INDEX
Abstract...1
Introduction...3
1. Campi Flegrei volcanic district...5
1.1. Campi Flegrei Caldera...9
1.2. Campi Flegrei caldera eruptive history...11
1.2.1. Pre-CI epoch: volcanic activity before the Campanian Ignimbrite eruption...16
1.2.2. From Campanian Ignimbrite activity to Neapolitan Yellow tuff eruption...20
1.2.3. Post-caldera activity: eruptions younger than ~15 ka...25
2. The Campanian Ignimbrite case study...31
2.1. Campanian Ignimbrite activity...31
3. Sampling strategy and analytical methods...44
3.1. Sampling strategy: collection and preparation...44
3.1.1. Sample preparation: cleaning procedure and cutting...47
3.1.2. Crystal separation...48
3.2. Geochemical measurements...50
3.2.1. Matrix glass and crystal major element compositions...50
3.2.2. Matrix glass trace element compositions...51
3.3. Sr and Nd isotopes on matrix glasses...51
3.3.1. Matrix glasses leaching procedure...53
3.4. Micro-Sr data...54
3.4.1. The Micro-drilling device...55
3.4.2. Milling procedure and sample collection...58
3.4.3. Sample dissolution and Sr purification...59
3.4.5. Sample loading onto filament...62
3.4.6. Measurement procedure reproducibility and accuracy...63
4. Matrix glass data on the San Martino stratigraphic sequence: clear evidence
for reactivation of the CI reservoir by the incoming of a new magma...65
4.1. San Martino stratigraphic sequence...65
4.2. Matrix glass geochemical compositions ...68
4.2.1. Major elements...68
4.2.2. Trace elements...70
4.3. Sr and Nd isotope compositions of matrix glasses...73
4.4. Discussion...77
4.4.1. Matrix glass geocheical heterogeneities...77
4.4.2. Matrix glass isotope heterogeneities...80
4.4.3. Origin of the UPFU magma composition...82
5. Heterogeneity of evolved magmatic components inferred from matrix glass
data of several proximal-CI outcrops...95
5.1. Description of the investigated proximal-CI stratigraphic sequences...97
5.1.1. Camaldoli...97
5.1.2. Punta Marmolite...99
5.1.3. Monte di Procida...101
5.1.4. Punta della Lingua...103
5.1.5. Pozzo Vecchio...105
5.1.6. Voscone...107
5.2. Selected samples from different outcrops...108
5.2.1. Juvenile products...108
5.2.2. Enclaves...112
5.3. Matrix glass geochemical compositions...114
5.3.2. Trace elements...119
5.4. Sr and Nd isotope compositions on matrix glasses...123
5.5. Discussion...127
5.5.1. Origin of the intermediate geochemical and isotope compositions whithin UPFU...128
5.5.2. Origin of the geochemical variation in PPF...137
5.5.3. SU incompatible trace element heterogeneity...139
5.5.4. Piperno enclaves...140
5.5.5. The CI plumbing system: a heterogeneous reservoir...142
6. Mineral chemistry and in-situ Sr-isotopes on crystals...148
6.1. Major element compositions...149
6.1.1. Sanidine...149
6.1.2. Plagioclase...151
6.1.3. Clinopyroxene...153
6.1.4. Other mineral phases...184
6.2. Micro-Sr analyses on feldspars...155
6.3. Discussion...157
7.
Summary and conclusions...163
Acknowledgments……….……….……….175
Bibliography...177
Appendix 1: Table 1...195Appendix 2: Table 2.1...198
Table 2.2...223
Appendix 3: Table 3.1...224Table 3.2...242
Appendix 4: Table 4…...248
Appendix 5: Table 5.1...249
Table 5.2...279
Appendix 6: Table 6……...280
Appendix 7: Table 7.1...281
Table 7.2...282
Appendix 8:...283
Appendix 9: Table 9.1...284
Table 9.2...285
Appendix 10 ………...286
1
Abstract
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 characterised 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.
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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.
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Introduction
Highly explosive caldera-forming eruptions, during which large volumes of magma are explosively evacuated into the atmosphere from shallow crustal reservoirs, represent one of the most dangerous natural events on Earth. The understanding of the mechanisms leading to the generation of large magmatic reservoirs at shallow crustal levels still represents a challenging and debated task for modern volcanology.
Caldera forming eruptions are generally related to zoned ignimbritic deposits that record significant compositional and thermal gradients within magma reservoirs, thus representing an excellent source of information on magma storage conditions and evolutionary processes. The origin of chemical and thermal zoning has been debated for years and the proposed hypotheses range from magma mixing to in-situ differentiation through crystal–liquid separation. Although mixing of different magma batches is a popular mechanism to account for the geochemical and isotopic heterogeneities detected in magma reservoirs, the importance of in-situ differentiation processes for the generation of chemical and physical gradients has been recently suggested (Bachmann and Bergantz 2008; Cashman and Giordano 2014; Cooper and Kent 2014; Cashman et al., 2017). Thanks to the advancing in analytical techniques, more complicated plumbing system architectures have been proposed in literature to explain geochemical and isotope characteristics within ignimbrite deposits. Recently, some authors have suggested that partially crystallised magmatic reservoirs (i.e., crystal mush) can be remobilized by partial melting processes, which lower their crystallinity and increase their buoyancy, when heat and volatiles are transferred from hotter and mafic recharges (e.g., Huber et al., 2012; Bachmann et al., 2014; Cashman et al., 2017). In this light, it is the combination of in-situ differentiation and interaction with less evolved recharges that generates gradients in zoned ignimbrites.
The classic bulk rock analytical approach is often not sufficient to recognize the possible presence of different magmatic components and their evolutionary processes, especially when complex magmatic dynamics are involved as in the case of the large plumbing systems related to caldera-forming eruptions. Therefore, the most powerful way to shed light into caldera-forming feeding systems is a more detailed approach, able to investigate separately the distinct components that made up volcanic rocks (e.g. matrix glasses and crystals). In this framework, isotopic data represent an essential and helpful mean to identify different magma components in volcanic plumbing system, as they are not affected by fractionating evolutionary processes.
For these reasons, we have applied a detailed geochemical and isotopic micro-analytical approach to study matrix glass and crystal compositions of the juvenile components belonging to
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proximal Campanian Ignimbrite deposits, as this volcanic activity represents a typical caldera-forming eruption. The Campanian Ignimbrite has been erupted at Campi Flegrei (Southern Italy) around 39 ka ago and represents the largest volcanic event in the Mediterranean area over the past 200 ka. Many authors have studied the Campanian Ignimbrite deposits with different methodological approaches and many previous works have been mainly focused on bulk rock geochemical and isotope compositions of medial/distal CI deposits (Civetta et al., 1999; Fedele et al., 2016; Forni et al., 2016; Smith et al., 2016), proposing the presence of geochemical and isotope gradients within the CI sequence. Few papers investigated the proximal CI outcrops (Melluso et al., 1995; Signorelli et al., 1999; Fedele et al., 2008) providing a detailed database of major and trace element contents of juvenile samples from the different units; however, isotope data are limited (e.g., Civetta et al., 1999; Arienzo et al., 2009; Gebauer et al., 2014) and have not been determined systematically throughout the entire stratigraphic sequence. In addition, almost all of the available data represent whole-rock measurements. Hence, at present, a complete dataset of the geochemical and isotopic composition of matrix glasses from samples that are representative of all the different phases of the eruption is still lacking.
