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Abstract: Campi Flegrei caldera, within the Neapolitan area of Italy, is potentially one of the most dangerous volcanoes in the world, and during the last decade it has shown clear signs of reactivation, marked by the onset of uplift and changes in the geochemistry of gas emissions. We describe a 30-year-long data set of the CO2–He–Ar–N2 compositions of fumarolic emissions from La Solfatara crater, which is located in the center of the caldera. The data display continuous decreases in both the N2/He and N2/CO2 ratios since 1985, paralleled by an increase in He/CO2. These variations cannot be explained by either processes of boiling/condensation in the local hydrothermal system or with changes in the mixing proportions between a magmatic vapor and hydrothermal fluids. We applied the magma degassing model of Nuccio and Paonita (2001, Earth Planet. Sci. Lett. 193, 467–481) using the most recent inert-gas solubilities in order to interpret these peculiar features in accordance with petrologic constraints derived from the ranges of the melt compositions and reservoir pressures at Campi Flegrei. The model simulations for mafic melts (trachybasalt and shoshonite) show a remarkably good agreement with the measured data. Both decompressive degassing of an ascending magma and mixing between magmatic fluids exsolved at various levels along the ascent path can explain the long-term geochemical changes. Recalling that (i) a sill-like reservoir of gases at a depth of 3–4km seems to be the main source of ground inflation and (ii) there is petrologic and geophysical evidence for a reservoir of magma at about 8km below Campi Flegrei, we suggest that the most-intense episodes of inflation occur when the gas supply to the sill-like reservoir comes from the 8km-deep magma, although fluids exsolved by magma bodies at shallower depths also contribute to the gas budget. Our work highlights that, in caldera systems where the presence of hydrothermal aquifers commonly masks the magmatic signature of reactive volatiles, inert gases are the preferred species to use when seeking information on the melt composition, dynamics, and structure of the plumbing systems. [Copyright &y& Elsevier]

Abstract: We present a study of high-temperature, uniaxial deformation experiments of natural, partially-crystallized magma from the Monte Nuovo (1538AD) trachytic eruption. The experiments were performed using a high-temperature uniaxial Geocomp LoadTrac II press at dry atmospheric conditions and under controlled deformation rates. Each experiment involved deforming cores of natural (i.e., crystal- and vesicle-bearing) scoriaceous samples isothermally (600 to 800°C) at constant displacement rates (CDR) corresponding to strain rates between 10−7 and 10−4 s−1. Measured viscosities vary between 1010 and 1013 Pas. The flow behavior of these complex natural materials are fully described by a simplified Herschel–Bulkely equation in terms of consistency K and flow index n. We estimate the combined effects of crystals and pores on the rheology of these multiphase suspensions. Our results demonstrate that the presence of pores has a major impact on the rheological response of magmas and may produce a marked decrease of their viscosity. At the same time, the presence of pores leads to a strong decrease in the strength of the magma inducing local and temporal variation in the deformation regimes (ductile vs. brittle). Brittle failure was in fact observed at T =600°C and strain rates of 10−5 s−1 and at T =800°C for the highest applied strain rate (10−4 s−1), respectively. This study constitutes an important step toward the estimation of multiphase rheological evolution of Monte Nuovo magmas and toward the general understanding of the full complexities governing the dynamics of magma transport in natural systems. [Copyright &y& Elsevier]

The 3.7 ka year-old Averno 2 eruption is one of the rare eruptions to have occurred in the northwest sector of the Phlegraean Fields caldera (PFc) over the past 5 ka. We focus here on the fallout deposits of the pyroclastic succession emplaced during this eruption. We present major and trace element data on the bulk pumices, along with major and volatile element data on clinopyroxene-hosted melt inclusions, in order to assess the conditions of storage, ascent, and eruption of the feeding trachytic magma. Crystal fractionation accounts for the evolution from trachyte to alkali-trachyte magmas; these were intimately mingled (at the micrometer scale) during the climactic phase of the eruption. The Averno 2 alkali trachyte represents one of the most evolved magmas erupted within the Phlegraean Fields area and belongs to the series of differentiated trachytic magmas erupted at different locations 5 ka ago. Melt inclusions record significant variations in HO (from 0.4 to 5 wt%), S (from 0.01 to 0.06 wt%), Cl (from 0.75 up to 1 wt%), and F (from 0.20 to >0.50 wt%) during both magma crystallization and degassing. Unlike the eruptions occurring in the central part of the PFc, deep-derived input(s) of gas and/or magma are not required to explain the composition of melt inclusions and the mineralogy of Averno 2 pumices. Compositional data on bulk pumices, glassy matrices, and melt inclusions suggest that the Averno 2 eruption mainly resulted from successive extrusions of independent magma batches probably emplaced at depths of 2-4 km along regional fractures bordering the Neapolitan Yellow Tuff caldera. [ABSTRACT FROM AUTHOR]

