The present review covers the two catastrophic events producing the Campanian Ignimbrite (~40ka) and the Neapolitan Yellow Tuff (~15ka) as well as all the recent eruptive activity within the CFC in the past 12 ka (Figure 1).
Campanian Ignimbrite (CI)
Although some authors point to a mainly closed-system evolution by fractional crystallization (e.g., Fedele et al., 2008; Fowler and Spera, 2008), many others propose magma mixing (i.e., open-system evolution) as a key process for the CI magma feeding system (e.g., Arienzo et al., 2009; Civetta et al., 1997; Pappalardo et al., 2008, 2002; Signorelli et al., 1999).
As an example, (Arienzo et al., 2009; Di Renzo et al., 2011) describe the existence of two isotopically distinct CI magmas based on Pd- and Nd-isotope data. Also, (Civetta et al., 1997) proposed that the compositional variation within the CI results from a combination of crystal-liquid fractionation and syn-eruptive magmatic interaction between magmas with different degrees of evolution, accounting for the disequilibrium evidence in crystals and the chemical heterogeneity of glass compositions.
In particular, some studies (Forni et al., 2016; Di Salvo et al., 2020) provided a detailed description of the CI feeding system, proposing the occurrence of a crystal-mush zone, characterized by a buoyant cap of evolved magma. In such scenario, the recharge of hotter and less evolved magma acted as the trigger for the CI eruption. Quantitative modeling by Forni et al., 2018 and Di Salvo et al., 2020, demonstrate that the geochemical and isotopic fingerprint of CI magmas resulted from multiple petrologic processes due to the evolution of the CI crystal-mush by fractional crystallization coupled with the physical and chemical interaction with new mafic magma that reactivated the CI system. In detail, both the cited suggested that the arrival of less evolved and hotter magmas at the base of the CI crystal-mush system produced the melting of low-Or sanidine and low-An plagioclase. In the scenario proposed by Di Salvo et al., 2020, the melting of sanidine and plagioclase reduced the crystallinity of the mushy system, then allowing the triggering of a sequence of complex petrologic processes, including mixing and crystallization.
Neapolitan Yellow Tuff (NYT)
As for the CI, the interpretation of the petrologic processes driving the pre-eruptive history of the Neapolitan Yellow Tuff (NYT; 14.9 ka, Deino et al., 2004) is not straightforward (e.g., Forni et al., 2018b; Orsi et al., 1995, 1992; Scarpati et al., 1993). As an example, Orsi et al., 1995 hypothesize three main magma compositions, separated by gaps in the eruptive sequence and interpreted the architecture of the magmatic system as a chamber filled with three distinct and stratified magmas. At the highest level, the magma was of alkali-trachyte composition, it was highly homogeneous and probably resulted from vigorous convection. The magma filling the intermediate layer was of trachytic composition, showing heterogeneities, also experiencing convection, but less intense than that of the uppermost level. The magma at the bottom of the system was compositionally zoned ranging from alkali-trachyte to latite downward. In such scenario proposed by Orsi et al., 1995, the three magmas filled the magmatic system sequentially, with the last one approaching the system shortly before the beginning of the eruption and possibly acting as the eruption trigger. Recently, Forni et al., 2018b reported a detailed micro-analytical investigation of the mineral phases and matrix glasses collected at different stratigraphic positions along the NYT pyroclastic sequence. In such scenario the compositional variations observed in the NYT do not reflect a vertically zoned magma chamber. Rather, it results from the complex interaction between different magmatic components stored in a heterogeneous upper crustal magma reservoir and progressively tapped. In particular, Forni et al., 2018b point to the occurrence of disequilibrium mineral phases, which suggests interaction with a less evolved recharging magma. Again, the same study indicates that the recharge of the magmatic system by a less evolved and hotter magma activated the convection and promoted the mixing between the refilling and host magmas. Such a scenario is also supported by the presence of intermediate rock compositions hosting crystals deriving from both the host and the refilling magmas. In agreement with Forni et al., 2018b, mixing among all the different components (i.e., evolved host magma, the one deriving from the melting of cumulates, and the less evolved refilling magma) might also explain the broad compositional ranges observed in the Upper Member of the NYT sequence.
Recent activity of the Campi Flegrei Caldera
In the recent activity of the CFC, the evidence of magma mixing is widespread (e.g., (Arienzo et al., 2010; Astbury et al., 2018; D’Antonio et al., 1999; Di Vito et al., 1999). The isotopic record provides many clues to unravel the open-system evolution of the recent activity within the CFC (e.g., Di Renzo et al., 2011; Di Vito et al., 1999). As an example, (D’Antonio et al., 1999) pointed to three isotopically and geochemically distinct magmatic components that erupted in the past 12 ka. They are a Campanian Ignimbrite component (CIc; 87Sr/86Sr ~0.70735–0.70740), a Neapolitan Yellow Tuff component (NYTc; 87Sr/86Sr ~0.70750–0.70757), and a Minopoli component (MIc 87Sr/86Sr of 0.7086), respectively. These three components (i.e., CIc, NYTc, and MIc) are similar to the trachytic magma that has been erupted during the first phase of the Campanian Ignimbrite, to the latitic–alkali–trachytic magma batches extruded the Neapolitan Yellow Tuff, and to the trachybasaltic magma of the Minopoli 2 eruptions, respectively. In agreement with D’Antonio et al., 1999, mixing processes occurred among the three components. In detail, the cited study proposes that the CIc and NYTc represent the residual portions of the long-lived and large-volume magmatic reservoirs developed since at least 60 and 5 ka, respectively, in agreement with Pappalardo et al., 1999. Finally, MIc could represent a magma coming from a deeper reservoir.