For the aforementioned reason, in this study we have analyzed different areas of matrix glass inside single clast, in order to assess the possible presence of geochemical (major and trace elements) and Sr and Nd-isotope heterogeneities in the magma components involved in the Campanian Ignimbrite plumbing system. Moreover, zoning of feldspar and clinopyroxene has been also investigated (major and trace elements and micro-Sr isotopes), to evaluate the condition of geochemical and isotopic (dis)equilibrium between the mineral phases and their host melts.
Data from this micro-analytical work, together with plumbing system configuration and evolutionary processes suggested in literature, will be interpreted in order to unravel geometric configuration of the Campanian Ignimbrite feeding system and the magmatic evolutionary processes that characterized the different melts involved during this caldera-forming eruption.
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1. Campi Flegrei volcanic district
Campi Flegrei caldera (CFc) together with Somma-Vesuvio, Ischia and Procida island constitute the Neapolitan District in the Campanian region, southern Italy (Fig. 1.1). This volcanic complex is located in the Campanian Plain, an area of regional extension related to the stretching and thinning of the continental crust by counter-clockwise rotation of the Italian peninsula and the contemporaneous opening of the Tyrrhenian Basin.
Fig. 1. 1 Map of the distribution of the potassic and ultrapotassic volcanism in Italy, grouped into the three different magmatic provinces: TMP, Tuscan Magmatic Province; RMP, Roman Province and LuMPP, Lucanian Magmatic Province. Within the RMP, the Neapolitan district volcanoes (i.e. ND) are shown with slightly different colour (light grey). From Avanzinelli et al. (2008).
The Campanian graben is delimited to the west by the Tyrrhenian Sea basin, and to the east, south and north by the southern Apennine belt (Doglioni 1991). This dominant
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basin structure, formed during delamination processes of the African plate imposing over the Apulian foreland (Peccerillo 2005), is composed of Mesozoic to Tertiary platform carbonates (Grasso 2001; Patacca and Scandone 2001). The graben is filled by a 2000-3000m thick sequence of Plio-Quaternary continental and marine sediments, which are intercalated with potassic volcanic deposits (De Vivo et al. 2001; Di Vito et al. 2008) (Fig. 1.2).
Fig. 1. 2 Geological sketch map of the Campanian Plain showing the location of volcanic and sedimentary deposits in the area. From Vitale and Isaia (2014).
The Campanian volcanoes lie at the intersection of NE-SW and NW-SE directed fault systems which formed during an extensional phase that generated the so called Apennine (NW-SE) and anti-Apennine (NE-SW) fault systems (Peccerillo 2005), involving various tectonic units with ages from Upper Triassic to Miocene. The extensional phase was preceded by a compressional phase during Upper Oligocene to Lower Pleistocene, leading to over-thrusting in the southern Apennine and generating a number of thrusts, locally covered by autochthonous shallow marine and continental sediments (Plio-Quaternary).
Neapolitan District magmatism is connected to the NW-directed subduction of the Ionian oceanic lithosphere (Doglioni et al., 2001), which is at a depth of ~450 km beneath the Campanian region (Frepoli et al., 1996). The related volcanoclastic successions, cropping out
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in several sectors of the southern apennines (Rolandi et al., 2003), are the geological record of volcanic activity within the Campanian Plain as old as 250 ka (Vitale and Isaia 2014). The genesis of Phlegrean Volcanic District magmatism has been discussed by many authors in the framework of Italian Quaternary potassic magmatism (Hawkesworth and Vollmer 1979; Beccaluva et al., 1991; D’Antonio et al. 1996, 1999a; Peccerillo 1999; Conticelli et al., 2002; Tonarini et al., 2004; Avanzinelli et al., 2008, 2009, 2018; Conticelli et al., 2015). The source of its magmatism has been located in a mantle variably enriched in K, incompatible trace elements, and radiogenic Sr with respect to a typical depleted peridotite. There is a vigorous scientific debate concerning the source of this enrichment and the main hypotesis can be resumed as follow: i) mantle-derived fluids in an intraplate tectonic environment (Hawkesworth and Vollmer, 1979; Cundari 1980; Vollmer 1989, 1991); ii) plume-derived melts (Ayuso et al., 1998; Gasperini et al., 2002; Bell et al. 2004); ii) and fluids/melts released by a subducting oceanic slab (Peccerillo and Manetti 1985; Rogers et al., 1987; Di Girolamo, 1987; Serri 1990; Beccaluva et al., 1991; D’Antonio et al., 1996, 1999a; Peccerillo 1999, 2001; Schiano et al. 2004; Tonarini et al., 2004, Avanzinelli et al., 2008, 2018). Furthermore, geochemical and isotopic variations of Phlegrean Volcanic District evolved rocks have been attributed to crustal contamination and/or mingling and mixing processes of mantle-derived magmas (Turi and Taylor, 1976; Beccaluva et al., 1990; Civetta et al., 1991b; D’Antonio et al., 1999a; de Vita et al., 1999; Piochi et al., 1999; Pappalardo et al., 2002b; De Astis et al., 2004). The island of Ischia is the emergent top of a large volcanic complex that rises more than 1500 m above the seafloor. Volcanic activity started before 150 ka (Gillot et al. 1982) and continued with both explosive and effusive phases, interspersed with periods of quiescence, until the last eruption 1302 A.D. Seismicity, diffuse fumarolic activity, and thermal springs testify that the Ischia magmatic system is still active. The volcanic history of Ischia was dominated by the caldera-forming Mount Epomeo Green Tuff eruption at ca. 55 ka. The erupted magmas at Ischia vary in composition from shoshonite to trachyte (Fig. 1.3). At Ischia, as in Campi Flegrei, the most-abundant rock type is trachyte. Temporal variations of the isotopic composition of the products emplaced in the past 55 ky indicate that the behaviour of the magmatic system has been characterized by phases of magma evolution under closed-system conditions and by arrivals of new, isotopically distinct magmas that show evidence for contamination and mixing processes (Civetta et al., 1991a;
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Piochi et al., 1999). A recent paper by Casalini et al. (2017) suggested the presence of pockets of melts in the feeding system of Ischia volcano, which were able to remain isolated and thus to develop high 87Sr/86Sr.
Fig. 1.3 Total alkali versus silica (TAS; Le Bas et al., 1986) showing compositions for Ischia, Procida, and Campi Flegrei (CF) volcanic rocks. From D’Antonio et al. (2007).
Volcanic activity at Procida Island started in the range 74–55 ka, with the emplacement of the Vivara, Pozzo Vecchio, and Terra Murata tuffs, and ended at ca. 17 ka with the Solchiaro eruption (Rosi et al., 1988; Lirer et al., 1991). The composition of the erupted magmas ranges from potassic trachybasalt through shoshonite to trachyte, with a predominance of mafic and intermediate rocks (D’Antonio et al., 1999a; De Astis et al., 2004) (Fig. 1.3). Lava xenoliths of high-K basaltic composition occur in the Solchiaro tuff and represent the least-evolved rocks of the whole Phlegrean Volcanic District (D’Antonio et al., 1999a). The Sr isotopic composition of the Procida products is variable; however, the scarcity of geochronological data does not allow identification of variation trends through time (D’Antonio et al., 2007).
CFc is the volcanic complex taken as case study for this work, so its description will be treated more extensively and with higher detail in the following paragraph.
