A CO2-rich magma source beneath the Phlegraean Volcanic District (Southern Italy):

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-rayequipped
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 lowcrystallized
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 2.69 wt.% and 2653 ppm, respectively, define a major degassing trend with small isobaric deviations. The
collected data allow recalculating entrapment pressures from ~350 MPa to b50 MPa and suggest that the
magma ascent was dominated by degassing. Crystallization was aminor 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 13–14 km (i.e., 350 MPa) 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.