rim faults are typically small and extend to mafic compositions; eruptions from vents in the central and eastern side of the caldera extend to evolved compositions and have produced Plinian columns; and vents along regional faults (also activated by caldera collapse) in the western caldera have produced sub-Plinian eruptions which are often relatively Na2O-rich and K2O-depleted. These compositional and eruptive differences suggest an intrinsic link between their volcano-tectonic setting and structure and/or processes operating within the sub-volcanic magmatic system. To investigate this, we compare post-15 ka erupted glass major element compositions to liquid lines of descent produced using the Rhyolite-MELTS thermodynamic model. To constrain magma storage conditions at Campi Flegrei, we systematically vary the crystallisation conditions in 1800 models before employing a new statistical approach to assess the quality of fit between natural glass compositions and model outputs. In simple (uncontaminated) fractional crystallisation models, we find that glass compositions in each volcano-tectonic setting are best reproduced by similar storage conditions: pressure of 110–160 MPa, liquidus oxygen fugacity of 0–1 log unit above the quartz-fayalite-magnetite buffer, and a liquidus H2O concentration of 2 wt% for northern, eastern and western caldera eruptions and 3 wt% for central caldera eruptions. However, the addition of an assimilant further improves the fit between predicted and observed major element compositions, with the amount and type of assimilant varying between volcano-tectonic settings. Best-fit models for vents along northern and eastern caldera rim faults include small (5–10%) amounts of Palaeozoic metamorphic basement, whereas those for vents in the centre of the caldera or along the western regional faults include larger quantities (~30%) of assimilated syenitic restite. The Fondi di Baia eruption is compositionally anomalous, and its evolution may reflect minor limestone or hydrothermal calcite contamination. Our results demonstrate a novel link between the spatial and structural location of vents within the Campi Flegrei caldera and the physicochemical processes operating within its magmatic system, providing important information for the assessment of future hazard scenarios.