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1.1 Campi Flegrei caldera
CFc is an active volcanic field that includes the urban area of the city of Naples in southern Italy and represents one of the most historically active volcanic complexes in Europe. In fact, it was the source of the most powerful eruption ever to be occurred in the Mediterranean area over the last 200 ka (Barberi et al., 1978): the ca. 39 ka Campanian Ignimbrite activity (CI). The volcanic activity in this area, as recorded in exposed sections, began in a period prior to 60 ka (Scarpati et al., 2012) and CFc is currently in a persistent state of activity, as testified by i) the last eruption of Monte Nuovo in A.D. 1538 (D ’Oriano et al., 2005), ii) the 1970–72 and 1982–84 A.D. unrest episodes and bradyseismic crises (Barberi et al., 1991), and iii) the presence of intense fumarolic activity and hot springs (Allard et al., 1991; Forni et al., 2018a) (Fig. 1.4). The volcanic hazard of the caldera is extremely high because of its explosive character and the occurrence of high-magnitude eruptions. The volcanic risk is very high too, due to the intense urbanisation of both the active portion of the caldera and its surroundings, as 1.5 million people are actually living within the caldera boundaries (Orsi et al., 2004).
CFc is identified as a resurgent nested caldera, formed as a consequence of two cataclysmic explosive eruptions, the CI (40Ar/39Ar age:∼39 ka, De Vivo et al., 2001 and Giaccio et al., 2017; (U–Th)/He age:∼42 ka, Gebauer et al., 2014) and the Neapolitan Yellow Tuff (NYT) (14.9 ka, Deino et al., 2004), during which ∼200 and ∼40 km3 DRE (Dense Rock Equivalent) of magma were respectively erupted (Civetta et al., 1997; Orsi et al., 1992) (Fig. 1.5; Fig 1.6). The exact location of caldera margins was the subject of various interpretations in recent literature (Rosi and Sbrana 1987; Orsi et al., 1996; Perrotta et al., 2006; Acocella 2008) that differ mainly in the definition of the eastern sector of the collapsed area, with particular reference to the inclusion or exclusion of the city of Naples in the area affected by the collapse. Some authors (Rolandi et al., 2003) exclude the formation of a caldera in relation to the CI eruption, identifying caldera formation at around 15 ka linked to the NYT (Deino et al., 2004) and other previous eruptions. The two caldera-forming eruptions are usually taken into account to subdivide CFc volcanic activity in pre-caldera eruptions (older than 39 ka) and post-caldera eruptions (younger than 15 ka). In fact, after the NYT eruption, volcanism and deformation have been very intense within the caldera. Volcanic activity continued with at least 70 eruptions, from vents located either along the structural
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boundary of the NYT caldera or along some of the boundaries of the resurgent block (Di Vito et al., 1999; Orsi et al., 1999a-b, 2004). The last epoch began after a significant change in the structural regime, that generated the apex of resurgence and variation of the eruption vents location with their concentration in the portion of the resurgent block under extension.
Fig. 1. 4 (a) Structural sketch map of the Campi Flegrei caldera and (b) the Campanian region including the Somma-Vesuvius volcano and the Phlegraean Volcanic District (Campi Flegrei, Procida and Ischia). (c) Geological cross section through the Campi Flegrei caldera and illustration of the proposed dynamic model (modified after Orsi et al., 1991, 1996). (d1) Vertical ground movement at the Serapeo roman market and (d2) at the most deformed benchmark of the levelling network in Pozzuoli. From Orsi et al. (2004).
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Fig. 1. 4 Map of the Campi Flegrei caldera displaying its major structures (in red) and location of volcanic deposits: i) pre-caldera, older than CI (age>39 ka), ii) between 15 and 39 ka (including NYT) and iii) post-caldera, younger than 15 ka. From Smith et al. (2016).
1.2 Campi Flegrei caldera eruptive history
The oldest dated Campi Flegrei volcanic unit yields an age of ca. 60 ka (Pappalardo et al. 1999), but the exact onset of volcanism in this area is still unknown. Indeed, the age of the magmatic activity has been extended to over 200 ka thanks to geochemical investigation of drillcores in the Campanian Plain (De Vivo et al. 2001). Usually, in literature, the age of the two large caldera-forming eruptions (CI and NYT) represent a sort of benchmark to identify distinct periods of volcanic activity in the system. For this reason, eruptions that took place from the onset of volcanism in the CFc since 39 ka ago (CI) are generally referred as “pre-CI” (I Period). In this light, there is an “intra-calderas” activity between the CI and the NYT eruption (39 - 15 ka ago; II Period). Instead, the “post-caldera” activity includes all eruptions younger than 15 ka (III Period). Furthermore, the latter phase of activity has been divided into three distinct epochs: between ~15 and 10.6 ka (Epoch I), 9.6 and 9.1 ka (Epoch II), and 5.5 and 3.5 ka (Epoch III) (Fig. 1.7).
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Fig. 1. 5 Chronostratigraphy of the volcanic activity in the CFc. The eruptive history is divided into three main Periods; the more recent III Period is moreover split into three epochs. On the left of the figure, the history of deformation through time is also reported. Modified from Orsi et al. (1996).
CFc products belong to the “Roman magmatic Province” (Beccaluva et al., 1991, Conticelli et al. 2002; Peccerillo 2005), and more specifically to its “Neapolitan District” (Avanzinelli et al., 2008, 2009, Conticelli et al., 2015), showing a typical potassic alkaine serial affinity. The composition of magmas erupted through time within the CFc varies from shoshonite to phonolite (Fig. 1.8) , with trachyte and phonolite as the most abundant (D’Antonio 2011). CFc rocks appear to be the product of magmas originated in a mantle modified by fluids/melts from the subducting Ionian slab (Tonarini et al., 2004; D'Antonio et al., 2007) and they show remarkable Sr and Nd isotope ranges (87Sr/86Sr = 0.7068 – 0.7088 and 143Nd/144Nd = 0.5125-0.5124) (Fig. 1.9). There is a general increase of 87Sr/86Sr through time from ca. 60 ka to 39 ka (CI eruption). From 39 ka until ca. 20 ka, the 87Sr/86Sr remained constant and then it increased again until 15 ka (NYT activity). After the NYT, the 87Sr/86Sr shows a large range between 0.7073 and 0.7086. Pb isotope ratios behave similarly to the
87
Sr/86Sr, whereas the 143Nd/144Nd ratio behaves the opposite way (Pappalardo et al., 2002b) (Fig. 1.9).
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Fig. 1. 6 Total alkalis vs silica (TAS, Le Maitre 1989) classification diagram for volcanic rocks of the CFc in the last 60 ka. Compositions of the caldera-forming eruptions (CI and NYT) and the least evolved products of Procida Island are grouped as separate fields. From D’Antonio et al. (1999a).
CFc glass dataset generally shows more diversity than the bulk data that are presented in literature (D’Antonio et al., 1999; Smith et al., 2011; Tomlinson et al., 2012; Voloschina et al., 2018). In fact, in the last 12 ka of volcanic activity, heterogeneous melt compositions have been found in the different eruptions. This suggests that multiple discrete melts are often tapped in a single eruption and that these melts must have had different upper crustal histories. Moreover, the presence of mingling features characterise several CFc deposits, without hybrid compositions that would be resulted from mixing processes, suggest that the contact between some of the different melts was short lived and only occurred immediately prior to eruption (Smith et al., 2011).
