Mineral-Melt Equilibria and Geothermobarometry of Campi Flegrei Magmas: Inferences for Magma Storage Conditions

Abstract

The eruptions of Campi Flegrei (Southern Italy), one of the most studied and dangerous active volcanic areas of the world, are fed by mildly potassic alkaline magmas, from shoshonite to trachyte and phonotrachyte. Petrological investigations carried out in past decades on Campi Flegrei rocks provide crucial information for understanding differentiation processes in its magmatic system. However, the compositional features of rocks are a palimpsest of many processes acting over timescales of 100–104 years, including crystal entrapment from multiple reservoirs with different magmatic histories. In this work, olivine, clinopyroxene and feldspar crystals from volcanic rocks related to the entire period of Campi Flegrei’s volcanic activity are checked for equilibrium with combined and possibly more rigorous tests than those commonly used in previous works (e.g., Fe–Mg exchange between either olivine or clinopyroxene and melt), with the aim of obtaining more robust geothermobarometric estimations for the magmas these products represent. We applied several combinations of equilibrium tests and geothermometric and geobarometric methods to a suite of rocks and related minerals spanning the period from ~59 ka to 1538 A.D. and compared the obtained results with the inferred magma storage conditions estimated in previous works through different methods. This mineral-chemistry investigation suggests that two prevalent sets of T–P (temperature–pressure) conditions, here referred to as “magmatic environments”, characterized the magma storage over the entire period of Campi Flegrei activity investigated here. These magmatic environments are ascribable to either mafic or differentiated magmas, stationing in deep and shallow reservoirs, respectively, which interacted frequently, mostly during the last 12 ka of activity. In fact, open-system magmatic processes (mixing/mingling, crustal contamination, CO2 flushing) hypothesized to have occurred before several Campi Flegrei eruptions could have removed earlier-grown crystals from their equilibrium melts. Moreover, our new results indicate that, in the case of complex systems such as Campi Flegrei’s, in which different pre-eruptive processes can modify the equilibrium composition of the crystals, one single geothermobarometric method offers little chance to constrain the magma storage conditions. Conversely, combined methods yield more robust results in agreement with estimates obtained in previous independent studies based on both petrological and geophysical methods.

The Whakamaru Magmatic System (Taupō Volcanic Zone, New Zealand), Part 2: Evidence from ignimbrite deposits for pre-eruptive distribution of melt-dominated magma and magma mushes

“…Large, explosive volcanic eruptions demonstrate that the crust must create and accommodate large volumes of melt-dominated magma prior to eruption. Substantial advances have been made to understand the pre-eruptive conditions of the melt-dominated magma bodies that feed such eruptions (Cashman & Giordano, 2014), including crystallization timescales (Simon and Reid, 2005;Charlier et al, 2008;Druitt et al, 2012;Allan et al, 2013;Barboni and Schoene, 2014;Chamberlain et al, 2014;Cooper and Kent, 2014;Pamukçu et al, 2015a;Gualda and Sutton, 2016;Fabbro et al, 2017;Reid and Vazquez, 2017;Shamloo and Till, 2019;Chakraborty and Dohmen, 2022); storage pressures (Blundy and Cashman, 2008;Hansteen and Klügel, 2008;Putirka, 2008;Ridolfi et al, 2010;Gualda and Ghiorso, 2013a;Bégué et al, 2014a;Bachmann and Huber, 2016;Gualda et al, 2018;Pitcher et al, 2021;Pelullo et al, 2022); volatile content (Moore et al, 1998;Papale et al, 2006;Ghiorso and Gualda, 2015;Waters and Lange, 2015;Iacovino et al, 2021;Wieser et al, 2022); and oxygen fugacity (fO2) conditions (McCanta et al, 2004;Kelley and Cottrell, 2009;Ulmer et al, 2018;Pitcher et al, 2021;Ghiorso et al, 2023). These eruptions have a variety of possible pre-eruptive storage configurations as they can erupt from one magma body, as in the 'mush' model…”

Editorial for the Special Issue “Minerals of Alkaline Igneous Rocks: Chemical and Isotopic Features as Tracers of Magmatic Processes”

“…The interaction (i.e., mingling/mixing) among different magma batches at Campi Flegrei was accompanied by other open-system processes such as crustal contamination and CO 2 flushing, as well as the removal of earlier grown crystals from their equilibrium melts. Another important finding of Pelullo et al [24] was the acknowledgment that using one single geothermobarometer does not constrain with certainty the magma storage conditions. Rather, a combination of various methods provides more reliable P-T estimates that, in the case of the Campi Flegrei complex volcanic system, are in agreement with independent estimates based on petrological and geophysical methods.…”

“…Pelullo et al [24] combined various equilibrium tests and geothermobarometric methods, which they applied to a suite of Campi Flegrei volcanic rocks and related minerals spanning the activity period from ~59 ka to 1538 A.D. (for a review see [25] and references therein), constituting a large set of mineral and bulk-rock chemistry data from the literature. The aim was to evaluate the reliability of the various methodologies in inferring pre-eruptive magma storage conditions.…”