Abstract: We present a study of olivine-hosted glass/melt inclusions (MIs) in the most primitive rocks erupted at Procida Island, within the Phlegraean Volcanic District (PVD), Southern Italy. MIs were analyzed by combined Scanning Electron Microscopy coupled with Energy Dispersive X-ray-detectors, Wavelength Dispersive X-ray-equipped Electron Microprobe and Fourier Transform Infrared (FT-IR) Spectroscopy; notably, the novel Focal-Plane-Array mode provided high-resolution FT-IR images evidencing the distribution of the C–H–O species across samples. Olivines range in composition from Fo85 to Fo90, do not show chemical zoning and are totally anhydrous. The majority of the MIs are glassy, while only few are completely crystallized. Some MIs, however, show the occurrence of crystal nuclei, i.e., nano- to micro-sized pyroxenes and oxides, and appear as low-crystallized MIs. The glass of crystal-free and low-crystallized MIs shows K-affinity and a compositional range along the basalt, trachy-basalt, shoshonite, tephrite basanite and phono-tephrite array. H2O and CO2 contents up to 2.69wt.% and 2653ppm, respectively, define a major degassing trend with small isobaric deviations. The collected data allow recalculating entrapment pressures from ~350MPa to <50MPa and suggest that the magma ascent was dominated by degassing. Crystallization was a minor process, likely also consequent to local CO2-fluxing. Mingling occurred between variable degassed and crystallized magma portions during decompression. The geochemical and isotopic data of Procida glasses and rocks, and the compositional relationship between our MIs and those from slightly more evolved and radiogenic Phlegraean products, indicate that Procida basalts are an adequate parental end-member for the PVD. Our data suggest that a CO2-rich magma source was stored at depths of at least 13–14km (i.e., 350MPa) beneath the PVD. Fast ascent of magma batches directly started from this depth shortly before PVD trachy-basaltic to shoshonitic eruptions. Such results have implication on volcanic hazard assessment in the PVD area. [Copyright &y& Elsevier]

Abstract: The solubility of H2O–CO2-bearing fluids in shoshonitic melts relevant to magmas of the Vulcanello peninsula and to mafic melts from magmas erupted at Campi Flegrei (Italy) was experimentally determined at pressures from 50 to 400MPa and at temperature of 1250°C. No quench crystals and less than 1vol.% bubles were observed in the rapid quenched glasses. H2O and CO2 contents in the experimental glasses were determined via Karl–Fischer Titration (KFT) and FTIR spectroscopy. For the quantification of volatile concentrations by IR spectroscopy we have calibrated the absorption coefficients of water-related and carbon-related IR bands for the shoshonitic composition. The determined absorption coefficients are 0.80±0.06L mol−1 cm−1 for the band at ~4500cm−1 (OH groups) and 1.02±0.03L mol−1 cm−1 for the band at ~5200cm−1 (H2O molecules). CO2 is bound in the shoshonitic glass as CO3 2− exclusively; its concentration was quantified by the peak height of the low wavenumber band of the doublet near 1430cm−1 using the calibrated absorption coefficient of 356±18L mol−1 cm−1. The H2O solubility in the shoshonitic melts is in the same range as observed for other natural aluminosilicate melts, i.e. 5.12±0.07wt.% at 200MPa and 7.92±0.07wt.% H2O at 400MPa. A non-linear variation of the H2O and CO2 solubility in the melts with increasing mole fraction of H2O (and thus decreasing mole fraction of CO2) in the fluid was observed at each investigated pressure. At 1250°C, the concentration of dissolved carbonate (expressed as CO2 component) in melts coexisting with nearly pure CO2 fluid increases from 307 to 2932ppm (±10% relative) as the pressure increases from 50 to 400MPa. The comparison of the dataset with available models predicting the H2O and CO2 concentrations in silicate melts coexisting with H2O–CO2-bearing fluids shows that the effect of melt composition is not calibrated appropriately in the models. The experimental data are used to re-evaluate the typical pressures of glass inclusions entrapment in phenocrysts from Minopoli2 eruption and the results indicate that the main magma chamber may have been located at a depth of ~4000m. [Copyright &y& Elsevier]