Identification of different magmatic components and their evolutionary processes through time in the CFc system have been mainly suggested thanks to isotope studies. Two isotopically distinct components have been recognized to characterize the feeding system between 60 and 15 ka ago: i) The first component is phonolitic and compositionally similar to the pre-CI erupted magmas but more enriched in radiogenic Sr and Nd. This component is supposed to represent the CI resident magma, stored at shallow depth, and it is considered
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the result of sediment assimilation by one of the magma batches feeding pre-CI activity (Arienzo et al., 2009); ii) The second component, trachytic and less-enriched in radiogenic Sr and Nd with respect to the first component, is isotopically different from all the pre-CI magmas. It seems to represent a new magma entering the CI reservoir (Civetta et al., 1997; De Campos et al., 2008; Arienzo et al., 2009). The pre-NYT/NYT component (D'Antonio et al., 2007; Pabst et al., 2008) arrives into the shallow CFc magmatic system a few thousand years before the NYT eruption, as testified by the isotopic composition of pre-NYT products. Residuals of both the CI second component and the NYT component were hypothesized to feed the CFc magmatic system in the past 15 ka (D'Antonio et al., 1999b). The geochemical and isotopic variations of the past 15 ka CFc products have been attributed to either crustal contamination or mingling/mixing among distinct magmas (Civetta et al., 1991; D'Antonio et al., 1999b; de Vita et al., 1999; Pappalardo et al., 2002b; Tonarini et al., 2004, 2009; D'Antonio et al., 2007; Di Vito et al., 2010; Perugini et al., 2010).
Fig. 1.9 87Sr/86Sr vs 143Nd/144Nd isotope trend of the CFc whole rocks through time (Pre-CI: pre Campanian Ignimbrite activity; CI: Campanian Ignimbrite; Post-CI – pre-NYT: volcanic activities between Campanian Ignimbrite and Neapolitan Yellow Tuff eruption; post-NYT: eruptions after Neapolitan Yellow Tuff). Database from literature (GeoRoc online database).
Geophysical investigations and drilling data suggest that the caldera is filled by pyroclastic deposits together with intercalated sandstone down to 2 km and that denser
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pyroclastic and sedimentary rocks occur between 2 and 3 km depth (AGIP 1987; Rosi and Sbrana 1987). Seismic reflection studies has evidenced the presence of thermo-metamorphic rocks bearing water and gas, located below 3 km depth and overlying a basement, whose top is located at 4 km (Zollo et al., 2008). This basement is homogeneous in terms of density between 4 and 8 km and some authors hypothesised that it consists of carbonate rocks (Berrino et al., 2008; Zollo et al., 2008), as Meso-Cenozoic limestones crop out all around the Campanian plain. On the other hand, others authors suggested that the basement, form 4 to 8 km depth, can be made by others lithologies that have similar physical properties of limestone: calc-alkaline volcanic rocks (Wohletz et al., 1999), skarn and feldspatoid-bearing syenites (Fowler et al., 2007). The basement is underlined by a low velocity layer highlighted by seismic data, which is interpreted as a partially molten rock zone (Zollo et al., 2008). This hypothesis is supported by melt inclusion data that provide evidence for CFc magmas of shoshonitic composition stagnated at around 7-9 km depth, then equilibrated and differentiating at progressively shallower depth up to 3-4 km (Marianelli et al., 2006; Arienzo et al., 2010; Esposito et al. 2018). Recently several works have evidenced the presence of crystalline igneous rocks (e.g. syenite s.l.) in CFc juvenile components (Fulignati et al., 2004; Fowler et al., 2007; Bohrson et al., 2006). Moreover, in CFc deposits linked to post-caldera eruptions (<15 ka) the occurrence of antecrysts (alkali-feldspars) with the same isotope composition as that of either CI or NYT volcanic products has been documented. In this light, a schematic CFc plumbing system architecture with a shallow magma reservoir located under a 3-4 km pile of sandstone and pyroclastics has been recently proposed. A crystalline basement, made by minerals related to magmas of older eruptions that accumulated through time in the system, occupies a depth between 4 to 8 km. Under the basement a partially solidified sill-like magmatic chamber is present (Pappalardo et al., 2002; D’Antonio et al., 2011) (Fig. 1.10).
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Fig. 1. 10 Recently suggested schematic view of the resurgent CFc and the underlying crust sector, down to 9 km depth. From D’Antonio et al. (2011). Two different magma reservoirs are located at different depths: the shallower and smaller one is present at 4 km depth while the larger sill-like and partially crystallized reservoir is placed at around 8 km. The basement in between is made of crystalline igneous rocks (syenite).
1.2.1 Pre-CI epoch: volcanic activity before the Campanian Ignimbrite eruption
Volcanic rocks older than the CI are the product of both effusive and explosive eruptions and are mostly alkali-trachyte to phonolite (Fig. 1.11), even if some samples display trachybasaltic compositions. The oldest dated CFc volcanic unit yielded an age of about 60 ka (Pappalardo et al., 1999) and is related to volcanism extending beyond the limits of the present caldera.
Pre-CI outcrops are exposed only along the scarps bordering the CFc depression and they include: i) the lava domes of Punta Marmolite and Cuma (47 ka; Cassignol and Gillot 1982), ii) the Tufi di Torre Franco pyroclastic deposits (42 ka; Alessio et al. 1973), iii) the remnant of the Monte Grillo tuff-cone and iv) a sequence of 12 pyroclastic units separated by paleosoils, outcropping at Trefola (inside the Campi Flegrei area, northeast of Quarto,
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Naples) (Orsi et al., 1996). The upper-most and the lower-most units of the latter sequence were dated by 40Ar/39Ar at 45.6 ± 0.7 ka and 58 ± 5 ka respectively (Pappalardo et al. 1999).
Fig. 1. 11 Major (A) and trace (B) element variations vs D.I. for pre-CI rocks (open triangles) together with volcanic samples attributed to Ischia activity (older than 39 ka; filled triangles). The dotted field represents CI compositions. Modified from Pappalardo et al. (1999). In box (C) the total alkalis vs silica classification diagram (TAS, Le Maitre 1989) showing compositions of pre-CI, CI, pre-NYT and NYT volcanic rocks, is presented. From Pabst et al. (2008).
SiO2, MnO and Na2O contents increase at increasing D.I. values (Differentiation
Index: normative: Or+Ab+Ne), whereas FeOtot, MgO, CaO and P2O5 show opposite
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decrease. Y, Nb, Zr, Rb, and Th plotted versus D.I. describe two trends both characterized by a positive correlation between incompatible element contents and the degree of differentiation. Instead Sr, Ba, Eu, and ferromagnesian elements display single depletion trends with increasing D.I. (Fig. 1.11). In general, pre-CI rocks display enrichments in both LREE and HREE at increasing degrees of differentiation and are characterized by negative Eu anomalies (Eu/Eu* = 0.21-0.98) (Pappalardo et al., 1999). They are also marked by the lowest Sr-isotope compositions (0.70679-0.70735) and the highest 143Nd/144Nd (0.51250-0.51265) displayed by CFc volcanic rocks .