Di Renzo et al., 2011, further refined the isotopic characterization of the end-members involved in mixing events during the recent activity of the CFC, hypothesizing three end-members defined by 143Nd/144Nd and 206Pb/204Pb ratios as reported in Figure 2. In particular, two components agree with the characterization reported by D’Antonio et al., 1999. They are the NYTc (87Sr/86Sr of 0.70750–53, 143Nd/144Nd ratio of ca. 0.51246, 206Pb/204Pb of ca. 19.04 and δ11B of ca. –7.9‰) and the MIc (87Sr/86Sr of ca. 0.70860, 143Nd/144Nd ratio of ca. 0.51236, 206Pb/204Pb of ca. 18.90, δ11B value of ca. –7.32‰). The third component in the characterization proposed by Di Renzo et al., 2011 is the Astroni 6 component (A6c) characterized by lower 87Sr/86Sr values than the CIc originally proposed by D’Antonio et al., 1999: in detail, the A6c points to 87Sr/86Sr values close to 0.70726, 206Pb/204Pb of ca. 19.08, 143Nd/144Nd of ca. 0.51250, and δ11B of −9.8‰.
To mention a few specific cases, Arienzo et al., 2010, pointed to two batches of magmas that mixed during the eruption of Agnano Monte Spina. This conclusion is supported by isotope data. In detail, one component was similar to the Minopoli shoshonite (i.e., the MIc proposed by D’Antonio et al., 1999), whereas the second agrees with the NYTc reported by D’Antonio et al., 1999. As a physical scenario, Arienzo et al., 2010 proposed that the mixing between the MI and NYT components was pushed by a gas phase which drove the ascent of magmas.
Also, for the case of Averno 2 fissure eruption (Di Vito et al., 2011), isotopic evidence supports the idea that magma mixing occurred between a more evolved and less radiogenic magma hosted in a shallow reservoir intruded by a less evolved and more radiogenic magma that triggered the eruption. In detail, the two components have been characterized by 87Sr/86Sr values close to 0.70750-0.70750 and 0.70753-0.70754, respectively.
Sr isotopic data on samples belonging to the Astroni 6 eruption reported by Arienzo et al., 2015, again suggest the presence of a magma similar to the NYTc reported by D’Antonio et al., 1999, i.e., 87Sr/86Sr close to 0.70750. Also, the same samples highlight the occurrence of a magma characterized by 87Sr/86Sr values close to 0.70724, interpreted as a magmatic component that entered the CFC feeding system in the past 5 ka, and described by Morgavi et al., 2017. Also, some crystals in Astroni 6 show peculiar 87Sr/86Sr values, in the range of 0.7060–0.7068 and high 143Nd/144Nd ratios, close to Ischia isotopic compositions D’Antonio et al., 2013. Arienzo et al., 2015, proposed that these compositions may represent another common magmatic component, as they have been found in most of the Phlegrean Volcanic District products emplaced over the past 75 ka. The same study proposed that the isotopic variations found in whole rocks, the evidence of isotopic disequilibrium between minerals and glasses, and the huge variation in the Sr isotopic record within single crystals can all be accounted for magma mixing processes. In addition to these isotopic constraints, Astbury et al., 2018, performed a combined use of textural investigations, geochemical data on glasses and crystals, and high-resolution trace element maps of the A6 crystal cargo to reveal the pre-eruptive dynamics occurred before the A6 eruption. Such study disclosed the evolution of the A6 plumbing system involving two separate magma bodies: (a) an evolved magma stored in a shallow system; (b) a less evolved magma represented originally stored at a depth of ~7 km, then raised to shallow levels. Also, Astbury et al., 2018, emphasized that a single recharge and mixing event occurred just before the beginning of the A6 eruption.
The isotopic record also points to open-system behavior for the Nisida Eruption (~ 4 ka BP; Arienzo et al., 2016). In detail, Arienzo et al., 2016, proposed the arrival of a volatile-rich, shoshonite–latite magma, close to the A6c (i.e., 87Sr/86Sr ~ 0.70730; 143Nd/144Nd ~ 0.51250) which triggered the Nisida eruption. Also, the same study suggested that emplacement of the A6c component activated the resurgence of the caldera floor, feeding most of the volcanic eruptions at CFC in the past 5 ka.
The geochemistry of melt inclusions also points to the open system evolution of the activity within the CFC. Esposito et al., 2018, focused on melt inclusions hosted in sanidine, clinopyroxene, plagioclase, biotite, and olivine belonging to the recent activity of the CFC (also including the NYT) and the Island of Procida volcanic systems. Combining analytical determinations and rhyolite-MELTS modeling, Esposito et al., 2018, highlighted a group of melt inclusions that recorded a polybaric fractional crystallization at depths ranging from ≥7.5 km to ~1 km of a volatile-saturated magma. Also, such study reported a group of melt inclusions recording the refilling of the shallow system by a less evolved magma which mixed with the resident magma before the eruption.
The only historical Eruption within the CFC, i.e., Monte Nuovo (1538 AD), also shows evidence of magma mixing: in particular, Di Vito et al., 2016, reported that the volcanic products belonging to the Monte Nuovo eruption result from the mixing between two magmas: the first stored at shallow levels, i.e., 4-5 km; the second possibly intruding the shallow reservoir.