Magmatic processes triggering eruptions at Campi Flegrei caldera (southern Italy) and their relationships with the widespread emissions of fluids and caldera unrest episodes, are poorly constrained. The 4.1kaB.P. Agnano–Monte Spina eruption, the reference event for a future large-size explosive eruption at Campi Flegrei, was investigated to shed light, through melt inclusion and isotope analyses, on the geochemical processes operating in the plumbing system. Chemical and isotopic data on whole rocks and glasses suggest that at least two magma batches mixed during the course of the eruption. Melt inclusion data highlight the pre-eruption storage conditions of two magmatic end-members. One end-member is like the less differentiated (shoshonitic) Campi Flegrei erupted magma, while the other could be a residual of the Neapolitan Yellow Tuff magma. Mixing between these two components was driven by a large gas phase which sustained the ascent of magmas of deep provenance. The H2O and CO2 contents in pyroxene-hosted melt inclusions yield entrapment pressures between 107 and 211MPa, corresponding to depths between 4 and 8km. The degassing trends reveal two extreme patterns. One pattern, already documented in the literature, is the volatile signature of poorly differentiated magmas ascending from more than 8km depth, while the other is related to a gas-dominated magma, flushed by a CO2-rich gas phase partly released from the deep reservoir. This study provides a conceptual frame for unrest phases at Campi Flegrei, such as the 1982–84 event. Uplift phases can be related to closed-system ascent of magmas and fluids from more than 8km depth, and their emplacement at shallow levels. This leads the shallow system to store, and then progressively release, the accumulated gas. In this view, both unrest episodes and eruptions could be strongly influenced by both the achievement of a critical upper limit of gas storage in the shallow magmatic reservoir and the stress and fracturing state of the roof rocks. The present results help to constrain the pre-eruptive conditions expected at Campi Flegrei caldera in case of a future large-size eruptive event. [Copyright &y& Elsevier]

Abstract: Campi Flegrei (CF) is a large volcanic complex located west of the city of Naples, Italy. Repeated episodes of bradyseism (slow vertical ground movement) near the town of Pozzuoli have been documented since Roman times. Bradyseismic events are interpreted as the consequence of aqueous fluid exsolution during magma solidification on a slow timescale (103–104 yr) superimposed upon a shorter (1–10yr) timescale for the episodic expulsion of fluid from a deep (~3–5km) lithostatically-pressured low-permeability reservoir to an overlying hydrostatic reservoir. Cycles of inflation and deflation occur during short duration transient events when connectivity is established between deep and shallow hydrothermal reservoirs. The total seismic energy released (4×1013 J) during the 1983–1984 bradyseismic crisis is consistent with the observed volume change (uplift) and consistent with the notion that seismic failure occurs in response to the shear stress release induced by volume change. Fluid transport and concomitant propagation of hydrofractures as fluid expands from lithostatic to hydrostatic pressure during decompression leads to ground surface displacement. Fluid decompression occurs along the fluid isenthalp (Joule–Thompson expansion) during transient periods of reservoir connectivity and leads to mineral precipitation. Each kilogram of fluid precipitates about 3×10−3 kg of silica along a typical decompression path along the isenthalp. Mineral precipitation modifies the permeability and acts to reseal connection paths thereby isolating lithostatic and hydrostatic reservoirs ending one bradyseism phase and beginning another. Crystallization and exsolution of the magmatic fluid generates ≈7×1015 J of mechanical (PΔV) energy, and this is sufficient to accomplish the observed uplift at CF. Although magma emplacement is the ultimate origin of bradyseism, fresh recharge of magma is not a prerequisite. Instead, short to intermediate timescale phenomena associated with fluid decompression and expansion in the crust with large variations in permeability, including permeability anisotropy, are the key elements at CF. [Copyright &y& Elsevier]

High resolution Vp and Vp/Vs tomography of the Campi Flegrei caldera is obtained using active and passive seismic data. We find a continuous ring of high Vp anomaly that defines the caldera rim associated to the last collapse. A sharp Vp/Vs decrease is observed between 2 and 4 km depth, suggesting the absence of magmatic fluids and the presence of rock volumes with over-pressured gas within the source region of uplift. Atmospheric water penetrating within the caldera and deep CO2 fluids are presumably heated by a magmatic body located at depth greater than 4 km nested within the limestone layer. Along the fractures bordering the shallow high Vp rim, deep gas and CO2 fluids up-raise and are released in the Pozzuoli solfatara. We hypothesize that the past unrest episode is more likely due to pressure changes within the shallow geothermal reservoir located at the top of the magma intrusion. [ABSTRACT FROM AUTHOR]

A supereruption that occurred in the Campi Flegrei area, Italy, ca. 39 ka had regional- and global-scale environmental impacts and deposited the Campanian Ignimbrite (CI). We attempt to shed light on critical aspects of the eruption (depth of magma chamber, intensive pre-eruptive magma conditions) and the large-volume magma plumbing system on the basis of information derived from analyzing melt inclusion (MI) data. To achieve these aims, we provide new measurements of homogenization temperatures and values of dissolved H2O within phenocryst-hosted MIs from pumices erupted during different phases of the CI eruption. The MI data indicate that a relatively homogeneous overheated trachytic magma resided within a relatively deep magma chamber. Dissolved water contents in MIs indicate that prior to the eruption the magma chamber underwent radical changes related to differential upward movement of magma. Decompression of the rising trachytic magma caused a decrease in water solubility and crystallization, and trachytic bodies were emplaced at very shallow depths. The proposed eruptive model links portions of the main magma chamber and apophyses with specific eruptive units. [ABSTRACT FROM AUTHOR]