Pre-CI rocks do not show a clear trend of increasing (or decreasing) differentiation with stratigraphic height and compositions seem to cluster in three distinct groups: i) the first erupted, highly differentiated magma batch (pre-CI 1), ii) a following slightly less differentiated batch (pre-CI 2) with intermediate SiO2 but lower Zr contents and iii) the last
component, the pre-CI 3 batch with higher SiO2 and Zr contents with respect to
component-2 (Pabst et al. component-2008). 87Sr/86Sr is largely variable and does not display a correlation with the stratigraphy, while 143Nd/144Nd shows a constant increase toward the CI activity. Pre-CI rock compositions match well with the highly evolved CI end-member (for MgO, Na2O, K2O, P2O5
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Fig. 1. 12 SiO2, MgO, K2O, Zr contents, and Sr and Nd isotopes variation along stratigraphy in the pre-CI,
compared to the following activities (CI, pre-NYT and NYT). Pre-CI reported compositions cover a time span between 60-39 ka ago. From Pabst et al. (2008).
Some authors, focussing on isotope variation in pre-CI volcanics, proposed a model of a continuously growing single magma chamber characterized by processes of fractionation, replenishment by fresh trachytic magma and mixing with newly arrived magma (Pappalardo et al., 1999). More recently, the existence of distinct and separated magma batches in the pre-CI system has been proposed, due to the presence of distinct rock composition groups which do not show systematic differentiation and variation with respect to stratigraphy, and thus, time (Fig. 1.12). In this light, different magma batches evolved in
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separated magma chambers, undergoing different evolutionary processes with distinct storage time. Moreover, the distinct pre-CI magma batches are geochemically and Nd-isotopically similar to the first erupted evolved CI magma, so a common magma source (heterogeneous large reservoir at depth; Fig. 1.13) for pre-CI and CI activities has been suggested (Pabst et al., 2008).
Fig. 1. 13 Schematic plumbing system model for pre-CI and CI activity, proposed by Pabst et. al (2008). Different magmatic sources, feeding pre-CI activity through time are reported as separated reservoirs. In this light the following CI eruption was fed by one of the pre-CI magma component, which was probably recharged by a new, less evolved batch.
1.2.2 From Campanian Ignimbrite activity to Neapolitan Yellow tuff eruption
The present nested resurgent caldera structure in CFc formed as a consequence of two cataclysmic eruptions: CI and NYT occurred around 39 and 15 ka ago, respectively. As the first one is the case study taken into account for this work, its description will be more extensively treated in Chapter 2. The CI is a voluminous (erupted volume 500 km3, bulk volume not dense rock equivalent; Fisher et al.,1993) and widespread pyroclastic sequence composed of a basal fallout deposit and distinct pyroclastic flow units (Civetta et al., 1997).
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CI deposits display very complex sequences in proximal, medial and distal outcrops (Barberi et al., 1978; Di Girolamo et al. 1984; Rosi and Sbrana 1987; Fisher et al., 1993; Perrotta and Scarpati 2003; Orsi et al., 1996; Rosiet al., 1999; Cappelletti et al., 2003; Perrotta et al., 2006; Fedele et al., 2008, 2016; Scarpati and Perrotta 2012, 2016; Engwell et al., 2014). Juvenile components are mainly represented by trachytic to phonolitic pumices and scoriae showing phenocrysts of sanidine, plagioclase, clinopyroxene, biotite, magnetite and apatite (Civetta et al., 1997; Fedele et al., 2008; Forni et al., 2016). General vertical and horizontal geochemical and isotope gradients (SiO2= 47.6-63.7; 87Sr/86Sr= 0.70728-0.70755; 143
Nd/144Nd= 0.51247-0.51257) (Fig. 1.4, 1.11, 1.12) in the CI deposits have been recognized since the early work of Di Girolamo (1970) on the basis of whole rock studies.
After the CI eruption, the intra-calderas II Period of CFc starts: from 39 to 15 ka ago the system was marked by a period of moderate eruptive activity. Juvenile products plot in the fields are trachytes and phonolites, even if some rocks display latitic and tephri-phonoliic compositions (Pappalardo et al., 1999). Generally, SiO2, and Na2O contents increase with
increasing D.I., whereas FeOtot, MgO, CaO, K2O and P2O5 contents decrease (Fig. 1.14). REE
except Eu , Y, Zr, Nb, Rb, Be, and Zn generally show positive correlations, whereas Sr, Ba, Eu, Sc, and V show negative correlations, relative to D.I. REE patterns are characterized by a high degree of enrichment of LREE, and relatively flat HREE patterns (Fig. 1.14), as well as negative Eu anomalies (Eu/Eu*= 0.94–0.29), which increase with increasing degree of evolution.
A link between CI activity and post-CI/pre-NYT eruptions has been argued. Some authors have found a good correlation between 87Sr/86Sr ratios with the stratigraphic position of post-CI/pre-NYT rocks (Orsi et al., 1995; Pappalardo et al., 1999). The lower part of the related deposits has low 87Sr/86Sr ratios (0.70728 – 0.70738) similar to that of some CI components (feldspar and least evolved pumices). At the same time, the upper part of stratigraphic sequence displays higher 87Sr/86Sr (0.70745 - 0.70756), with values that increase towards the top of the sequence reaching the value of magmas feeding the first phase of the NYT eruption. In this light, Pappalardo et al. (1999) proposed that the beginning of post-CI/pre-NYT activity was fed by alkali-trachytic magmas isotopically similar to the last-erupted CI magma, from which they presumably derived by 70% fractionation of the least-evolved CI trachytic magma left in the system. Subsequently, magmas isotopically similar to
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those erupted during the first phase of the NYT eruption (0.70756) reached the system and mixed with the resident magmas (0.70730), producing hybrid magmas with intermediate isotope ratios.
Fig. 1. 14 (A) Bulk rock post-CI/pre-NYT (Period II) major and trace element compositions vs D.I. Open squares represent post-CI/pre-NYT samples while the dotted field shows CI compositions. (B) REE patterns for CI/pre-NYT samples compared to NYT. Modified from Pappalardo et al. (1999). (C) Sr and Nd isotope compositions for CI/pre-NYT (open squares) compared to pre-CI, CI and NYT. From Pabst et al. (2008).
The presence of three different magma components has been also suggested within pre-CI activity (Fig. 1.15). In fact, three distinct rock compositions, mostly distinguished by geochemical compositions and degree of evolution, have been recognize (Pabst et al., 2008). One of these batches (the least evolved) has affinities with the more evolved NYT magma, also showing similar 143Nd/144Nd values (0.51247). Moreover, geochemical and isotope compositions show a continuous trend and decreasing evolution upward toward values displayed by NYT rocks: due to this evidence, post-CI/pre-NYT and NYT magmas have been suggested to evolve from one parent magma, which is similar to the later erupted most mafic NYT end member and completely unrelated to CI magmatic components (Pabst et al., 2008).
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Fig. 1. 15 Schematic plumbing system model for post-CI/pre-NYT and NYT activity, proposed by Pabst et.al (2008). Different magmatic sources, feeding post-CI/pre-NYT activity through time are reported as separated reservoirs. Those magma components interact between them, through mixing processes and possibly represent the second end-member interacting with the NYT melt.
The presence in the system of three different magma components, mostly distinguished by geochemical compositions and degree of evolution, suggests a plumbing system configuration for the post-CI/pre-NYT eruptive period similar to that proposed for pre-CI activity (Pabst et al., 2008) (Fig. 1.15).
