Origin of magmas feeding the Plinian phase of the Campanian Ignimbrite eruption, Phlegrean Fields (Italy): constraints based on matrix-glass and glass-inclusion compositions

Abstract

Glass inclusions hosted by clinopyroxene and matrix glasses of pumices from the Plinian fall deposit associated with the Campanian Ignimbrite eruption were analysed in order to investigate processes that occurred at various stages of the magmatic evolution and to characterise the magma-withdrawal dynamics during the eruption. Petrographic, SEM–EDS, and electron microprobe analyses were performed on pumice samples, glass inclusions, and host minerals. Primary non-evolved glass inclusions were found both in homogeneous salite and in zoned diopside-salite clinopyroxenes. All the studied glasses are peralkaline alkali-trachytic in composition. The variation trends of glass inclusions and matrix glasses overlap for most elements except for alkalies and Cl. The least evolved compositions, however, are shown by glass inclusions found in salitic clinopyroxene at interfaces with diopside from the early erupted Plinian pumices (Lower Fall), whereas the most evolved compositions were found in the matrix glasses of the late-erupted Plinian pumices (Upper Fall). Excluding the most mafic glass inclusions, the observed compositional range is closely comparable to the range for ignimbrite juvenile clast compositions, indicating that the two eruptive phases drained the same magmatic reservoir. The compositional evolution of the Campanian Ignimbrite magmatic system seems to have resulted from different processes occurring at different stages: pre-eruptive magma mixing, due to an input of mafic magma into the reservoir, and crystal fractionation starting from different hybrid parental magmas to produce the most evolved compositions. These processes could have generated a vertically and laterally zoned magma chamber with multiple layers of variably hybrid magma extending from the input points of mafic magma. This type of zoning seems to better account for all the complex chemical and mineralogical characteristics of the Campanian Ignimbrite magmatic system, instead of a simple vertically zoned magmatic reservoir. A syn-eruptive mingling between the least evolved hybrid magmas and the most evolved melts characterised the first phase of Plinian eruption. This uncommon emptying of the magma chamber could be explained by the particular geometry of the reservoir.