Unravelling plumbing system dynamics linked to explosive eruptions by geochemical and isotopic micro-analyses: the case study of Campanian Ignimbrite, Campi Flegrei, Italy

Caldera-forming eruptions, related to explosive outpouring of large volumes of magma from shallow crustal reservoirs, are one of the most dangerous natural events on Earth. The Campanian Ignimbrite (CI; Campi Flegrei, Italy) represents a typical example of such kind of eruption, and it is associated to a voluminous pyroclastic sequence of trachytic to phonolitic magma emplaced in southern-central Italy, around 39 ka ago. The CI deposits (PPF: Plinian Pumice Fallout, USAF: Unconsolidated Stratified Ash Flow, Piperno: highly welded tuff, LPFU: Lower Pumice Flow Unit, BU/SU: Breccia Unit and Spatter Unit, UPFU: Upper Pumice Flow Unit; Fedele et al. 2008) are characterised by geochemical and isotope variations and its proximal outcrops reveal more significant compositional heterogeneities, as well as a more complete stratigraphy, with respect to medial/distal sequences. Abundant data on bulk rock compositions of CI-proximal deposits are available in the literature, but glass composition had not been determined with the same detail yet. In this study, we tackled this issue through a detailed micro-analytical geochemical and isotopic study of all the units recognized for the proximal CI. The scale of the observation were reduced down to analyzing different areas of matrix glass inside single clasts in order to recognise the presence of geochemical (major and trace elements) and Sr and Nd-isotope heterogeneities in the magma components of the CI plumbing system. Moreover, zoning of feldspar and clinopyroxene were also characterized for major and trace elements and micro-Sr isotopes, to reveal the possible presence of solid/liquid geochemical and isotopic disequilibria. This micro-analytical approach has shown that the CI proximal deposits do not display smooth vertical geochemical and isotope gradients. Samples from all units, with the notable exception the last erupted one (UPFU), have i) low crystal content, ii) evolved matrix glasses, iii) negative Eu anomalies (0.2-0.6), iv) strong micro-scale geochemical and isotope heterogeneities and v) phenocrysts mostly showing disequilibrium textures. On the other hand, the last erupted unit UPFU, shows significant differences with respect to the products of the previously erupted units, namely i) a marked higher phenocryst content, ii) “less evolved matrix glass compositions, iii) positive Eu anomalies (1.0-1.4), iv) less Sr- and Nd-radiogenic signatures and iv) high-Or83-87sanidine with equilibrium textures. Glasses from the crystal-poor evolved units represent compositionally heterogeneous liquids which have been stored separately within the resident crystal-mush, after the arrival of the UPFU “mafic” melt, which represents a new magmatic component entering and un-locking the resident-cumulate system. The evidence of micro-scale heterogeneities in the products of the crystal-poor evolved units (from PPF to BU/SU), suggests that several evolved melts were present at the same time within the magmatic system, but they have been stored individually in distinct areas of the reservoir, separated by the crystal framework of the cumulate. In this light, different evolved melts begin to be in contact between them only shortly before eruption, when the drainage of the liquid portion in the mush has started. Mineral chemistry and micro-Sr isotopes on feldspars point out that the crystal cargo within the mush reservoir is composed by a mixture of i) minerals in equilibrium with CI evolved melts and ii) antecrysts deriving from pre-CI activity, having really wide chemical and 87Sr/86Sr ranges, thus constraining the heterogeneity of the crystal-mush. Data from the UPFU matrix glasses can be explained by a complex evolution involving 1) mixing between the entering “mafic” magma and mush-derived melts made up by i) a large proportion (about 80%) of high-degree melts of sanidine hosted in the crystal mush and ii) a lower amount (about 20%) of interstitial melt hosted within the crystal mush, but also 2) a minor but significant amount of sanidine crystallisation induced by cooling of the “mafic” magma during partial melts of mush components. Evidences of i) chemical mixing between the “mafic” new magma and the resident crystal-poor evolved melts are provided by some intermediate geochemical and isotopic matrix glasses within UPFU samples; ii) the same process plus sanidine fractional crystallisation account for the presence of the hybrid glasses in the BU. Overall, the micro-analytical geochemical and isotopic data presented in this work add new and important constraints to the structure and evolution of the reservoir of a large explosive eruption such as the CI. They suggest an extremely complex scenario where crystal-poor evolved magmas related to previous CFc activity, remain isolated within the CI reservoir, which is thermally re-activated by the arrival of new batches of fresh “mafic” melt. The incoming magma is also involved in the eruption, directly interacting with the lower part of the mush through a complex process that include partial melting of mush-derived crystals, mixing and fractional crystallisation.