The NYT activity, dated 14.9 ka by Deino et al. (2004), is the second largest eruption that took place in the CFc system. Magma composition varies from latite to trachyte and at least three compositionally different magma components fed this eruption: i) an evolved magma displaying low-Ca–Mg–Ba–Sr, negative Eu anomalies and the highest 87Sr/86Sr values (0.70756) (Magma1); ii) an intermediate magma showing low-Ca–Mg, high-K, intermediate Ba and Sr and slightly negative to positive Eu anomalies (Magma2) and iii) a more mafic magma with high-Ca–Mg–Ba–Sr, low-K and 87Sr/86Sr (0.70720), as well as slightly negative Eu anomalies (Magma3). Mineral phases characterizing products of the NYT activity are
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sanidine, clinopyroxene, biotite, magnetite and apatite (Orsi et al., 1992, 1995; Forni et al., 2018b) (Fig. 1.16).
The NYT sequence consists of two main units called Lower and Upper Member (Orsi et al., 1992, 1995) which correspond to Members A and B, respectively, of Scarpati et al. (1993). The Lower Member consists of a sequence of pumice-and-ash fallout deposits and dilute pyroclastic density currents (PDCs) deposits related to phreatoplinian and magmatic explosions from a central vent (Orsi et al., 1995). The Upper Member comprises both massive and thick pumice-and-ash deposits from highly concentrated PDCs and stratified ash beds from dilute PDCs linked to phreatomagmatic activity from multiple vents. A coarse lithic breccia corresponding to the onset of the caldera collapse phase was identified at the base of the Upper Member in the proximal areas (Scarpati et al., 1993). Published works indicate that the eruption was fed by a compositionally zoned reservoir hosting trachytic to trachytic magmas (Magma1 and Magma2 components) and triggered by the arrival of a less evolved recharge magma (Magma3 component) (Orsi et al., 1995; Forni et al., 2018b). It has been suggested that the compositional variations observed in the NYT do not reflect the inversion of a vertically zoned magma chamber but rather a complex interaction between different magmatic components stored in a heterogeneous upper crustal magma reservoir and progressively tapped (Orsi et al., 1992, 1995; Forni et al. 2018b). Those batches interacted prior and during eruption generating the wide compositional ranges observed in the NYT bulk rocks and matrix glasses. In this light, the more evolved melt (Magma1) fed the initial stages of eruption but was also erupted immediately after the caldera collapse. In addition, the relative more mafic component, showing slightly lower Sr-isotope ratios is representative of recharge magmas with partially melted low temperature mineral phases of the resident cumulate pile in the reservoir. These processes generated a compositionally intermediate melt that mixed with the resident evolved melt. Following this model of evolution, NYT represents a combination of melt extraction, cumulate mush melting and mixing/mingling with recharge magmas. A similar scenario has been proposed for the CI plumbing system (Forni et al., 2016), but a connection between the two caldera-forming eruptions has been ruled out (Pappalardo et al., 1999; Forni et al., 2018b).
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Fig. 1. 16 Simplified section of the NYT pyroclastic sequence (after Orsi et al. 1992) and variation of some major and trace elements along the deposit. Grey symbols refer to data from Orsi et al. (1992) and (1995), Scarpati et al. (1993), Wohletz et al. (1995), whereas coloured symbols indicate data from Forni et al. (2018). From Forni et al. (2018b).
1.2.3 Post-caldera activity: eruptions younger than ~15 ka
During the last 15 ka, volcanism continued with at least 70 eruptions (Di Vito et al., 1999), the most recent of which is represented by the 1538 A.D. eruption of Monte Nuovo (Di Vito et al., 1987). The CFc magmas erupted between 15 ka and 1538 AD, range in composition from shoshonite to phonolite, with trachyte and phonolite as the most abundant (D'Antonio et al., 2007) (Fig. 1.17).
They define a silica-undersaturated potassic alkaline series, with degree of silica undersaturation increasing along with DI. After the NYT eruption, activity occurred in three
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epochs (Fig. 1.7): i) between 15 and 10.6 ka (Epoch I), ii) between 9.6 and 9.1 (Epoch II) and iii) between 5.5 and 3.5 ka (Epoch III) (Smith et al., 2011). Location of vents shows a correlation with activity of the different Epochs. During Epoch I, they were located along the structural boundary of the NYT caldera (with the exception of some events). Vents in Epoch II were placed along the north-eastern boundary of NYT caldera, except that of the Baia-Fondi di Baia eruption. Epoch III was characterized by vents located in the north-eastern sector of the caldera floor, subjected to extensional stress (except Averno eruption that occurred in a compressive-regime sector) (Fig. 1.18). The subsequent and last eruption built up the Monte Nuovo tuff cone in 1538, after a quiescence period of about 3 ka (D’Antonio et al., 1999).
Fig. 1. 17 Total alkali-silica classification diagram (T.A.S.; Le Maitre, 1989) for NYT and CFc post 15 ka volcanic rocks. Symbols inside the two gray fields (NYT and post 15 ka) referto samples from different eruptiove Epochs in post-NYT activity. From (Di Renzo et al. 2011).
Relevant unrest episodes (bradyseism) occurred in 1969–1972 and 1982–1984, accompanied by a net uplift of 3.5 m in the central part of the caldera, followed by minor episodes, with a maximum uplift on the order of a few cm (Orsi et al., 1999; Del Gaudio et al., 2010). Intense degassing through fumaroles and hot springs as well as ground deformation and seismicity testify that the magmatic system is still active, and this caused some periods of unrest that have lasted even decades (Chiodini et al., 2001, 2015; Bianco et al., 2004; D’Auria et al., 2011).
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Fig. 1. 18 Location of vent position of the post-NYT eruptions in the structural map of CFc. Different colours identify the dominant (or dominants) isotopically distinct magmatic components. Modified from Di Renzo et al. (2011).
Major and trace element compositions define a single, continuous trend of evolution as a function of DI values: SiO2, Na2O, Rb, La, Th and Pb increase with increasing DI
while MgO and CaO decrease. Different eruptions generally display quite homogenous geochemical compositions. The least evolved rocks have slightly positive Eu anomaly
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whereas usually other more evolved rocks have a negative Eu anomaly and Eu/Eu* together with evolution (Eu/Eu*= 1.04-0.31) (D’Antonio et al., 1999).
Whole-rock Sr-isotope ratios display a large variability, from 0.7073 to 0.7086, and they are negatively correlated to the degree of evolution (Civetta et al., 1991a; D’Antonio et al., 1999). Micro-analytical isotope studies have evidenced the presence of crystals with low Sr/Sr (from 0.7068 to 07060) (Arienzo et al., 2015). Nd-isotopes show lower values (0.51235-0.51250) if compared to pre-NYT activity, and they correlate positively with 87Sr/86Sr during Epoch I and II whilst displaying a negative correlation in Epoch III (Di Renzo et al., 2011) (Fig. 1.19).
Fig. 1. 19 Whole rock variation of Sr-, Nd-, Pb-, B-isotopes through time at Campi Flegrei in the last 15 ka. From Di Renzo et al. (2011).
In order to identify magmatic components involved in the last 15 ka CFc activity, several published papers have focused on isotope composition studies instead of geochemical works. The reason is related to the fact that post-caldera activity extruded geochemically rather homogenous magmas, preventing the use of major oxide and trace element variations to unequivocally establish the role of variable magma evolution processes (Arienzo et al., 2015). On the base of isotopic information, several authors have identified at least three distinct magmatic components occurring in the post-caldera system.
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A first hypothesis (D’Antonio et al., 2011) argues for the presence of the following melts in the last 15ka at CFc: i) a less Sr-radiogenic magma (0.70735-0.70740) ranging in composition from trachyte to trachyte to phono-trachyte, representing CI trachytic melts (named CI component), ii) trachytic to phono-trachytic magma, with high 87Sr/86Sr (0.70750-0.70757) representing a residual portion of NYT reservoir (named NYT component) and iii) a trachybasaltic magma, with the highest Sr-isotope ratio (up to 0.7086) named the MI component (D’Antonio et al., 2011). In this light the CI and NYT components represent residual portions of long-lived and large-volume magmatic reservoirs generated over at least 60 and 15 ka, respectively (Pappalardo et al., 1999). On the other hand, the less evolved MI component could represent magma coming from a deeper reservoir, which was tapped by regional fault system reactivated after NYT caldera collapse. Sr-isotope disequilibria between whole-rocks, minerals and glasses testify that batches of the three magmatic components mixed during their rise to the surface, in variable proportion through time.
Despite being in agreement with the number of distinct magmatic components in the post-caldera system, other authors suggest other geochemical and isotopic characteristics to identify different melts (Di Renzo et al., 2011; Arienzo et al., 2015, 2016). This hypothesis argues that the last 12 ka of activity was dominated by a i) magmatic component similar to the NYT magma (as already proposed by D’Antonio et al., 2011). Moreover, at least two new magmatic components invaded many times the shallow feeding system: ii) Minopoli 2, more enriched in radiogenic Sr (since ca. 10 ka), and iii) Astroni 6, less enriched in radiogenic Sr than NYT in terms of Sr isotopes (since 4 ka). This view also suggests a link between post-caldera and pre-NYT activity in the CFc. Thus, since 15 ka ago, CFc volcanic activity was fed by batches of magma resulting from mixing between the NYT residue and new magmatic components. The Minopoli 2 magmatic component was generated mostly through crustal contamination of mafic magma prior to 10 ka. After, volcanic activity until 8.2 ka (end of the II epoch) was fed by magma batches resulting from mixing between NYT and Minopoli 2, and closed-system evolutionary processes. Other authors, on the base of a melt inclusion coupled together with a geochemical and isotopic matrix glass study of the Baia-Fondi di Baia activity (onset of Epoch II), argued that this eruptive period originated within the CI reservoir (6-9 km depth), thus involving CI components in this post-Caldera volcanic event (Voloschina et al., 2018). Finally, the III epoch was marked by the involvement of at least one new magmatic component, Astroni 6,
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which was significantly less radiogenic in 87Sr/86Sr and more radiogenic Nd (with respect to both Minopoli 2 and NYT). The Astroni 6 isotopic features can be attributed to a smaller role for crustal contamination and a greater role for source heterogeneity with respect to that required for Minopoli 2 and NYT magmatic components (Di Renzo et al. 2011). Arienzo et al. (2015) evidenced the presence of very low 87Sr/86Sr values (0.7060) in some diopsides inside the products of epoch III (Astroni 6 activity), clearly in disequilibrium with the host glass/rock. According to this evidence, Mg-rich pyroxenes of Astroni 6 could represent xenocrysts/antecrysts crystallized from mantle-derived mafic magmas with 87Sr/86Sr ~0.70751 and 143Nd/144Nd ~0.51275. Such low Sr isotope ratios are only recognizable in the CI and pre-CI products (Pappalardo et al., 1999; Di Renzo et al., 2007), suggesting that this primitive component occurred in the CFc system several times over a long time period. In this light, diopside crystals might represent cumulates that formed by prolonged fractional crystallization processes at depth (D’Antonio et al., 2011) and this crystalline material may have been intercepted several times by later upwelling magmas.
These findings provide important constraints for the understanding of the behaviour of the volcano and its magmatic feeding system. The renewal of the activity can be triggered by the arrival in the feeding system of fresh magma (hot and gas-rich) that would mingle/mix with a resident magma batch.
Following those hypotheses, activity at CFc might have been triggered by the arrival of a hot, gas-rich and less evolved magma in a shallow reservoir that would have mixed/mingled with the resident and partially-crystallized magma batch, causing the re-melting of the crystal-mush (Arienzo et al., 2015). Such an event might be able to start an unrest phase. In fact, there are several geophysical lines of evidence suggesting that the unrest event occurred at Campi Flegrei between 1982–1984 might have been caused by the emplacement at shallow depth of a small volume of magma (De Siena et al., 2010; Trasatti et al. 2011; Amoruso et al., 2014). Discovering a detectable surficial signal (geophysical or geochemical), to recognize the arrival of fresh magma at shallow depth, would greatly improve capability to mitigate the risks associated with a future volcanic eruption. This seems to be very important as recent studies have suggested that time-scales between mixing events and eruptions in the recent (< 12 ka) CFc activities are in the order of few days or less than a day (Perugini et al., 2010; Astbury et al., 2018).
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2. The Campanian Ignimbrite case study
In this chapter the Campanian Ignimbrite activity will be described more in detail as it represents the case study taken into account for this work. We have focussed our attention on the study of this highly explosive and large-scale eruption, applying a geochemical and isotopic microanalytical approach, to underline the powerful help that these methods can provide to unravel plumbing system dynamics and their evolutionary processes even when complex volcanic architectures are involved.
2.1 Campanian Ignimbrite activity
The Campanian Ignimbrite (CI) occurred around 39 ka ago (De Vivo et al., 2001; Wood et al. 2012) and is the product of the most explosive eruption occurred in the Mediterranean area over the past 200 ka (Barberi et al., 1978). A recent paper provides a high-precision 40Ar/39Ar age of the CI at 39.85 ± 0.14 ka (Giaccio et al., 2017). This super-eruption was accompanied by a huge volcano-tectonic collapse that gave birth to the CFc (Rosi and Sbrana, 1987; Perrotta et al., 2006; Scarpati et al., 2012). The CI eruption emplaced a pyroclastic fall and flow deposit, consisting in a complex sequence composed of a basal fallout deposit and distinct pyroclastic flow units (Civetta et al., 1997). A widespread tephra layer (Y5), extending from the eastern Mediterranean Sea to Russia, is associated with the products of the CI (Sparks and Huang, 1980; Paterne et al., 1988; Engwell et al., 2014; Smith et al. 2016) (Fig. 2.1).
The total volume and tephra dispersal linked to the CI eruption still represents a debated topic in literature. The plinian phase erupted about 25 km3 of tephra or 20 km3 of DRE (Dense Rock Equivalent. Rosi et al., 1999; Perrotta and Scarpati 2003; Marti et al., 2016. Costa et al. 2012 suggests a greater volume up to 104-125 km3). Co-ignimbrite ash estimations range from 30 to 300 km3 (Giaccio et al., 2005; Marti et al. 2016). However, recent papers suggest the total volume to be around 180-280 km3 DRE (Costa et al., 2012; Marti et al., 2016).
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Fig. 2. 1 A) Locations where distal Campanian Ignimbrite tephra is found (red points on the map) and the general dispersal area (shaded). B) Isopach map of the Plinian fall deposits (in cm) and locations where the flow units are exposed. C) Map of the Campi Flegrei caldera and its major structures. Outcropping areas of proximal and medial/distal CI deposits is also shown. From Smith et al. (2016).
Many published papers have described the stratigraphy of the CI deposits, taking into account both proximal and medial to distal outcrops (Barberi et al., 1978; Rosi and Sbrana 1987; Orsi et al., 1996; Rosi et al., 1996; Cappelletti et al., 2003; Fedele et al., 2008, 2016; Scarpati e Perrotta 2016).
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The proximal CI deposits, called the “Breccia Museo sequence” (Melluso et al., 1995; Fedele et al., 2008), can be observed in scattered outcrops along and close the Campi Flegrei caldera rim, up to ~10 km from the vent. Recent works have shown that those deposits i) display a more complex stratigraphy with respect to medial/distal outcrops and ii) their geochemical compositions and crystal contents extend the respective ranges of medial/distal outcrops (Fedele et al., 2008, 2016; Forni et al., 2016). Following the description of Fedele et al. (2008), the proximal CI type-section, from the bottom to the top of the sequence, is composed by (Fig. 2.2):
Fig. 2. 2 Stratigraphic type-section of the proximal-CI deposit, following the description of Fedele et al. (2008). See text for further details about single units and juvenile and lithic components.
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PPF: Plinian Pumice Fall, moderately to well sorted, stratified coarse pumice deposit, including both the upper and the lower fallout unit of Signorelli et al. (1999). The lower fallout unit displays reverse size grading of pumice and lithics, good sorting, occurrence of free crystals, and rare obsidian fragments. Pumice clasts, up to 5 cm in size and with irregular shapes, are light in colour at the bottom and pale pink at the top of the sub-unit. The lithic content (<5 vol. %) is dominated by fresh to slightly altered trachytic and aphyric lavas. The passage to the upper fallout unit is sharp, indicating a change in eruptive dynamics. It is characterised by normal grading of pumice and lithics, weak sorting and minor abundance of free crystals. Pumice fragments may reach up to 6 cm in size but the fine fraction is more abundant than in lower fallout unit. Lithics, compositionally and dimensionally similar to those of lower fallout unit, are more than twice as abundant (10 vol. %).
USAF: Unconsolidated Stratified Ash Flow level that varies in colour from white at the bottom to red at the top and it is found only in a few proximal locations, whereas it is ubiquitous in distal locations.
Piperno: highly welded tuff, consisting of altered beds of welded ash and flattened scoriae (fiamme) and a coarse breccia with grey lava fragments in ashy matrix. This unit is exposed only in the eastern sector of the Campi Flegrei caldera. Lateral transition to sintered grey ignimbrite has been recognized and it is considered to be the equivalent of the distal WGI of Cappelletti et al. (2003).
LPFU: Lower Pumice Flow Unit, a poorly sorted and pumice-rich level. This unit is locally stratified, passing from an unlithified, weakly welded level at the base to a lithified upper portion. LPFU thickness varies from tens centimetres to tens of metres, being strongly dependent on topography and it shows strong secondary alteration features.
BU/SU: Breccia Unit and Spatter Unit. This level is a coarse, crudely stratified lithic breccia containing juveniles of pumices, spatters and obsidians (BU), which is interlayered in the middle part by welded spatter deposits (SU) which can occur up to
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three times inside the breccia. Due to its characteristics, this unit is considered to be associated to the climactic caldera collapse. Based on the position of BU/SU within the sequence, the caldera collapse likely occurred in the late phases of the CI eruption after the deposition of the main ignimbrite body (WGI and LPFU).
UPFU: Upper Pumice Flow Unit is the uppermost one, consisting in an unlithified and poorly sorted level with abundant degassing structures. Pumices, scoriae and lithics are
the main components, showing a slight normal grading.
Medial outcrops are the most extensively studied in literature and they can be found from ~10 km up to ~80 km east from the inferred CI vent, in the Campanian Plain and on the Southern Appennines. In the past, they were described in a simplified stratigraphic model consisting in a single ignimbritic layer made by two main ignimbritic bodies (Fisher et al., 1993). Recently in literature, a more complex description of CI medial outcrops, based on stratigraphic and compositional correlation between different units of proximal deposits and medial sequences, have been suggested (Fedele et al., 2008; Fedele et al., 2016) (Fig. 2.3). The latter sequences are composed of four units which overlie the basal plinian fallout (PPF). From the bottom to the top, the sequence is characterized by i) a stratified and unconsolidated mainly ashy layer (USAF) followed by the most voluminous ii) welded ash deposit containing reverse graded scoriae, lapilli and blocks (WGI, Welded Grey Ignimbrite). This unit displays lateral different lithofacies, showing sintered-unconsolidated features in distal outcrops while being welded in proximal ones (Piperno unit). WGI grades upward into iii) the Lithified Yellow Tuff (LYT), representing its zeolitized upper facies (Cappelletti et al., 2003; Langella et al., 2013). On top of medial sequence an iv) incoherent pumice deposit (Coarse Pumice Flow, CPF) is founded (Fedele et al., 2016).
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Fig. 2.3 Schematic geological cross-section (not to scale) showing the distribution of the proximal-CI (“Breccia Museo”, from 0 to 10 km) and medial units (from 10 to 80 km) as a function of the distance from the Campi Flegrei source area and of palaeotopography. From Fedele et a.,l (2016).
The CI eruption began with the onset of a substained plinian plume, up to 39 km high, producing a stratified pumice lapillus fall deposit (PPF) dispersed toward the East (Martì et al., 2016; Scarpati and Perrotta 2016). The plinian phase was followed by a collapsing column phase (Scarpati et al., 2014) during which a large pyroclastic density current was generated, spreading over an area of more than 30,000 km2 and managing to surmount mountain ridges up to 1000 m high (Fisher et al., 1993). This phase produced the CI intermediate portions of the sequence (WGI and LPFU); those massive, mostly welded, ignimbrite sheets (Barberi et al., 1978; Fisher et al., 1993; Scarpati and Perrotta, 2012; Scarpati et al., 2015a) represent units that are variously affected by post-depositional glass recrystallisation (Cappelletti et al., 2003). After these, a welded horizon and unconsolidated pumiceous deposits were emplaced in proximal areas, resulting in accumulation of lithic breccia along the caldera rim (Perrotta and Scarpati, 1994; Melluso et al., 1995; Fedele et al., 2008). In this scenario, the main caldera collapse event likely occurred in the late phase of the CI activity, after the deposition of the main ignimbritic bodies, and during the emission of BU (Fig. 2.4) (Rosi et al., 1996; Fedele et al., 2008).
CI juvenile samples are represented by trachytic to phonolitic pumices (Fig. 2.5), scoriae, obsidians and spatters with glassy, aphyric to weakly porphyritic textures (Civetta et al. 1997; Fedele et al. 2016). Their paragenesis, in decreasing order of abundance, is made of sanidine, plagioclase, clinopyroxenes, biotite, oxides and accessory apatite (Barberi et al. 1978; Civetta et al. 1997).
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Fig. 2.4 Eruptive stages and vent evolution during the CI eruption. A)a Plinian fallout stage; b) pyroclastic flow stage leading to deposition of a high-grade ignimbrite and lithic-rich breccias; c) pyroclastic flows producing the accumulation of fines-rich, low-grade ignimbrite and sintered ignimbrite; d) emplacement of the most important breccia layer and associated spatter agglutinate deposit; e) emission of moderately dispersed pumice and ash flows from various vents within the caldera. From Rosi et al. (1996).
Fig. 2. 5 Totali alkalis vs silica (TAS; Le Maitre 1989) classification diagram for volcanic rocks representative of the whole Campi Flegrei Volcanic District over the past 60 ka. The red field identifies bulk rock compositions relative to CI juvenile components. Modified, from D’Antonio (2011).
